Iceland Rift Platinum

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Platinum-group elements in the Icelandic rift system:

melting processes and mantle sources beneath Iceland

Peter Momme a,*, Nels Oskarssona, Reid R. Keaysb

aNordic Volcanological Institute, Grensasvegur 50, 108 Reykjavik, IcelandbDepartment of Earth Sciences, Monash University, Clayton, VIC 3800, Australia

Abstract

Thirty new platinum-group element (PGE) analyses of various basalt types from Iceland are presented in this study. The

analysed samples are divided into three groups based on their Mg-contents, high-Mg tholeiites (1014 wt.% MgO), composed

of primitive olivine tholeiites and picrites, evolved olivine tholeiites (7 10 wt.% MgO) and evolved basalts (4 7 wt.% MgO)

consisting of FeTi basalts, quartz tholeiites and alkaline basalts.

The high-Mg tholeiites have a range of compositions between a relatively Cu-rich and Pd-poor (120 ppm Cu and 6 ppb Pd)

end-member and a relatively Cu-poor and Pd-rich (74 ppm Cu and 17 ppb Pd) end-member. There is a positive correlation

between the highly siderophile elements, whereas Cu and Pd correlate negatively between the Cu-rich primitive olivine tholeiite

and the Cu-poor picrite end-member. Negative correlation between Cu and the PGEs cannot be reconciled in a model where the

two end-members form by varying degrees of partial melting of a common source. The sub-primitive mantle Cu/Pd ratio of the

picrite end-member (f 4300) could indicate a strongly depleted mantle source, where Pd was efficiently retained in mantle

sulphide, relative to Cu, during previous melt extraction episodes. In mantle melting models presented here, the picrite

composition can be approximated by 25% melting of a mantle source that previously lost a normal MORB component

(Ff 15%). The olivine tholeiite end-member (Cu/Pdf 19000) can be approximated in mantle melting models by assuming

derivation from a separate source, one slightly Cu-enriched source undergoing melting beneath a rift. Based on Cu and Pd

variations, the majority of high-Mg tholeiites are interpreted to be mixtures between melts derived from highly depleted to

slightly enriched sources, which is consistent with previous studies of SrNd isotopic variations.

The similar range in Cu/Pd ratios of the high-Mg tholeiites and the evolved olivine tholeiites could infer that the latter

evolved from parental liquids similar to the high-Mg tholeiites under S-undersaturated conditions. Relatively high Pd and low Ir

concentrations in the evolved olivine tholeiite samples (f 7 wt.% MgO) indicate, too, that these magmas underwent S-

undersaturated fractionation during which Pd accumulated in the melt whereas Ir was incorporated into the fractionating

assemblage. This is in contrast to the group of evolved basalts with less than 7 wt.% MgO with low PGE contents and high Cu/

Pd ratios that strongly suggest that these samples have experienced S-saturation.We propose a scenario for melt generation beneath the Icelandic rift zone, where picritic magma is generated from a highly

depleted mantle, whereas primitive olivine tholeiite magma forms from the depleted as well as a Cu-enriched source. It is

proposed that Cu-enriched mantle domains were exhausted at depth since their melting products are absent in the most depleted

picrite derived from the uppermost part of the central melting region. The olivine tholeiites can be approximated by around 11

12% partial melting in a triangular melting regime beneath the Icelandic rift zone. The olivine tholeiites are interpreted to reflect

0009-2541/02/$ - see front matterD 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0009-2541(02)00414-X

* Corresponding author. Present address: Department of Civil Engineering, Aalborg University, Sohngaardsholmsvej 57, DK-9000

Aalborg, Denmark.

E-mail address: [email protected] (P. Momme).

www.elsevier.com/locate/chemgeo

Chemical Geology 196 (2003) 209234


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efficient melt collection from the deepest and most distal parts of the melting triangle. In contrast, the picrite represent a high

degree of melting (Ff 25%) without incorporation of melt batches from the Cu-enriched mantle source.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Platinum-group elements; Icelandic rift system; Melting processes and mantle sources

1. Introduction

Iceland is suitable for investigating the interaction

between a ridge system and an upwelling mantle

plume. The ridge system normally produces mid-

ocean ridge basalt (MORB), whereas a mantle plume

can bring multiple components to the melting region

(e.g. White and McKenzie, 1989; Langmuir et al.,1992; Hemond et al., 1993; Hanan and Schilling,

1997; Ito et al., 1999; Hanan et al., 2000). Studying

the platinum-group elements (PGE) in Iceland can

reveal such interactions, because normal MORB is

PGE-poor (Hamlyn et al., 1985; Rehkamper et al.,

1999) whereas plume-derived magmas such as flood

basalts are usually PGE-rich, and sometimes associ-

ated with economically important PGE deposits

(Barnes et al., 1988; Naldrett et al., 1992). The

difference in PGE contents between MORB and

plume-derived basalts may reflect highly different

source compositions or relate to the degree of partial

melting by which the magmas form. PGE-rich mag-

mas are known to form by high degrees of partial

melting (F>2025%, Keays, 1995) compared to

MORB (F=820%, Klein and Langmuir, 1987).

Platinum-group elements and Re Os isotopes

have been studied in Hawaiian basalts derived from

intra-plate hot-spot volcanism (Brandon et al., 1998,

1999; Bennett et al., 2000; Crocket, 2000). There,

positive correlations between 186Os/188Os and cOshave been suggested to reflect incorporation of

PGE-rich outer core material, including PGEs, intothe Hawaiian mantle plume (Brandon et al., 1998).

Koolau picrites stand out with near-chondritic186Os/188Os but radiogenic 187Os/188Os, which could

represent the presence of ancient crustal material in

the plume (Brandon et al., 2000). Platinum-group

element concentrations in Hawaiian picrites and

basalts are relatively low (e.g. 0.34 ppb Pd) (Bennett

et al., 2000; Crocket, 2000), which seems consistent

with the low degrees of partial melting 310% by

which Hawaiian magmas are derived (Norman and

Garcia, 1999). The PGE concentrations of Hawaiian picrites have been interpreted to reflect varying

amounts of residual sulphide in the mantle (Bennett

et al., 2000).

Continental flood basalts were derived from mantle

melting during continental breakup above the Ice-

landic mantle plume at 55 Ma and were deposited at

the volcanic rifted margin in central East Greenland

(Lawver and Muller, 1994; Pedersen et al., 1997;

Tegner et al., 1998a). There, isotopically depleted

low-Ti basalts are found interlayered with enriched

high-Ti basalts (Tegner et al., 1998a). Previous studies

have shown that the high-Ti basalts are relatively rich

in PGE (e.g. up to 24 ppb Pd and 250 ppm Cu) even

though they represent low to moderate degrees of

partial melting f 39% ( Nielsen and Brooks, 1995;

Tegner et al., 1998a; Momme, 2000; Momme et al.,

2001, 2002). The distribution of PGEs in the primitive

Ti-rich flood basalts is best explained by spreading-

controlled upwelling, and mantle melting during con-

tinental rifting (Tegner et al., 1998a; Momme, 2000).

The low-Ti basalts are also rich in the PGEs (up to 24

ppb Pd) and isotopically they are similar to the most

depleted MORB in the North Atlantic region, and areconsequently interpreted to have formed beneath the

earliest plume-influenced segments of the North

Atlantic oceanic rift (Tegner et al., 1998a). One of

Fig. 1. (a) Map of Iceland showing the Eastern Rift Zone (labeled ERZ), the adjoining propagating rift, the South-Eastern volcanic Zone (labeled

SEZ) and the Western Rift Zone (labeled WRZ). The Reykjanes and Kolbeinsey Ridges are labeled RR and KR, respectively, and the Tjornes

Fracture Zone is labeled TFZ. Glaciers are shown as shaded areas. The inset in the figure denotes the plume area shown in b. Sample locations

are marked in both figures. (b) The plume area in Iceland is outlined on the map where Holocene volcanics are shown by shaded areas. The area

of the figure is approximately coincident with the 3He/4He anomaly in Iceland. Encircled B marks the position of the Bardarbunga volcano. The

axial rift segment overlying the plume area is composed of several volcanic centers and rifting fissure swarms.

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the main aspects of this study is to investigate if

recent, isotopically depleted basalts, derived from

plume-influenced rift segments in Iceland are PGE-

rich and S-undersaturated similar to their 55-Ma-oldequivalents.

In this paper we present PGE data, mantle melting

modeling and a scenario in which the observed

chemical and physical relationships between the Ice-

landic primary magma types can be explained by

melting processes, magma mixing and mantle source

heterogeneity beneath Iceland.

