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The inception and progression of melting in a monogenetic eruption: MotukoreaVolcano, the Auckland Volcanic Field, New Zealand
Lucy E. McGee a,⁎, Marc-Alban Millet b,1, Ian E.M. Smith a, Károly Németh c, Jan M. Lindsay a
a School of Environment, The University of Auckland, Private Bag 92019, Auckland, New Zealandb School of Geography, Environment and Earth Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealandc Volcanic Risk Solutions, Massey University, PO Box 11222, Palmerston North, New Zealand
Compositional variation through basaltic monogenetic eruptive sequences provides a unique view into theprocesses and source heterogeneity of small-scale magmatic systems. A well-exposed, continuous sequenceon Motukorea volcano in the Auckland Volcanic Field, New Zealand, consists of an early tuff ring, scoriaceousdeposits and late lava flows which allow the evolution of the eruption to be studied at very high resolution.The deposits show a spectrum of basaltic compositions from Mg# 60 nephelinite (early tuff ring) to Mg# 70alkalic basalt (lava). Within the deposits of each main eruptive phase (i.e. tuff, scoria and lava) very little var-iation is observed in major element chemistry, suggesting that fractional crystallisation has a limited effect.Systematic changes in trace element chemistry, however, are significant through the sequence. The majorand trace element features observed through the sequence are inferred to be primarily due to the mixingof two magma batches, with a two-fold increase in the degree of melting between these. Variation inPb-isotopic compositions up-sequence indicates subtle changes in mantle source with samples representingthe start of the eruption displaying higher 207Pb/204Pb than the latter parts of the eruption. This chemicalchange coincides with a switch in the mode of eruption, with the arrival at the surface of magmas producedby larger degrees of partial melting resulting in the beginning of a more effusive eruption phase. Thesilica-undersaturated, high total alkali, low Al2O3 and higher 207Pb/204Pb nature of the samples from thetuff units suggests that these samples were produced by melting of relatively young eclogite domains. Thelower 207Pb/204Pb, higher silica, lower total alkali nature of the samples from the scoria and lava reflectsthe exhaustion of these domains and the resultant melting of the surrounding garnet-peridotite matrix.This detailed study shows that the petrogenesis of small volcanic centres may be far more complex thantheir physical volcanology suggests.
Monogenetic eruptions of basalt where there has been only limitedmodification in magma chambers or by crustal assimilation providevaluable information regarding the deeper processes ofmagma genera-tion and extraction. The term ‘monogenetic’ describes small volume,typically basaltic volcanoes that have been built in a continuous erup-tion sequence within a relatively short time span (on the scale of daysup to several decades) (Connor and Conway, 2000; Kienle et al., 1980;Valentine and Gregg, 2008). An assumption is that monogenetic volca-noes are built by the eruption of a single compositionally discrete batchof magma. Recent detailedwork on individual centres in volcanic fields,however, shows that they are more often the product of relatively
ac.nz (I.E.M. Smith),d.ac.nz (J.M. Lindsay).ysical Sciences, University ofnue, Chicago, IL 60637, USA.
rights reserved.
complex magmatic processes, commonly involving more than onemagma batch within a single eruptive episode (e.g. Bradshaw andSmith, 1994; Brenna et al., 2010, 2011; Needham et al., 2011;Valentine and Gregg, 2008; Valentine and Hirano, 2010; Valentine andKeating, 2007). Detailed studies of volcanic sequences have investigatedsystematic relationships of chemical composition to stratigraphic posi-tion (and therefore to time in an eruption sequence) and found thatthey are far from simple. For example, Strong and Wolff (2003) docu-mented the compositional changes through several monogenetic se-quences in the Southern Cascades, USA, and described differencesboth within scoria deposits, and between the scoria and lavas of thesame centre. This was attributed to the involvement of several distinctsources over the course of one eruption. This study (in addition toothers, e.g. Blondes et al., 2008; Brenna et al., 2010; Cebriá et al., 2011;Garcia et al., 2000; Reiners, 2002; Smith et al., 2008) demonstrates thesignificance of monogenetic volcanism in sampling heterogeneoussource regions, and also the complexity of simple sequences.