2. Geological background

Iceland forms the junction between the Reykjanes

Ridge to the South and the Kolbeinsey Ridge to the

North. The plate boundary in Iceland is composed of

several rift segments (Fig. 1a). Two major rift zones

are active in Iceland; the Western Rift-Zone (WRZ)

that connects to the Reykjanes Ridge by the Rey-

kjanes Peninsula, a leaky transform structure, and the

Eastern Rift-Zone (ERZ), which is connected to the

Kolbeinsey Ridge to the north by the Tjornes Frac-

ture-Zone (TFZ). In central Iceland, the WRZ and

ERZ are connected by a wide volcanic transform

zone. During the Quaternary, south propagation of

the ERZ formed the South-Eastern volcanic Zone(SEZ). In contrast to the exclusively tholeiitic rocks

of the rift zones, the SEZ produces FeTi basalts with

alkalic affinities at the junction to the ERZ and alkali-

olivine basalts further from the rift zone (Oskarsson et

al., 1982).

The presence of a large mantle plume beneath

Iceland is inferred from anomalous volcanic produc-

tivity and an 3He anomaly along the central and

southern confines of the ERZ (Hilton et al., 1990,

2000; Breddam et al., 2000). Crustal thickness in the

plume area is estimated at some 4045 km (Menke et

al., 1996; Darbyshire et al., 2000). The plume area is

outlined in Fig. 1b, which shows the ERZ north of the

SEZ propagating rift. An outstanding feature of the

regional petrology of the plume area is the enormous

production of high-Mg olivine tholeiites along the

central plate boundary. This contrasts the production

of evolved, generally quartz-normative basalts along

the SE rift margin. Mantle sources for the Icelandic

tholeiites are known to be heterogeneous; the olivine

Fig. 2. (ad) Iron (all Fe as FeO), TiO2, Al2O3 and CaO plotted against MgO for the Icelandic high-Mg tholeiites, evolved olivine tholeiites and

evolved basalts.



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tholeiites are slightly more evolved than normal

MORBs in terms of SrNd systematics, while the rift

margin magmas and basalts from the SEZ propagating

rift are significantly more evolved (Hemond et al.,1993). The SrNd variations implies that the anom-

alous productivity above the plume apex is mainly

due to extensive degree of melting of a normal

depleted mantle source. The isotopically enriched

magmas of the rift margin may thus represent lower

degree of melting of a deeper source that contains a

higher proportion of the mantle plume component(s)

(Hemond et al., 1993; Chauvel and Hemond, 2000).

The pervasive 3He anomaly in rocks and geothermal

fluids of the plume area (Hilton et al., 1990, 2000),

regardless of petrochemical evolution, has to be

assigned to very deep degassing that reaches the

surface through the plate boundary. A tentative con-

clusion regarding petrogenesis of the plume area is

that the upwelling mantle material suffers larger

degree of relatively shallow melting beneath the plate

boundary as compared with the rift margin volcanism.Accordingly, the largest melt fractions above the

plume contain only a small fraction of fertile plume

material whereas the fertile plume component in the

marginal volcanism is less diluted by melts of shallow

origin (Oskarsson et al., 1994).

Productivity of the rift zones was particularly high

for 2000 years following the last deglaciation (10000

years ago). During this period of fast crustal rebound,

several large (up to 12 km3) olivine tholeiite lava

shields were formed as well as extremely productive

large volcanic fissures. Volcanic centers in the Ice-

landic rift zones are active for about 0.51 million

years. During the late Quaternary, these centers pro-

Fig. 3. (af) Copper, Cr, Ni, Y, Sr and Zr plotted against MgO for the Icelandic high-Mg tholeiites, evolved olivine tholeiites and evolved

basalts.



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duced hyaloclastite ridges and table mountains that

dominate the volcanic topography in the plume area,

indicating that the production anomaly is a long

standing feature.The axial rift segment overlying the plume area is

composed of several volcanic centers and rifting

fissure swarms. The Bardarbunga volcano (encircled

B in Fig. 1b) and the adjoining Veidivotn fissure

swarm is the largest rifting unit in the plume area

(Fig. 1b). North-East of Bardarbunga a lineament of

large basaltic eruption forms the definitive production

anomaly of the Icelandic rift system. Trolladyngja is

the largest lava shield in Iceland, about 12 km3. Two

volcanic centers (Grimsvotn and Kverkfjoll, Fig. 1b)

and their associated fissure swarms make up the SE

margin of the axial rift zone.

In the present study, we examine samples that

represent the production anomaly of the plume area

in Iceland. High-Mg tholeiites as well as evolved

basalts from the rift margin are also included in the

study. The propagating rift south of the ERZ is

represented by two FeTi basalt samples (SAL626

and BALK). The distribution of the samples is shown

in Fig. 1. The normal rift zones (Fig. 1a) are repre-

sented by high-Mg tholeiites from the Theistareykir

volcanic center in the North (TH40) and from the

Reykjanes Peninsula (D7 and D27). The olivinetholeiites from the Reykjanes Peninsula are repre-

sented by sample STAP, and the sample BTHO that

represents an interglacial lava shield in the WRZ. The

selected sample set has been chosen to be representa-

tive of the North Atlantic mantle plume as well as the

normal rift segments of Iceland.

Phase assemblages of the olivine tholeiites are

simple; olivine and plagioclase phenocrysts dominate

and a few grains of clinopyroxene are found. Chromite

is found as microphenocrysts in picritic rocks, but more

commonly as inclusions in olivine. Minor amounts(< 5%) of chromian diposide xenocrysts are found in

some Icelandic picrites and they rarely make up as

much as 20% of the lava mass. The xenocrysts most

likely represent deep-crustal or upper mantle material.

The evolved basalts that make up the rift zone margin

are aphyric or contain minor amounts of plagioclase

microphenocrysts. The samples analysed for this study

are relatively aphyric with up to around 5% phenoc-

rysts and were specifically chosen so they do not

contain significant abundances of xenocrysts. When

olivine phenocrysts are found, they are relatively fresh

and unaltered. Strong positive correlation between, for

example, Zr and Ba (R2 = 0.9, n = 30, not shown; Ba is

generally known to be mo bile during hydrothermalalteration; Rollinson, 1993) in the data set, as well as

the systematic variations between various trace and

major elements (Figs. 2 and 3) infer that the rocks are

relatively unaffected by hydrothermal alteration, and

representative of the magma that was deposited.

3. Samples and analytical methods

A total of 30 samples that included high-Mg

tholeiites, evolved olivine tholeiites and evolved

basalt samples were analysed for PGEs at the Geo-

science Laboratories in Sudbury, Canada (Table 1).

Fifteen grams of sample powder was used for Ni-

sulphide fire assay. The Ni-sulphide beads were dis-

solved in acid and the PGE determined by ICP-MS

(Jackson et al., 1990). Lower limits of detection

(average blank + 3 S.D.) for Ru, Rh, Pd, Ir, Pt and

Au are given in Table 2. The concentrations of PGEs

in the majority of the samples are above the detection

limits for all elements except Ir, Ru and Rh in the

evolved basalts (47 wt.% MgO).

Rehkamper et al. (1999) presented multiple PGEanalyses of sample BTHO, using isotope dilution,

Carius tube digestion and multi collector ICP-MS

analysis, and reported an average BTHO composition

of Ir (0.189F 0.005 ppb), Ru (0.646F 0.019 ppb), Pt

(5.38F 0.5 ppb) and Pd (7.58F 0.3 ppb). Sample

BTHO was also analysed for this study using the

Ni-sulphide fire assay-ICP-MS method (Table 2). Our

analyses produced lower Ir (29%), Pt (28%) and Pd

(21%) compared to the isotope dilution procedure.

Ruthenium analyses are within error of each other.

Replicate analyses of standard WPR-1, using the fireassay-ICP-MS procedure, are also reported in Table 2.

Relative standard deviations do not exceed 7% except

Rh and Au where the relative standard deviations are

15% and 10%, respectively. The average element

concentrations of replicate analyses of WPR-1 are

within 15% of the certified values for all elements.

Major and trace elements were analysed by ICP-

OES. Dissolution of samples was made, as described

in Govindaraju (1993), after fusion of the rock powder

with lithium metaborate. The borate beads were dis-



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solved in complexing acid mixture of 5% nitric acid,

1.3% hydrochloric acid and 1.3% oxalic acid. Sample/

flux ratio was 1:2 and dissolved solids in the solution

were adjusted to 1% for standards and samples. ICP-

OES analysis was made on a sequential high resolu-

tion instrument (Thermo Jarell-Ash AtomScan25) and

background correction was made on all minor and

trace elements. International geochemical reference

samples (USGS series W1, G1 W2, G2, AGV-1,

BCR-1, GSP-1, PCC-1, DTS-1, BHVO, RGM-1 and

BIR-1) were used for calibration of the method. Total

iron of the analysis is reported as wt.% FeO.

4. Major and lithophile elements

The 30 samples span a relatively large range of

MgO content (f 4 14 wt.%). Between 14 and 8

wt.% MgO, the iron concentrations do not vary

significantly, whereas they increase up to 15 wt.%

FeO (all Fe as FeO) in the interval from 8 to 5 wt.%

MgO (Fig. 2). The most Mg-rich samples contain 0.5

wt.% TiO2, which increase steadily (reaching 1.2

wt.%) at 8 wt.% MgO (Fig. 2b). From 8 to 5 wt.%

MgO, the rate of TiO2 increase is much greater,reaching 4 wt.%. Al2O3 increase from 12 to 16

wt.% in the interval from 14 to 10 wt.% MgO (Fig.

2c), whereas CaO concentrations are relatively con-

stant around 12 to 14 wt.% CaO (Fig. 2d). In contrast,

both CaO and Al2O3 concentrations decrease with

MgO from 9 to 5 wt.% MgO. The Icelandic samples

form reasonably coherent trends (Fig. 2ad) with an

inflection point atf 89 wt.% MgO.

Plots of various trace elements against MgO (Fig.

3af) also show an inflection point at 89 wt.% MgO,

except Ni and Cr that show strong positive correlations

with MgO throughout the sequence. Strontium con-

centrations in samples with 1014 wt.% MgO are

between 50 and 200 ppm, whereas Sr concentrations

in the most Mg-poor samples with f 5 wt.% MgO

range between 150 and 420 ppm. Yttrium concentra-

tions increase about a factor 4 (10 to 40 ppm) from 14 to

5 wt.% MgO. Zirconium, on the other hand, increases

by a factor 10 from (25 to 250 ppm) from the most

primitive to the most evolved samples.

Assuming that crystal fractionation was the domi-

nating process in the evolution of the basalts, the

inflection point atf 89 wt.% MgO represents the

stage where clinopyroxene joined plagioclase andolivine as a fractionating phase. It is evident from the

increase in Sr concentrations during fractionation that

plagioclase fractionation must be relatively limited,

and that olivine is dominating the fractionating assem-

blage. There is, however, strong evidence that the

various basalt types of the Icelandic rift system are

formed by variable degrees of partial melting and from

different sources. Segregation of mantle-derived melts

beneath the thick (1540 km) crust and the largely

similar PTconditions during ascent of these magmas

favour similar fractionation trends during their lateevolution. As a whole, the high-Mg tholeiites, the

evolved olivine tholeiites and the evolved basalts can

be described as series of different mantle melts modi-

fied by similar late-stage crystal fractionation.

5. Platinum-group elements

Samples with MgO>7 wt.% generally have higher

concentrations of PGEs (e.g. average 9 ppb Pd)

Table 2

Parameters describing the analytical quality of the PGE data

WPR-1a F 2 S.D. Certified

value

F BTHO F 1 S.D. Average

blankbF 3 S.D. D.L.

Ru ppb 19.9 2.3 22 4 0.91 0.34 0.03 0.1 0.13

Rh 12.9 3.7 13.4 0.9 0.38 0.05 0.01 0.07 0.08

Pd 214 26 235 9 6.26 0.38 0.02 0.04 0.06

Ir 14.7 2.1 13.5 1.8 0.15 0.02 0.01 0.03 0.04

Pt 253 27 285 12 4.20 0.20 0.05 0.1 0.14

Au 35.4 7.4 42 3 3.61 0.26 0.2 0.18 0.38

D.L.: detection limit = background + 3 S.D.a Running average of all WPR-1 analyses carried out at Ontario Geological Survey between 5/2000 and 1/2001.

b ppb equivalent in 15-g sample.



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compared to Mg-poor samples (MgO < 7 wt.%; aver-

age 1 ppb Pd; Fig. 4af). Ruthenium, Rh and Ir

concentrations in some of the evolved basalts samples

(< 7 wt.% MgO) are below detection limits (Fig.4b,d,e).

The high-Mg tholeiite samples (1014 wt.% MgO)

have a wide range of Pd (317 ppb), Ru (0.20.9

ppb) and Rh (0.15 0.66 ppb) concentrations (Fig.

4a,b,d). The evolved olivine tholeiites with 7 10

wt.% MgO show a similar range in Pt (310 ppb)

and Pd concentrations (518 ppb) as observed in the

high-Mg tholeiite group. Iridium shows an overall

positive correlation with MgO (Fig. 4e). Gold con-

centrations generally increase slightly from 2.5 to 4

ppb from 14 to 10 wt.% MgO in contrast to the

scattered positive correlation between Au and MgO

at lower MgO contents (Fig. 4f). In this respect, the

variations in Au and Cu concentrations (Fig. 3a) are

relatively similar.Compatible behaviour of Ir and incompatible

behaviour of Pd during S-undersaturated fractionation

makes the Pd/Ir ratio, represented as the slope of

mantle normalized metal patterns (Fig. 5ac), a useful

fractionation index. The high-Mg tholeiite samples

(Fig. 5a) have relatively primitive metal patterns with

Pd/Ir ratios in the range 1772, which is similar to

other high-MgO basalts (Barnes et al., 1988). The

metal patterns of samples with 710 wt.% MgO are

slightly more fractionated with Pd/Ir ratios between

38 and 220 (Fig. 5b). In this suite, concentrations

Fig. 4. (a f) Palladium, Ru, Pt, Rh, Ir and Au plotted against MgO for the Icelandic high-Mg tholeiites, evolved olivine tholeiites and evolved

basalts. Lower limits of detection are shown in b, d and e (stipled line) where some samples points are not plotted because they have

concentrations below the detection limit.



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range from 74 to 142 ppm Ni, 0.06 to 0.19 ppb Ir, 3 to

11 ppb Pt, 4 to 18 ppb Pd, and 2 to 6 ppb Au

(excluding one exceptionally PGE-poor sample

NAL-620). Four samples have Ru concentrations

below detection limit; one of which (sample NAL-

620) has Ir and Rh concentrations below detection as

well. The low PGE concentrations of this sample

result in a metal pattern quite different from the other

olivine tholeiites, and suggest that this melt equili-

brated with sulphide. Such silicate sulphide equili-

bration would extract Ir, Ru, Rh, Pt and Pd more

efficiently than Au, Cu and Ni from the silicate melt

(Barnes et al., 1988), and lead to an elevated Cu/Pd

ratio as observed for this sample (Cu/Pd 376000). In

the group of samples with < 7 wt.% MgO, the

majority of the samples have Ir, Ru and Rh concen-

trations below detection limit (Fig. 5c) and low Pt

(< 2.2 ppb) and Pd (< 2.4 ppb) concentrations as well.

Fig. 5. Primitive mantle-normalized metal patterns of Icelandic samples with (a) high-Mg tholeiites, (b) evolved olivine tholeiites and (c)

evolved basalts. Detection limit is shown in the diagrams (thick solid line). Normalizing values from Barnes et al. (1988).



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Low PGE abundances are characteristic of all ana-

lysed samples with MgO < 7 wt.%.

6. Discussion

In the following we present two mantle melting

models and in that framework, we discuss and interpret

PGE abundances in the Icelandic samples and explore

the possible processes governing the PGE abundances

in the three sample suites. We also present a scenario

that explains the observed CuPd data of high-Mg

tholeiites and olivine tholeiites by extraction of melts

from various parts of the melting region beneath the

Iceland rift. First, however, the possible genetic rela-

tions between the individual samples, effects of magma

mixing and fractionation will be evaluated for each

sample suite. Hereafter we will use mantle melting

models to approximate the identified primary magma

candidates, and use the results to generate a scenario for

melt generation beneath the Icelandic rift system.

7. The high-Mg tholeiite suite (MgO=1014 wt.%)

The Mg numbers of this sample suite ranges

between 67 and 75 (Table 1), and corresponds toequilibrium olivine compositions from Fo87 to Fo91,

indicating that these samples may be primary (Mg

numbers 7075) to slightly fractionated (Mg numbers

6769) magmas of mantle origin. The most Mg-rich

samples are picrites (TH40, D7 and D27) with olivine

and minor spinel phenocrysts. Due to the relatively

unfractionated nature of these picritic samples, some of

the phenocrysts may potentially be xenocrysts derived

from the upper mantle or the lowermost crust(Sigurds-

son et al., 2000). The most Mg-rich melts found in

Iceland contain up to 12 wt.% MgO (Sigurdsson et al.,2000; Gudfinnsson and Oskarsson, in preparation), so

the most Mg-rich samples analysed (14 wt.% MgO) for

this study may represent only f 5% accumulation of

olivine phenocrysts (12 wt.% MgO melt + 5% Fo84

88 = 14 wt.% MgO magma). Therefore the high-Mg

tholeiite sample suite, as a whole, is interpreted to

represent melt compositions in mantle melting model-

ing presented below.

Palladium concentrations in these samples range

from 3 to 17 ppb, and there is positive correlation

between Pd and Pt, Ir, Rh (Fig. 6a). In contrast, there

is negative correlation between Pd and Au and Cu

(Fig. 6b). Olivine tholeiite samples BTHO and

TRO08 (both have f 10 wt.% MgO and similarNi, Cu and PGE abundances) and the picrite sample

TH40 (14 wt.% MgO) constitute end-members in

this linear relationship in the present data set (in the

following these end-members will be referred to as

BTHO and TH40, respectively). Such negative

correlation between chalcophile elements Pd and Cu

cannot result reflect various degrees of partial melting

from one mantle source (see Mantle melting models).