The observation that chemical compositions, melting processes andsource characteristics can be highly variable from one eruptive phase to
another in a single episode shows the importance of detailed samplingthrough a complete volcanic sequence. Here we present the results of ahigh resolution volcanological and geochemical study of the deposits ofa single monogenetic eruptive centre in the Auckland Volcanic Field(AVF): Motukorea. The AVF is a Quaternary basaltic intraplate volcanicfield consisting of c. 50 volcanic centres which take the form of scoriacones, tuff rings and maars. The field covers an area of c. 360 km2 onthe isthmus of Auckland in the North Island of New Zealand (Fig. 1A),and has been active for c. 250 ka (see Lindsay et al., 2011 and referencestherein). Individual volcanic centres in the field are of very small vol-ume and have distinct geochemistry (McGee et al., in review).Motukorea volcano (Fig. 2) has an exceptionally well-preserved se-quence of deposits spanning the duration of the eruption with no evi-dence of hiatuses, and thus can be considered monogenetic in termsof its physical volcanology. A striking feature of the deposits atMotukorea is their primitive nature (shown by MgO contents of9–13 wt.%, Table 1), which allows direct investigation of mantle pro-cesses. A uranium (U-) series study which focussed on the AVF byHuang et al. (1997) included some samples from Motukorea whichwere used tomodel amelting scenario for the AVF as awhole, however,a complete geochemical and isotopic study of the sequence has untilnow been lacking. We document compositional variations in majorand trace elements and Pb isotopes through all phases of the eruption,including the tuff sequence, overlying scoriaceous unit, scoria coneand lava flows in order to investigate the magmatic conditions thatled to the eruption of a small volume, monogenetic volcanic centre.
2. Geomorphology and physical volcanology of Motukorea
Motukorea is awell-preserved basaltic volcano, theminimum age ofwhich has been estimated as 7000–9000 years B.P, based on an early
5km
Rangitoto channel
Waitemata harbour
Manukau harbour
AucklandCBD
N
Motukorea
200kmN
TVZAVF
Volcanic centre
A
CraterHill
Fig. 1. (A) Schematic map of the Auckland Volcanic Field (AVF) showing the location of voSection 5) is also shown. Inset shows position of the AVF relative to other major volcanic(B) Geological sketch map of Motukorea Island showing the main volcanic features. Locatio
Holocene high stand terrace built over lava flows (Bryner, 1991). The is-land covers an area of just 0.75 km2 and is c. 900 m wide at its widestpoint (Fig. 1B). Its outcrops display a complete volcanic sequencewhich is composed of a tuff ring, a central scoria cone (66 m in height,c. 180 m in diameter), lava flows and several rafted spatter mounds(referred to here as ‘scoriamounds’) (Figs. 1B and 2). Volume estimateswere recently calculated from a Digital SurfaceModel created from spotheight detected by Light Detection and Radar (LiDAR) techniques. Thecalculated dense rock equivalent (DRE) volumes for the various volca-nic units are the following: 0.9×106 m3 for the tuff ring, 2×106 m3
for the scoria cone, and 2.8×106 m3 for the lava flow including therafted spatter/scoria mounds. Correction values used for calculation ofDRE are 30%, 50% and 95% respectively, consistent with previous esti-mates for the Auckland Volcanic Field (Allen and Smith, 1994; Allen etal., 1996) (G. Kereszturi, pers. comm; Kereszturi et al., 2012).
We refer to the following eruptive units, in order of eruption: 1) thebasal tuff ring-forming unit (comprising the ‘lower tuff sequence’,‘mid-scoriaceous unit’ and the ‘upper tuff sequence’) outcropping as10–25 m high cliffs of interbedded tephra layers on the north andnortheast side of the volcanic island, which have been partially erodedfrom the northeast at Crater Bay (Figs. 1B and 2), 2) the overlying unitof scoriaceous lapilli beds (‘upper scoriaceous unit’: USU) inferred tohave erupted from the intra-tuff ring scoria cone, 3) the intra-tuff ringscoria cone edifice including the rafted mounds (‘scoria mounds’), and4) the lava flows emitted from the central scoria cone region. Thereare no soil horizons between eruptive units or other evidence of timebreaks that would suggest that the deposits represent anything otherthan a continuous sequence. Because of this we interpret Motukoreaas having formed during a single eruptive episode, and thereforemonogenetic in terms of its physical volcanology (e.g. Németh, 2010;Valentine and Gregg, 2008).