The positive correlation between the highly sidero-

phile elements could, on the other hand, infer that

sulphide addition or fractionation is a likely explan-

ation for this variation. If, for example, BTHO is a

fractionation product derived from TH40, it requires

that sulphide was segregated in so small fractions, that

the bulk partition coefficient for a sulphide-bearing

fractionation assemblage was below unity for Cu and

Au (Dsulph silib20000) but above unity for the PGE

(Dsulph sili>20000). Alternatively, TH40 could rep-

resent the addition of sulphide-bearing silicate and

oxide phenocrysts to a BTHO-composition. In suit-

able models, however, the variations in Cu (12474

ppm) suggest that the two end-members (10 and 14%

Fig. 6. (a) Pt, Ir and Rh plotted against Pd. (b) Au and Cu plotted

against Pd for Icelandic high-Mg tholeiites.



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MgO, respectively) are related by either f 45%

fractionation or f 45% phenocryst accumulation.

Fractional crystallization models calculated by the

computer program COMAGMAT (Ariskin et al.,

1993) indicate that the major element variations

between the end-members can be explained by 15%,

but not as much as f 45% fractionation, and the

fractionation model is therefore rejected. The high-Mg



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tholeiite trend in a FeO versus MgO plot (Fig. 2a)

could be interpreted as an olivine accumulation trend,

but is suggestive of only f 11% addition of Fo84

olivine to an equilibrium liquid. It is therefore notlikely that TH40 represent addition off 45% phe-

nocryst material to a primitive olivine tholeiitic

magma. The Cu-PGE correlations outlined here there-

fore exclude the possibility that Icelandic high-Mg

tholeiites are genetically related, i.e. derived from one

primary magma type. We consider the simplest alter-

native interpretation to be a mixing model in which

the trends in Fig. 6 represent mixing lines between

olivine tholeiitic BTHO magma with f 10 wt.%

MgO, 36 ppb Pd and 114124 ppm Cu and a

picritic TH40 magma with f 14 wt.% MgO, f 17

ppb Pd and 74 ppm Cu. Based on the mixing scenario,

it follows that we interpret the two samples BTHO

and TH40 to be primary magma candidates, and the

samples in between to be mixtures. Three samples

belonging to the high-Mg tholeiite suite, with the

lowest PGE contents (e.g. < 5 ppb Pd; Mg numbers

6974), deviate from the linear mixing relationship,

however, and along with samples BTHO and TH40,

they may represent near-primary magma composi-

tions. In the light of this analysis, we therefore

consider it relevant to approximate only 5 of the 11

high-Mg tholeiites in mantle melting models, becausethe other compositions do not represent primary

magmasthey are most likely mixtures.

8. The evolved olivine tholeiites (MgO=710

wt.%)

The majority of the evolved olivine tholeiites are

PGE-rich relative to normal MORB. However, one of

the most Mg-poor (f 7 wt.% MgO) olivine tholei-

ites, NAL 620, has a high Cu/Pd ratio of 376000reflecting fractionation under S-saturated conditions.

Chalcophile element concentrations in the other sam-

ples vary significantly with Pd and Cu abundances

(418 ppb and 100150 ppm, respectively). Iridium

concentrations range between values close to the

detection limit (0.04 ppb) and 0.2 ppb.

A previous study by Rehkamper et al. (1999) presented five PGE analyses of Icelandic tholeiite

samples, and they showed that PGE concentrations

decreased with MgO, and interpreted this to reflect S-

saturated fractionation. During such S-saturated differ-

entiation, Pd and Ir concentrations wouldboth decrease

due to similar chalcophile behaviour (Peach et al.,

1990, 1994). Since Cu is much less chalcophile than

Pd, Cu/Pd ratios increase during S-saturated differ-

entiation (Barnes and Maier, 1999). If the silicate melt

differentiates without reaching sulphide saturation, Pd

and Cu will accumulate in the magma whereas Ir will

be fractionated. It is not clear which phase hosts Ir

during S-undersaturated differentiation. It has been

suggested that Ir-compatibility reflects (i) incorpora-

tion into the olivine lattice (Brugmann et al., 1987,

1993), (ii) incorporation into spinel (Capobianco and

Drake, 1990, 1994), or (iii) co-precipitation of alloys in

main fractionating silicate phases (Keays, 1995).

The majority of olivine tholeiites in this study are

slightly evolved melts with Mgnumbersof 62to 64and

Pd/Ir: 40 120 (Fig. 7a). Three samples, however, form

a trend of increasing Pd/Ir ratios from 110 to 220 with

decreasing whole-rock Mg numbers (64 54) (Fig. 7a),indicating that the most evolved olivine tholeiites (Mg

number 6054) differentiated under S-undersaturated

conditions. The highest Pd concentration (18 ppb Pd)

found in this study is also found in a differentiated

olivine tholeiite sample (sample MIL-83; Mg number

58, Pd/Ir: 145) and not a primitive picrite sample. The

range of observed Cu/Pd ratios of the evolved olivine

tholeiites is similar to that of the high-Mg tholeiite

suite. If the high-Mg tholeiites differentiated under S-

saturated conditions, then their differentiation prod-

ucts, the evolved olivine tholeiites, would have signifi-cantly higher Cu/Pd ratios, which they have not. In

combination, the variations in Cu, Pd and Ir infer that

the evolved olivine tholeiites differentiated under S-

Fig. 7. (a) Pd/Ir ratios plotted against whole-rock Mg number for Icelandic evolved olivine tholeiites. (b) Ir plotted against Pd concentrations in

the evolved olivine tholeiites. The inserted model of S-undersaturated fractional crystallization (050%) of BTHO and TH40 mixtures,

assumes DIrsolid, liquid =4 and DPd

solid, liquid = 0.01. The arrow indicates that Ir contents decrease whereas Pd contents increase during S-un-

dersaturated fractionation. Partition coefficients used in modeling are from Momme et al. (2002). They estimated Pd and Ir partitioning between

liquid and the average fractionating assemblage in basalts derived from the Tertiary Iceland plume during S-undersaturated differentiation. (c)

Mg number plotted against amount of fractionation (1F), estimated for each individual sample, from the fractionation model presented in b.



14/26

undersaturated conditions. Suitable differentiation

models must therefore involve S-undersaturated differ-

entiation withoutsegregation of sulphide liquid. Eight

evolved olivine tholeiite samples have Pt/Pd (0.2 0.8)and Cu/Pd ratios (700017000) (S-saturated sample

NAL 620 is omitted) similar to the high-Mg tholeiite

suite (Fig. 8a,b). Therefore the evolved olivine tholei-

ites are possibly differentiated counterparts to the high-

Mg tholeiite suite, which constitutes mixtures between

the most depleted high-Mg tholeiite TH40 and prim-

itive olivine tholeiite BTHO (Figs. 7b, 8a,b and 9a,b).

The varying Cu/Pd ratio of the olivine tholeiites has to

be considered in a differentiation model, because this

indicates that their parental magmas had varying initial

Pd concentrations. Assuming that parental olivine

tholeiitic melts lie on the BTHO to TH40 mixing

line (Fig. 7b), their present range of Cu/Pd ratios (Fig.

8b) indicates initial Pd contents between 6 and 15 ppb

and that the olivine tholeiites represent up to f 30%

fractionation (Fig. 7b). While there is no correlation

(R2 = 0.3) between Pd concentrations and whole-rock

Mg number (not shown), correlation is strong between

amount of fractionation in the model, 1F, and whole-

rock Mg number (R2 = 0.8; Fig. 7c). In other words, a

fractional crystallization model, which assumes S-

undersaturation based on Cu, Pd and Ir concentrations,

and the mixing relation between the two distinctprimary magma types, correlates well with whole-rock

Mg numbers. This infers that the olivine tholeiites were

possibly derived from a spectrum of parental magma

compositions, which is consistent with the conclusions

of various isotopic studies (e.g. Hemond et al., 1993;

Chauvel and Hemond, 2000).

Traditionally, basalts like the evolved olivine tho-

leiites from Iceland with Cu/Pd ratios higher than

primitive mantle would be considered S-saturated.

However, S-undersaturated fractionation has also been

reported in East Greenland basalts with similarlyelevated Cu/Pd ratios (Fig. 8b) that formed during

Paleogene Iceland plumerift interaction (Momme et

al., 2002).

9. The evolved basalts (MgO< 7 wt.%)

The low PGE concentrations and the high Cu/Pd

ratios of this Mg-poor suite of samples probably

reflect fractionation of immiscible sulphide liquid in

Fig. 8. (a) Platinum plotted against Pd for all Icelandic samples.

Solid line represents primitive mantle Pt/Pd ratio of about 2 (Barnes

et al., 1988) and stipled line is linear regression of the Icelandic

samples with Pt/Pd = 0.5. (b) Cu plotted against Pd. Fields

representing East Greenland flood basalts are inserted for

comparison (see Momme et al., 2002). Two arrows indicate the

effects of S-undersaturated differentiation in the East Greenland

basalts where Pd and Cu are equally incompatible (Momme et al.,

2002). Two melting curves, representative of depleted mantle

[F(1) = 150 ppm S, 19 ppm Cu, 4.3 ppb Pd] melting in a columnarmelting regime and enriched mantle [ F(2)= 200 ppm S, 35 ppm Cu,

4.3 ppb Pd] undergoing melting in a triangular melting regime are

inserted (calculated assuming a primary magma capacity of 1000

ppm S). The TH40 picrite composition can be approximated by

F= 25% partial melting of depleted mantle in a columnar melting

regime. The BTHO olivine tholeiite composition can be approxi-

mated by slightly Cu-enriched mantle melting in a triangular

melting regime with F= 11 12% (Fmax = 22 24% in the center of

this triangular melting regime). Three high-Mg tholeiites with high

Mg numbers 6974 have relatively low Pd contents, which could

be explained by slightly lower degrees of melting F=10% in the

F(2) melting model.



15/26

samples with < 7 wt.% MgO. The high concentrations

of incompatible elements in this suite of samples are

equally likely to represent lower degrees of partial

melting and the low PGE concentrations could there-fore represent segregation from a S-bearing residual

mantle.

Three evolved basalts with about 5 wt.% MgO

and the highest Cu/Pd ratios also have significantly

elevated Pt/Pd ratios which could indicate that Pd,

here, is slightly more chalcophile than Pt (Fig. 9a

and b).

It is noteworthy that even though this suite is

generally considered S-saturated due to low Pd

concentrations and high Cu/Pd ratios, some of these

samples are among the most Cu rich samples (up to

155 ppm; Table 1) analysed for this study. If these

Cu-rich samples with relatively low Mg numbers

(5052; Table 1) have undergone fractionation and

concurrent segregation of an immiscible sulphide

liquid, they must have had very high initial Cu

concentrations.

10. PGE behaviour during partial melting

This section gives an introduction to the columnar

and triangular melting regime models that approxi-mate PGE concentrations in melts during mantle

melting. The calculations follow the principles out-

lined by Rehkamper et al. (1999) to which the reader

is referred for further details on the modeling. In both

models, it is assumed that primary magma dissolves

1000 ppm S (Mathez, 1976; Keays, 1995; Rehkamper

et al., 1999) and that the undepleted mantle contains

250 ppm S (f 714 ppm sulphide; sulphides contain

f 35% S), 4.4 ppb Pd and 28 ppm Cu (Barnes et al.,

1988). Pd and Cu are sulphide hosted (Barnes et al.,

1988), and partitioning between mantle sulphides and

silicate melt is approximated by Dsulph sili of 30 000

and 800, respectively (Barnes and Maier, 1999 and

references herein).

The columnar melting regime model (Fig. 10a)

assumes that a rectangular section of primitive mantle

undergoes melting and that all volumes melt to the

same degree (Langmuir et al., 1992). Melt is removed

in 1% increments, and as the melt is extracted, mantle

sulphide is dissolved by the partial melt. At 25%

partial melting, the sulphide is exhausted in the mantle

and Pd and Cu will be released into the melt (Fig. 10a;

Pd = 17 ppb, Cu = 108 ppm). During further partialmelting, Pd and Cu concentrations in the magma will

be diluted.

Melt generation beneath mid-ocean ridges is prob-

ably best approximated by a triangular melting regime

model (Fig. 10b) reflecting spreading controlled,

decompressional melting during upwelling (Langmuir

et al., 1992). In this model, the amount of liquid

extracted from the mantle decreases from the central

parts of the melting triangle to the distal parts of the

triangle; the mean degree of partial melting, F, equals

0.5 times the maximum degree of partial melting(Langmuir et al., 1992). With respect to PGE concen-

trations of the residual mantle and the partial melts,

this model is very different from the columnar melting

regime model. When Pd is released into melts formed

in the central part of the melting triangle (F>13%;

Fig. 10b), these melts mix with Pd-poor magmas,

formed by lower degrees of melting, to form hybrid

magma compositions representative for the entire

melting triangle. Platinum-group element concentra-

tions in the primary magmas and the mean degree ofFig. 9. (a, b) Pt/Pd ratio and Cu/Pd ratio against MgO for Icelandic

high-Mg tholeiites, evolved olivine tholeiites and evolved basalts.



16/26

melting by which they form are consequently lower

compared to the columnar melting regime model. The

first magma with significant PGE concentrations (>1

ppb Pd) derived from a triangular melting regime

forms by 13% partial melting and contains 2 ppb Pd

(Fig. 10b). The differences between the two modelstherefore relate to the maximum concentrations in

observed magmas (e.g. 8 versus 17 ppb Pd) and the

degree of partial melting by which PGE-rich magmas

form (13 versus 25%) (Fig. 10a and b).

11. Mantle melting models

The TH40 picrite end-member has a Cu/Pd ratio

off 4350 (Fig. 9b; Table 1), which is significantly

lower than the estimate of primitive mantle (Barnes etal., 1988). Because Cu is less chalcophile than Pd, this

element is less efficiently retained during mantle

melting, and it follows that Cu/Pd ratios of refractory

mantle sources will be lower than more fertile sources

as inferred in the models (Fig. 11a). The Cu/Pd ratio

of TH40 is similar to a residual mantle that has

previously experienced f 15% melt extraction (Fig.

11a). The estimated degree of source depletion needed

to approximate the sub-mantle Cu/Pd ratios of TH40

is a function of the sulphidesilicate partition coef-

ficients used in the calculations; it follows that the

estimated 15% melt depletion of the picrite source is a

loosely constrained figure. This estimate of mantle

depletion, however, falls right in the degree of melting

range by which normal MORB forms f 820%

(Klein and Langmuir, 1987). Normal MORB extrac-tion could therefore be the background for the

depleted character of the picrite source. The depleted

picrite source appears to be much more depleted than

a normal depleted upper mantle MORB source, and

is more likely to represent re-melting of a MORB

residue. The high Pd concentration (17 ppb) of the

TH40 end-member clearly indicates that sulphide

was completely exhausted in the residual mantle

during picrite formation; at that point, magmas would

have Cu/Pd similar to the mantle from which they

were derived (Fig. 11a). Pd and Cu concentrations of picrite TH40 can be approximated by 25% melting

in a columnar melting regime assuming a homoge-

neously depleted mantle source (primitive mantle

depleted by extraction of 15% melt: Pd = 4.3 ppb,

Cu = 19 ppm and 200 ppm S; F= 25%, picrite melt:

17 ppb Pd and 76 ppm Cu; Figs. 8b and 11b).

Rift-related melting is assumed to occur in a

triangular melting regime where mantle upwelling

and decompressional melting is modulated by plate

separation (Langmuir et al., 1992). Olivine tholeiites

Fig. 10. Calculated Cu and Pd concentrations in partial melts plotted against degree of partial melting of primitive mantle (250 ppm S, 4.4 ppb

Pd, 4.4 ppb Ir and 28 ppm Cu) for (a) a columnar melting regime and (b) a triangular melting regime. Both models assume a primary magma S

capacity of 1000 ppm. See text for modeling details.



17/26

constitute the most abundant magma type in the rift

system, and is for modeling purposes assumed to be

derived from a triangular melting regime. With respectto Cu/Pd ratios, aggregate melts (primary magmas)

with Pd concentrations >1 ppb derived from such a

melting triangle are characterized by Cu/Pd ratios

ranging between 8400 and 37 000 (Figs. 10b and

11a), similar to the observed variation in Icelandic

olivine tholeiites (700038000). The evolved olivine

tholeiites are interpreted to be differentiated equiva-

lents to the high-Mg tholeiite suite (i.e. derived from

mixtures of TH40 and BTHO; Fig. 7b). It is not

possible, however, to approximate the observed Cu

and Pd concentrations in BTHO by melting of the

depleted mantle composition, from which TH40 is

derived, in a triangular melting regime. In such amodel, the Cu/Pd ratio can be approximated but the

actual Cu and Pd concentrations (3.8 ppb Pd and 80

ppm Cu) are significantly lower than those of BTHO

(6.3 ppb Pd and 120 ppm Cu). Using an undepleted

primitive mantle composition (28 ppm Cu, 4.3 ppb,

250 ppm S) also fails to approximate the BTHO

composition. Since the Iceland plume has a history of

producing very Cu-rich basalts (up to 300 ppm Cu;

see below), we find the least exotic explanation to be

that the plume may contain a Cu-enriched source.

Fig. 11. Calculated Cu/Pd ratios in partial melts and residual mantle versus degree of partial melting in a columnar (CMR) and triangular (TMR)

melting regime. The residual mantle compositions are representative of the most depleted upper parts of a columnar melting regime. The

composition of TH40 is illustrated to show that melting models based on a normal primitive mantle starting composition is unable to generate

the low Cu/Pd ratio of this sample. Models in a and b assume primary magma S-capacity = 1000 ppm. (a) This model assumes melting of

undepleted primitive mantle: 250 ppm S, 28 ppm Cu and 4.4 ppb Pd. (b) This model is based on a depleted mantle starting composition: 200

ppm S, 19 ppm Cu and 4.3 ppb Pd. The initial mantle composition in this model is similar to a MORB ( F= 15%) residue.