N
100m
?
Lava flowsScoriaTuff
Scoriaceoushills
Scoriaceoushills
? Log 1
Log 2B
CraterBay
Scoria cone
Crater
lcanic centres, including Motukorea. The location of Crater Hill volcano (referred to incentres in the North Island of New Zealand, including the Taupo volcanic Zone (TVZ).n of logs (Fig. 3) is indicated by dashed boxes.
Fig. 2. Aerial photograph of Motukorea Island taken from the Northeast, showing the well-exposed tuff sequence at Crater Bay, the scoria cone and associated mounds. Photographcourtesy of Bruce Hayward.
362 L.E. McGee et al. / Lithos 155 (2012) 360–374
2.1. Basal tuff ring sequence and upper scoriaceous unit
The basal tuff ring sequence was logged at two locations: ‘SoutheastCrater Bay’ (Log 1) and ‘Northwest Crater Bay’ (Log 2) (Figs. 1B and 3).Samples were taken from both logged sections in order to obtain mate-rial representing the entire tuff sequence (and some units were moreaccessible in one than the other). Due to the northeast dip of the tuff de-posits Log 2 begins at a higher level (B1, correlating with S12 in Log 1),with the lower section of this unit outcropping below wave base. Wenote thatMotukorea is thought to have erupted in a non-marine setting(although possibly estuarine, Allen et al., 1996), therefore the base ofthe exposed deposits in Log 1 is unlikely to be the base of the sequenceand information regarding the initial eruptive deposits cannot thereforebe obtained. Several units in the tuff sequence have characteristic fea-tures which helped with correlation between Logs 1 and 2, for examplethe large plastically deformed lithics in S14/B2, the juvenile-dominatedlapilli unit S17 (‘mid-scoriaceous unit’: MSU) and shell fossils in S18(B5) (Fig. 3); however, correlation was more challenging further up-sequence (S23/B9 upwards) as the characteristics of the two logs dif-fered. S22 to S29 in Log 1 is dominated by planar beds of juvenile lapilliinterbedded with cross-bedded ash-rich layers, whereas the corre-sponding units in Log 2 (B6 to B15) are more chaotic and contain abun-dant large blocks with directional indicators and impact sags. Theseobservations led to the conclusion that logs 1 and 2 represent laterallycontinuous distal and proximal sections, respectively (Fig. 3).
We have divided the basal tuff ring sequence into upper and lowersections, separated by the distinctive marker bed of scoriaceous ba-saltic lapilli (S17 – MSU) which ranges from a c. 20 cm thick layerat Crater Bay to a c. 1.5 m thick bed further to the southeast. In gen-eral, the tuff sequence comprises fine-grained, cream-beige coloured,typically cross-bedded layers of weakly to moderately indurated,poorly to moderately sorted, tuff and lapilli tuff beds, containingabundant dense juvenile and accidental lithic lapilli. These beds alter-nate with planar-bedded to massive black-brown, cm to few dmthick, relatively well-sorted, laterally continuous, juvenile-lapilli-rich units, an example being the mid-scoriaceous unit, inferred torepresent pyroclastic fall beds (Fig. 3). Interbedding between acci-dental lithic-rich, dune and cross-bedded tuff and lapilli tuff andfall-dominated juvenile pyroclast-rich lapilli beds is commonly rhyth-mic, especially in the upper tuff ring deposit (i.e. above the MSU).Accidental lithic fragment-rich tuff and lapilli tuff beds and juvenile-lapilli-rich beds are typically 5–30 cm and 20–100 cm thick, respec-tively; the thickness of individual layers varies laterally. Within this
stratigraphic framework, we interpret the lower and upper tuff se-quences as dominantly phreatomagmatic surge deposits.