18/26

Assuming a slightly Cu-enriched mantle starting

composition with 35 ppm Cu (primitive mantle: 28

ppm Cu), on the other hand, the high-Mg tholeiite

samples with Cu/Pd ratios between 19200 (BTHO)and 38000 (NAL606) can be interpreted to represent

F= 1 0 12% partial melting in a triangular melting

regime (Fig. 8b). It is noteworthy that the three Pd-

poorest high-Mg tholeiite samples, which deviate

from the linear mixing relation between BTHO

and TH40, can be approximated by f 10% partial

melting of the same source that forms BTHO by

11 12% partial melting (Fig. 8b).

The model melt which was extracted from the

picrite source, prior to generation of TH40, was

relatively rich in Cu (87 ppm) and poor in Pd (0.5

ppb), similar to normal MORB (Fig. 11a). The same

type of melt would be required to cause the Cu-

enrichment of the BTHO source. It would require,

however, a 20% addition of such an oceanic crustal

component with 90 ppm Cu to the depleted mantle to

elevate the Cu concentration needed for successful

mantle melting models.

The scenario of melt generation beneath the Ice-

landic rift system presented in the following assume

that (1) efficient melt focusing in the triangular melt-

ing regime leads to olivine tholeiite formation, (2) the

olivine tholeiites contain melt from a Cu-enrichedsource, and (3) the picrites, derived from the upper-

most and central melting column, are exclusively

derived from a depleted, Cu-poor source.

A scenario that explains these facts and assump-

tions involves melting of enriched mantle portions at

depth whereas more shallow melting, forming pic-

rites, only involves melting of depleted, refractory

mantle portions. One way to explain such a scenario

could be that the enriched mantle regions are present

beneath the melting region but are exhausted at depth,

leaving only refractory mantle sources in the upper-most melting regime from which the picrites form

(Fig. 12).

Based on this hypothesis, the centrally formed

picrites, and the olivine tholeiites, representing the

entire melting regime, can be generated beneath the

same rift zone. In this scenario, the mixing relations

between BTHO and TH40 compositions could be

explained by more or less efficient focusing of low-F

melts from distal and lowermost parts of the triangular

melting regime. In addition, high-Mg tholeiites are

consequently extruded close to the contemporary rift

axis (picrite sample localities TH40, D7 and D27

in Fig. 1a), which is in accord with high-Mg tholeiite

melts forming only in the central (and uppermost)

parts of the melting regime (Fig. 12). The scarcity of

such picritic lavas in Iceland suggests, however, that

they are rarely extruded prior to mixing with ascend-

ing low-F melts; a mixing that results in an olivine

tholeiitic aggregate magma composition with higherCu/Pd ratio (Fig. 12). The picrites reflect the highest

degrees of mantle melting, and are found near the

northern and southern ends of the Iceland rift system

(Theystareykir, northern Iceland, and Reykjanes Pen-

insula, southern Iceland). We interpret the picrite

melts to be extracted from a highly depleted mantle

source undergoing melting in a triangular melting

regime, where melts at the top of the central melting

column escape prior to mixing with low-F melt

batches. In that sense, we interpret the picrites to have

Fig. 12. The figure shows a triangular melting regime (left), which

produces a residual melting column (to the right). To explain the

formation of picrites and the BTHO end member in the melting

region beneath Iceland, Cu (MORB)-enriched mantle portions must

be exhausted at depth. The shallow melting of the depleted mantle

forms the picrites, is limited to shallow melting of a highly depleted

source. The relatively high Pd concentrations in Icelandic high-Mg

tholeiites and evolved olivine tholeiites infer that mantle melting

exhaust mantle sulphide in the upper central parts of the melting

region beneath Iceland. The mantle solidus is given as a range, since

the enriched and depleted, refractory mantle portions are likely tocross their solidii at various depths.



19/26

formed at rift segments where unusual conditions,

perhaps glacial rebound and associated tectonic activ-

ity, may have facilitated tapping of these mantle melts

(Gudmundsson, 1986).The present modeling indicates that the melting

mantle beneath the Iceland rift is heterogeneous with

respect to its Cu-contents and that compositions

probably range from very depleted to slightly

enriched. A similar heterogeneity is also inferred from

isotopic studies (Hemond et al., 1993; Kempton et al.,

2000). In terms of Sr Nd isotopes, the Icelandic

picrites and olivine tholeiites range between very

depleted to slightly enriched compositions (e.g. Fig.

4 in Hemond et al., 1993). Quartz tholeiites and alkali

basalts from Iceland overlap the isotopic compositions

of the picrites and olivine tholeiites but extend to more

evolved compositions with lower 143Nd/144 Nd and

higher 87Sr/86Sr ratios (Hemond et al., 1993).

Modeling of Cu and Pd in partial mantle melts

indicates that normal MORB extraction could have

caused the depletion of the depleted picrite mantle

source beneath Iceland. In addition, the enriched

mantle portions can be explained by addition of a

similar Cu-rich and Pd-poor normal MORB compo-

nent to the depleted mantle. Based on trace elements

and Pb isotope systematics, Chauvel and Hemond

(2000) suggested that the mantle heterogeneity beneath Iceland reflected melting of varying parts of

an entire section of recycled oceanic crust present as

distinct compositional reservoirs within the Iceland

plume. They proposed that Icelandic alkali basalts

were derived from the basaltic (enriched) portions

whereas the picrites formed from the gabbroic por-

tions with a contribution from a harzburgitic (highly

depleted) source. The olivine tholeiites were inter-

preted to represent mixtures of melts derived from the

two sources. There are several areas of agreement

between the scenarios of melt generation beneathIceland suggested by Chauvel and Hemond (2000)

and in this study; enriched mantle portions are inter-

preted to represent MORB-enriched mantle domains.

This (MORB) enriched source is melted out at depth.

The picrites possess trace element and isotopic fea-

tures that connect them to melting of gabbro (Chauvel

and Hemond, 2000), but the low Cu/Pd of TH40

observed here could infer that they formed from

highly depleted sources such as the harzburgitic

(MORB residue) part of the oceanic crust. The most

abundant lava type in Iceland, the olivine tholeiites,

represent mixtures of magmas derived from the

enriched and depleted sources respectively. In combi-

nation, Cu, Pd and radiogenic isotope variationsindicate that processes known from present-day mid-

ocean ridge systems could have caused the hetero-

geneity of the mantle sources beneath Iceland by

recycling of oceanic crust.

12. PGE in flood basalts in East Greenland derived

from the Paleogene Iceland plume

The f 55 Ma Tertiary plateau lavas in central East

Greenland consist of the low-Ti, high-Ti and very

high-Ti series, which are found intercalated in the >6-

km-thick volcanic sequence (Tegner et al., 1998a).

The on-shore Ti-rich and Ti-poor basalts are inter-

preted to have formed in continental and oceanic rift-

settings, respectively, influenced by the Paleogene

precursor of the Iceland plume (Tegner et al.,

1998a,b). The two Ti-rich series in East Greenland

possess enriched geochemical features such as LREE

enrichment (La/Sm)N = 1.1 2.0 and relatively low

eNd= + 5 to + 7, whereas the low-Ti series is LREE-

depleted (La/Sm)N = 0.5 0.7 and have highereNd= + 8

to + 11 similar to the most depleted picrites in IcelandeNd < + 9 . 7 (Hemond et al., 1993; Tegner et al.,

1998a). The two Ti-rich series from East Greenland

are interpreted to have formed in a continental rift-

setting by low degrees of melting (F=39%) com-

pared to the low-Ti series (F= 15 25%) that formed

beneath the early North Atlantic oceanic rift(Tegner

et al., 1998a). Five primitive (1316 wt.% MgO) low-

Ti series basalts average 10 ppb Pd, 10 ppb Pt and 0.7

ppb Ir. The evolved low-Ti basalts (78 wt.% MgO,

four samples) average 21 ppb Pd, 8 ppb Pt and 0.19

ppb Ir(Momme et al., 2002). The compatible behav-iour of Ir and incompatible behaviour of Pd and Cu

indicate S-undersaturated differentiation of the East

Greenland low-Ti series (Fig. 8b; Momme et al.,

2002). The near-plume segments of the early Tertiary

North Atlantic ridge produced PGE-rich basalts capa-

ble of significant S-undersaturated differentiation

(Momme et al., 2002). The two Ti-rich series have

elevated Cu/Pd ratios (higher than mantle) similar

to recent Icelandic olivine tholeiites (e.g. BTHO;

Fig. 8b).



20/26

Based on the mantle melting models presented here

(Fig. 8b), the East Greenland Ti-rich series can be

interpreted as S-undersaturated differentiated equiva-

lents of parental magmas similar to BTHO (i.e.formed by f 11 12% partial melting; Fig. 8b).

Different La/Sm ratios ofBTHO ([La/Sm]N = 0.4; data

from Hemond et al., 1993) and the Ti-rich series from

East Greenland ([La/Sm]N = 1.1 2.0) indicate, how-

ever, that a possible relationship between the Tertiary

and the Holocene tholeiites derived from near-plume

ridge segments is relatively complex.