Juvenile material in the ash-dominated layers of the tuff sequenceranges from fresh, black, vesicular, sometimes glassy, ragged clasts(selected for geochemical analysis) to moderately weathered, brown-ish, non-vesicular, smoother clasts, the latter of which may representdegassed, recycled juvenile material. Both juvenile clast types rangein size from lapilli to bomb size; the bombs often show plastic defor-mation and rest in large sags, and many contain small (c. 0.5–2 cm)lithics of sandy-clay material. Petrographically, the juvenile bombsare glassy, with few olivine phenocrysts, and small inclusions of acci-dental lithics. Lithic material is abundant throughout the tuff se-quence; the majority of clasts are a pale sandy to clay materialwhich sometimes contains relict bedding. These occur as the coresof bombs, as small inclusions in bombs, or as individual clasts rangingfrom c. 1 cm up to large blocks ≤20 cm in diameter. Some of theseblocks are plastically deformed and have impacted surroundinglayers due to water release (Fig. 3). These lithics are thought to be de-rived from the underlying Waitemata Group sedimentary sequence(Bryner, 1991), the uppermost country rock in much of the Aucklandarea (Edbrooke, 2001). Also present as lithics are larger (c. 10–25 cm)clasts of orange-coloured sedimentary conglomerate. These are typi-cally rounded and smooth, but also occur as less coherent clasts.This type of lithic clast is more abundant in the lower part of thelower tuff sequence, although it also occurs in the top part of theupper tuff sequence. These clasts show characteristics such as alarge range in clast colour and type that are typical of the so-called‘Parnell Volcaniclastic Conglomerate’ facies of the Waitemata Groupsedimentary sequence, and are therefore interpreted to have been de-rived from this unit. A rare clast of potentially ultramafic materialwhich may be sourced from the Brook Street volcanics terrane –
part of the Dun Mountain Ophiolite Belt which passes beneathAuckland – was found at the southernmost extent of the tuff se-quence of the island.
Although the abundance of juvenile basaltic bombs and lapilli in-creases in the uppermost units of the upper tuff sequence (Fig. 3),there is a more abrupt change in style in the top unit of the cliff section(‘upper scoriaceous unit’: USU). This comprises a 4–10 m thick se-quence of moderately to highly vesicular fine lapilli and coarse blockyscoria in diffusely massive beds with very rare lithic clasts (Fig. 3). Thedeposit does not vary laterally where exposed. Juvenile blocks in thislayer range from 10 cm up to 70 cm, commonly have chilled marginsand are elongate in shape and plastically deformed. Scoria samples are
Table 1Representative whole rock major and trace element analyses for rock samples from Motukorea.
highly vesicular, and contain olivine phenocrysts. Titaniferous (Ti)-augite is present as a minor groundmass phase.
The characteristics of the basal tuff ring sequence at Motukorea(Fig. 3) allow some generalisations to be made about the conditions ofthe initial phase of the eruption. There is a general transition frommatrix-supported facies at the base to clast-supported facies in theupper parts of the sequence, corresponding to an increase in clast vesic-ularity upwards (Allen et al., 1996). This has been interpreted to repre-sent a trend from a dominantly wet phreatomagmatic fragmentationeruptive style early in the eruption history of the tuff ring, to a domi-nantly dry phreatomagmatic to magmatic fragmentation style in thewaning phase of the tuff eruption (e.g. Németh, 2010). This is inferredto correspond to the gradual exhaustion of the local water supply lead-ing to an increase in the magma:water ratio (Allen et al., 1996; Bryner,1991). Similar eruption style changes have been inferred on the basis ofchanges in sedimentological features from base to top at other tuff ringsin the AVF (e.g. Crater Hill, Houghton et al., 1999). The relative abun-dance of thin phreatomagmatic tuff and lapilli tuff layers in the uppertuff sequence indicates cyclic but localized recharge of water to the sys-tem to fuel explosive magma-water interaction. The gradual increase ofscoriaceous ash and lapilli layers in the top of the basal tuff ring unit in-dicate a rapid subsequent drying out of the system allowing explosivemagma fragmentation to dominate. The transition to the purely mag-matic upper scoriaceous unit indicates further drying out of the system,either due to the complete exhaustion of water or a significant increasein the magma:water ratio.
2.2. Scoria cone, rafted scoria mounds and lava flows
The intra-tuff ring scoria cone is an unbreached, circular edificec. 70 m high in the northern part of the island. It has a c. 10 m deep cra-ter, and the rim is higher on the northeast side. There is a small group oflowhills (c. 10–25 mhigh, Figs. 1B and 2) to the south and southwest ofthe scoria cone composed of scoriaceous agglutinate and lapilli com-monly displaying lava spatter layering and moderate agglutinationand welding. The implication is that the mounds are rafted sections ofthe scoria cone transported on top of lava flows issued from the crater
towards the end of the eruption. The present day circular shape of theintra-crater scoria cone is inferred to be the result of subsequent healingof the cone due to penecontemporaneous explosive activity, similarto those processes described from Red Crater, Arizona (Riggs andDuffield, 2008).
Lava flows cover the southern part of island (Fig. 1B), and have aminimum thickness of c. 1.5 m. The lavas have flowed to the south forc. 1 km; their terminus now lies underwater to the south and south-west. They are dominantly pahoehoe with rounded vesicles. Polygonaljointing on the top surface is visible in some outcrops. Lavas are denseandmoderately non-vesicular. Olivine and Ti-augite are present as phe-nocryst phases – olivine being the larger and most abundant of these.Plagioclase, Ti-augite, oxides and glass make up the groundmass.
3. Methodology
Thirty-four samples of juvenile material were collected from all keyunits of the Motukorea eruption sequence for geochemical analysis.Twenty-one samples of juvenile material (typically blocks and bombsfrom the ash-rich layers and lapilli clasts from the scoriaceous layers)were collected from the tuff sequence, with samples taken everyc. 0.5–1 m up-sequence, including several from the mid-scoriaceousunit (Fig. 3). Four samples were taken from the unit overlying the tuffsequence (USU), five from the scoria mounds, and four samples fromthe lava flows. No samples were taken from the scoria cone itself dueto lack of exposed material. When selecting blocks and bombs for geo-chemistry, an attempt was made to select inclusion-free, glassy, freshmaterial, interpreted to represent magma at the time of the eruption.Seventeen of these samples were analysed for Pb isotopes (Table 1).To the suite of 34 samples, 10 previously collected and analysed,well-located samples (from the upper scoriaceous unit, scoria moundsand lava flows) were included to augment the dataset.
Samples were washed in distilled water and dried at 80 °C beforebeing crushed in a tungsten carbide mill to b200 μ mesh. Major ele-ments were analysed by XRF on fused glass discs made using LithiumBorate Spectrachem 12–22 flux, using a Siemens SRS3000 sequentialX-ray spectrometer with a Rh tube at the University of Auckland.
BASE
2m
4m
6m
8m
10m
12m
14m
16m
18m
~36m
S1
S2
S3
S4
S5
S6
S7*
S8
S9
S10
S11
S13
S15
S16*
S17
S18
S19
S20S21
S22
S23
S24
S25S26
S27
S28
S29
S30
S14
S12
Mid scoriaceous
unit
Lower tuff
sequence
Upper tuff
sequence
Upper scoriaceous
unit
B1 (S12)
B2 (S13-15)
B3 (S16)
B4 (S17)
B5(S18-19)
B6 (S20)
B7 (S21)
B8 (S22)
B9 (S23)
B10-13(S24-27)
B14 (S28)
B15 (S29)
B16 (S30)S31
S32
S33
S34
BASE
1m
2m
3m
4m
5m
6m
7m
8m
9m
10m
11m
12m
13m
14m
15m
~18m
~26m
Directional indicators
Lithic sagsand directional
indicators
Bomb sags
Bomb sagsCross beddedash-rich layers
Rhythmic juvenileand ash beds
Plastic deformationof lithics, sags
Dune/crossbedding
Dune/crossbedding
Log 1: Southeast Crater Bay Log 2: Northwest Crater Bay
SE
Lower tuff Mid-scoriaceous unitUpper tuff
Upper scoriaceous unit
Samples taken:
Fig. 3. Simplified logs of the tuff sequence in two locations at Motukorea (see Fig. 1B for location of logs) with descriptive features used in determining proximal and distal deposits.Units in the sequence are labelled ‘S’ at Southeast Crater Bay and correlated with units labelled ‘B’ in Northwest Crater Bay until S30/B16 is reached then ‘S’ labelling system is re-sumed. Dashed lines indicate correlation of units across logs. Samples were taken from units marked with coloured symbols – these symbols are used in later geochemical plots.Note change of scale in Log 2 from 9 m. Gap below Upper Scoriaceous Unit in Log 1 is due to the height of the cliff at this point; the unit is correlated with that in Log 2 where thedeposit was more accessible.