In a previous section we have argued that the

evolved olivine tholeiitic samples from Iceland differ-

entiated under S-undersaturated conditions, and it is

noteworthy that all except two of these samples plot

within or near the fields delineating the Cu Pd

distributions of the S-undersaturated basalts from East

Greenland in Fig. 8b.

Due to the S-undersaturated differentiation ob-

served in Tertiary and recent Icelandic olivine tholei-

ites derived from plume-influenced North Atlantic

ridge segments, the maximum Pd concentrations (East

Greenland: 25 ppb, Iceland: 18 ppb) occur in signifi-

cantly differentiated samples with about 7 wt.% MgO.

In these basalts, the Pt/Pd ratio is significantly lower

than that of primitive mantle (Fig. 8a). In Icelandic

olivine tholeiites, the Pt/Pd ratio range between 0.2and 0.75 (Fig. 9a), similar to the East Greenland low-

Ti series. In the Icelandic evolved olivine tholeiites,

the Pt/Pd and the Cu/Pd ratios are interpreted to reflect

compositional variations of their parental magmas.

These two ratios are both elevated in the evolved

basalts, and this is interpreted to reflect the order of

chalcophile behaviour Pd>PtHCu (Fig. 9a and b). As

mentioned earlier, some of the S-saturated evolved

basalts with high Cu/Pd ratios have among the highest

Cu concentrations in the analysed set of samples. The

East Greenland high-Ti series also have high Cuconcentrations up to 300 ppm Cu (East Greenland

high-Ti series in Fig. 8b). It therefore seems that the

Icelandic plume has a history of producing extraordi-

nary Cu-rich basalts that could indicate a Cu-enriched

source within the plume through time.

The near-plume segments of the early mid-Atlantic

ridge at 55 Ma produced S-undersaturated, PGE-rich

low-Ti tholeiites in East Greenland similar to recent

(< 10 000 years) olivine tholeiites derived from the

Icelandic rift. This reflects the high degrees of partial

melting up to 2025% associated with the Iceland

mantle plume (e.g. Fig. 8b). In addition, the plume

may contain S-poor sources, as discussed in the

following section, that produce S-poor melts capa- ble of significant S-undersaturated differentiation

(Momme et al., 2002) before S-saturation occurs at

around 7 wt.% MgO.

13. PGE in Icelandic versus normal mid-ocean

ridge basalts

Several studies have shown that normal MORB

magmas are S-saturated at the time of extrusion (e.g.

Mathez, 1976; Rehkamper et al., 1999). The average

MORB composition is PGE-poor with f 0.8 ppb Pd

and f 70 ppm Cu (Hamlyn et al., 1985). This has

been interpreted to reflect segregation from a sul-

phide-bearing mantle residual that retains the PGEs

efficiently or segregation of an immiscible sulphide

liquid. One Kolbeinsey Ridge MORB sam ple, how-

ever, shows high Pd concentration of 6 ppb (Rehkam-

per et al., 1999). Elevated degrees of partial melting

(>20%) beneath Kolbeinsey Ridge, compared to those

normally associated with MORB formation (820%)

(Klein and Langmuir, 1987), is most likely the cause

for the high Pd content. The most evolved, PGE-poorsamples from Iceland reach concentrations as low as

those observed in MORB.

The S-undersaturated differentiation of Icelandic

tholeiites shown here contrasts the S-saturated behav-

iour of normal MORB basalts (Hamlyn et al., 1985)

and previous studies of PGE in Icelandic olivine

tholeiites (Rehkamper et al., 1999). A fundamental

difference between S-saturation status of normal

MORB (F=820%) (Klein and Langmuir, 1987)

and PGE-rich Icelandic basalts is likely the elevated

degree of partial melting up to 25% associated withthe Icelandic mantle plume (Fig. 8). In addition to the

difference in degree of partial melting, the difference

between normal MORB and plume-derived basalts

may involve melting of highly depleted, S-poor sour-

ces in or entrained by the plume (e.g. Fitton et al.,

1997; Hardason et al., 1997; Kempton et al., 2000).

Probably 1% to 3% of melt has been extracted from

the depleted normal-MORB source (Hart and Zindler,

1986; Hofmann, 1988) whereas the Cu/Pd ratio of the

Iceland picrite end-member indicates that the previous



21/26

partial melting events were about 15%. Assuming a

normal 200 250 ppm S primitive upper mantle

source, and a depletion by extraction of 15% melt

prior to picrite formation would only leavef 50100 ppm S in the source assuming a primary magma

S-capacity of 1000 ppm (Mathez, 1976; Rehkamper et

al., 1999). Formation of the picrite by 25% partial (re-

)melting of such a strongly depleted source would

result in 200-400 ppm S in the picrite magma.

Normally, primitive magmas are able to dissolve

f 1000 ppm S (Poulson and Ohmoto, 1990), so the

Iceland picrite end-member is likely to be highly S-

undersaturated. A highly refractory mantle source is

only likely to cross its solidus after significant adia-

batic decompression and suggests a deep origin for

this source. So, in addition to high degrees of partial

melting, plume-tholeiites may be S-undersaturated

due to derivation from S-poor sources (50100 ppm

S) in or entrained by the plume in contrast to MORB

derived from a less depleted upper mantle (f 250

ppm S) source.

14. PGE in Icelandic versus Hawaiian basalts

Platinum-group element concentration studies

from Hawaiian basalts are quite limited (e.g. Bennettet al., 2000; Crocket, 2000). A Re-PGE concentra-

tion study from Hawaiian high-Mg tholeiites (Ben-

nett et al., 2000) reveals Pt and Pd concentrations

significantly lower than observed in Icelandic olivine

tholeiites and high-Mg tholeiites. The low concen-

trations, e.g. Pt = 2 6 ppb, Pd = 1.4 7.7 ppb, are

interpreted to reflect segregation from mantle with

varying amounts of residual sulphide retaining the

PGEs to varying degrees (Bennett et al., 2000; they

are interpreted to be S-saturated at source). The

assumption that melts formed beneath Hawaii, andsegregated from a sulphide-bearing mantle source

appears reasonable, because the degree of melting

beneath Hawaii is assumed to be moderate around

510% partial melting (Norman and Garcia, 1999).

Koolau picrites have low PGE concentrations (f 4

ppt Pt and 1.5 ppb Pd) similar to MORB (Fig. 1 in

Bennett et al., 2000). This leads to the varying and

relatively high Cu/Pd ratios (10000100000) of the

Hawaiian picrites (Bennett et al., 2000) significantly

higher than the most depleted Icelandic picrite

f 4300 (TH40). The observed range in MgO

concentrations (827 wt.%) in the Hawaiian samples

reflect olivine accumulation to a large degree, and

since the highest PGE concentrations (7.7 ppb Pd) isfound in some of the most Mg-rich (23 24 wt.%

MgO) olivine cumulates, it is difficult to evaluate

PGE concentrations of the silicate melts. The very

high Mg, Ni and Ir contents of the Hawaiian samples

implies an olivine cumulative background for the

majority of these samples.

Eighteen PGE analyses of fresh basalts with 611

wt.% MgO from Kilauea are presented in Crocket

(2000) with an average of about 2.4 ppb Pd and 0.38

ppb Ir. Compared to Icelandic olivine tholeiites with

79 wt.% MgO, which average 11 ppb Pd and 0.13

ppb Ir, the Kilauea samples have lower Pd but higher

Ir concentrations. The most primitive basalts in both

suites have similar MgO contents but whereas Ice-

landic olivine tholeiite primary melts probably have

1012 wt.% MgO, Hawaiian basalts may be derived

from primary melts with 1317 wt.% MgO (Norman

and Garcia, 1999) and may therefore, at 10 wt.%

MgO, have experienced significant differentiation,

including segregation of sulphide. The relatively high

Ir concentrations and low Pd concentrations of Hawai-

ian samples result in very low Pd/Ir ratios f 324

(Bennett et al., 2000; Crocket, 2000) where the most primitive Icelandic olivine tholeiite (BTHO) and

picrite (TH40) samples have Pd/Ir ratios off 41

and f 53, respectively.

15. Ni, Cu, Pd and Ir variations in mafic and

ultramafic magmas

Diagrams of base metal to PGE ratios, e.g. Ni/Pd

against Cu/Ir ratio (Fig. 13) can be used to discrim-

inate between various mantle-derived rock typessuch as komatiites, high-MgO basalts and flood

basalts (Barnes et al., 1988; Fig. 13). Komatiites

are characterized by high Ni/Pd and low Cu/Ir ratios

whereas high-MgO basalts and flood basalts have

lower Ni/Pd and higher Cu/Ir ratios (Fig. 13). A

negative correlation between the two ratios in this

diagram is interpreted to reflect the decreasing

amount of mantle partial melting associated with

the formation of komatiites and flood basalts, respec-

tively (Barnes and Maier, 1999). Compared to the



22/26

flood basalt field, sulphur undersaturated East Green-

land flood basalts are displaced towards higher Ni/Pdand Cu/Ir ratios. In the East Greenland flood basalt

sequence, the most primitive low-Ti basalts have the

h ig he st N i/ Pd (f 4 104) a nd l ow es t C u/ Ir

(f 1.5 105) ratios, probably reflecting formation

by high degrees of partial melting f 1525%

(Tegner et al., 1998a; Momme, 2000). The contem-

porary high-Ti basalts formed by lower degrees of

partial melting around (f 39%; Tegner et al.,

1998a) and the most primitive samples of the high-

Ti basalts have lower Ni/Pd (f 1.5 104) a nd

higher Cu/Ir (f

8

10

5

) ratios compared to primi-tive low-Ti basalts (see Momme et al., 2002). All

three flood basalt types from East Greenland differ-

entiated under sulphur undersaturated conditions

(Momme et al., 2002) and have similarly low Ni/

Pd (f 4 103) and high Cu/Ir (f 6 106) ratios. It

is thus clear that decreasing degrees of partial melt-

ing leads to lower Ni/Pd and higher Cu/Ir ratios,

whereas S-undersaturated differentiation increases

the Cu/Ir ratio and decreases the Ni/Pd ratio. In this

diagram, basalts from the North Atlantic region have

higher Ni/Pd and Cu/Ir ratios compared to the fields

representative of high-MgO basalts and flood basaltsand it is unclear whether this is related to mantle

source composition or mantle melting processes. The

positive correlation between the two ratios in MORB

samples from Kolbeinsey Ridge reflects their S-

saturated state during differentiation. The most

PGE-rich Kolbeinsey Ridge MORB sample have

similar ratios as the primitive olivine tholeiitic sam-

ple BTHO from Iceland, and the evolved (47

wt.% MgO) sulphur-saturated Icelandic basalts are

broadly similar to the sulphur-saturated Kolbeinsey

Ridge MORB.A study of major, trace and platinum-group ele-

ments in the Paleogene seaward-dipping reflector

sequence off-shore central East Greenland was carried

out by Philipp et al. (2001). These basalts were

deposited shortly after the low, high and very high-

Ti series flood basalts found in East Greenland that

formed during continental breakup (Tegner et al.,

1998a,b). Philipp et al. (2001) found that the sea-

ward-dipping reflector sequence basalts initially dif-

ferentiated under S-undersaturated conditions, but

Fig. 13. Ni/Pd versus Cu/Ir diagram for Icelandic high-Mg tholeiites (high-Mg), evolved olivine tholeiites (Ol-thol) and evolved basalts

(Evolved) as well as Hawaiian basalts (data from Crocket, 2000) and basalts from the East Greenland seaward-dipping reflector sequence (EG

SDRS ; data from Philipp et al., 2001). Fields representing komatiites, high-MgO basalts, flood basalts (from Barnes and Maier, 1999),

Kolbeinsey Ridge MORB (data from Rehkamper et al., 1999) and East Greenland tholeiites (EG tholeiites; data from Momme et al., 2002) are

inserted. The three arrows indicate the relative effects of increased degrees of partial melting and/or phenocryst accumulation, sulphide

segregation, and silicate, chromite and platinum-group mineral fractionation, respectively.



23/26

reached S-saturation at around 10 wt.% MgO and

segregated an immiscible sulphide liquid that led to

very low PGE contents in basalts with < 10 wt.%

MgO. They furthermore concluded that the basaltsformed from a normal MORB source. Pd and Cu

reach maximum concentrations of 16 ppb and 262

ppm, respectively (Philipp et al., 2001), and especially

the high Cu concentrations cannot be approximated

by normal melting models (Fig. 10), inferring a high-

Cu source. The most depleted sample of the Icelandic

high-Mg tholeiite, TH40 (Cu/Ir f 230 500), has a

similar Cu/Ir ratio and slightly lower Ni/Pd ratio

(f 21600) compared to the primitive low-Ti basalts

from East Greenland (Fig. 13). The Cu/Ir ratio of

BTHO (f 820000) is similar to the most primitive

East Greenland tholeiites.

There is no visible difference in the Cu/Ir versus

Ni/Pd diagram between Paleogene, continental Ti-rich

tholeiites, derived from melting of the Iceland plume

head, Paleogene, North Atlantic oceanic basalts, and

Holocene Icelandic olivine tholeiites. There is, how-

ever, significant differences between the Ti-rich con-

tinental type and Ti-poor oceanic basalts with respect

to isotopic signatures, incompatible trace-element

contents and Cu/Pd ratios (Fig. 8b).

16. Conclusions

The most primitive Icelandic magmas, picrites and

olivine tholeiites with 10 14 wt.% MgO, form a

mixing trend between distinct picrite and olivine

tholeiitic parental magma compositions.

Copper and PGE concentrations in the most

depleted Icelandic picrite can be approximated by

high degrees (f 25%) of partial melting from a highly

depleted mantle source in mantle melting models.

Assuming a triangular melting regime beneath theIcelandic rift, such melts would be formed in the

central, uppermost part of the melting zone. A con-

dition for the eruption of such melts is an efficient

extraction of the melts from the upper part of the

melting triangle without mixing with lower melt

fractions formed further down.

The Cu/Pd ratios of the olivine tholeiites indicate

derivation from a slightly Cu-enriched source by

degrees of partial melting of around F=1012%

(Fmax =2024%).

In a combined scenario of melt generation beneath

the Iceland rift, the picrites represent the central

melting column of a melting triangle whereas the

olivine tholeiite end-member represents the entiremelting triangle (i.e. efficient melt focusing). In this

model, Cu-enriched mantle portions must be melted

out at depth, beneath the zone of picrite formation.

The relatively high degrees of melting and forma-

tion of PGE-rich magmas beneath Iceland are readily

explained by presence of a hot mantle plume.

The olivine tholeiites differentiate under S-under-

saturated conditions to Mg number 54 (MgO f 7

wt.%) leading to high Pd and low Ir concentrations.

The differentiated olivine tholeiites (7 10 wt.%

MgO) evolved from mixtures between the picrite and

olivine tholeiite end-members.

The extrusion of picrites, limited to periods of

post-glacial uplift, indicates selective tapping and

extrusion of melts from the central part of the melting

triangle.

Highly differentiated Icelandic FeTi, alkaline and

quartz tholeiitic basalts with MgO < 7 wt.%, are char-

acterized by low PGE concentrations reflecting resid-

ual sulphides in the mantle and/or S-saturated

differentiation.

The interaction between the Iceland plume and a

mid-ocean ridge leads to extensive melting of mantlesources beneath Iceland and formation of PGE-rich

and S-undersaturated melts in contrast to the Hawai-

ian intra-plate hot-spot volcanism where magmas

form by moderate degrees of partial melting around

5 10%. The S-undersaturated differentiation of the

Icelandic olivine tholeiites leads to much higher Pd

concentrations (up to 18 ppb) than S-saturated Hawai-

ian basalts (average 2.4 ppb Pd).

The Tertiary East Greenland low-Ti series and

recent Icelandic olivine tholeiites have similar Cu/Pd

ratios, suggesting derivation from a similarly depletedsource by similar degrees of partial melting beneath

plume-influenced segments of the North Atlantic

oceanic rift present day and 55 Ma ago. Independent

trace element and isotope geochemical evidence sup-

port the notion that the early Tertiary plume head

supplied material with the same composition as the

current Iceland plume (Thirlwall et al., 1994; Saun-

ders et al., 1997; Trnnes et al., 1999). Interaction

between the mantle plume presently situated beneath

Iceland and a continental rift zone 55 Ma ago and an



24/26

oceanic rift at present has led to significant production

of PGE-rich magmas.

There is largely consistency between the model of

Thirlwall (1995, 1997), Chauvel and Hemond (2000)and the model needed to explain the Cu and Pd

variations in Icelandic picrites and olivine tholeiites;

the enriched mantle component is interpreted to rep-

resent MORB-enriched mantle domains. This

enriched source beneath Iceland must be melted out

at depth. The Cu/Pd ratio of the Iceland picrite end-

member clearly indicate that the mantle sulphide

component is characteristic of strongly depleted sour-

ces such as the harzburgitic part of the oceanic crust or

entrained highly refractory lower mantle.

Acknowledgements

Staff and colleagues, especially Rikke Pedersen,

Carolina Paglia, Amy Clifton, Rosa Olafsdottir, Anna

Eiriksdottir, Matt Jackson, Daniel Larsson, Tor

Sigvald, Paul Frogner and Reidar Tronnes, at the

Nordic Volcanological Institute are gratefully ac-

knowledged for constructive discussions and helpful

input. Karl Gronvold is thanked for access to samples

from northern Iceland. Richard Wilson, Kent Brooks,

Peter Danielsen, Christian Tegner, Jens Chr. Andersenand Troels Nielsen are thanked for constructive input

to various parts of this project. An informal review by

Reidar Tronnes greatly improved an earlier version of

this manuscript. Constructive reviews by Sarah-Jane

Barnes, Bill Chazey and one anonymous reviewer are

gratefully acknowledged. [RR]

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