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Trace elements were measured on a Laser Ablation InductivelyCoupled Mass Spectrometer (LA-ICP-MS) at the Australian NationalUniversity following the procedure of Eggins et al. (1998). XRF discsof samples were glued with epoxy into stacks of 15, cut, mounted(30 samples per mount) and polished on the side to be ablated. A103 μm spot size was used to track down each half of the mount;
NIST 612 was run every 15 samples and used for calibration, andthe silica content obtained by XRF used in data reduction as an inter-nal standard. BCR-2G was used as a secondary standard. The sampleswere run over three sessions (the 10 previously collected samples inJune 2007, the others in December 2010 and June 2011) and each ses-sion included samples from all eruptive phases; precision in these
Fig. 4. Major element geochemistry of juvenile clasts, scoria and lava from Motukorea vs. MgO, all in wt%. A total alkali vs. silica plot shows the variety of basaltic rock types (afterCox et al., 1979). Samples from Crater Hill volcano, also in the AVF (Fig. 1A), are shown for comparison (Smith et al., 2008), see discussion. Arrow denotes progression of the CraterHill eruptive sequence. Vectors for crystallisation of olivine and Ti-augite (percentages refer to amount of crystallisation) are shown (calculated using compositions from Deer et al.(1966) and Tschegg et al. (2011) respectively).
366 L.E. McGee et al. / Lithos 155 (2012) 360–374
sessions is ≤5%, ≤9% and ≤14% (2SD) respectively, and accuracy isb7% (b9% for Y, Cs and Tb), b4% (b6% for Sc and Y) and b10% respec-tively. Precision across all BCR-2G data (n=64) is b12% (2SD) for allelements except Y, Lu, Yb and Hf which are b14% (2SD). Accuracy is≤10% for all elements. XRF data is reported for Cr, V, Ni and Zr. TheSupplementary Data (sheet 2) presents all BCR-2G analyses.
Pb isotopes were prepared and measured at Victoria University ofWellington (VuW) in ultra-clean lab conditions, using Optima™ acids.Sample powders were leached in hot 6 M HCl for one hour then rinsedto remove un-bonded Pb after Millet et al. (2008). Powders weredigested in hot concentrated HNO3 and HF for 24 h, then dried down
and nitrified once. Samples were then taken up in 0.8 M HBr twice,and centrifuged. Pb was separated in a double pass through a pipettetip column filled with AG1-X8 resin (see Baker et al. (2004) for proce-dure). Pb isotopesweremeasured on aNu® instrumentsMulti CollectorICP-MS at VuW in static mode. Pb isotope measurements used asample-standard bracketing method with NBS-981 as the bracketingstandard to correct for instrumental mass-bias and drift (Bakeret al., 2004). JB-2 was run as a secondary standard and measuredas 206Pb/204Pb=18.3402, 207Pb/204Pb=15.5621 and 208Pb/204Pb=38.2755 with 2SD of 166, 239 and 288 ppm respectively (based onone digestion measured 5 times) close to the reference value of Baker
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et al. (2004) (206Pb/204Pb=18.3435±17, 207Pb/204Pb=15.5619±16,208Pb/204Pb=38.2784±50). Error bars are shown in Figs. 7 and 8 forthe 2SD of replicate analyses of JB-2. Pb isotope standard data arepresented in the Supplementary Data (sheet 3).
4. Geochemistry
Analyses from representative samples (all those analysed for Pbisotopes) are presented in Table 1. The full dataset is presented inthe Supplementary Data.
4.1. Major elements
Motukorea rocks range from basalt, through basanite to nephelin-ite, with low SiO2 values (39–44 wt.%), and total alkali (Na2O+K2O)values that range from 4 to 6 wt.% (Fig. 4). Samples from the tuff se-quence have higher total alkalis and lower SiO2 than the scoriamounds and lava, and samples from the USU plot between thesetwo groups (Fig. 4). MgO varies from 9.5 to 13.5 wt.%, with samplesfrom the tuff sequence displaying the lowest values. Generally, SiO2
increases and TiO2, FeO and P2O5 decrease with increasing MgOthrough the sequence (Fig. 4). CaO concentrations are constant
4
6
8
10
Th
150
200
250
300
350
400
Zr
250
300
350
400
450
500
Cr
0.18
0.19
0.20
0.21
9 10 11 12 13
Sm
/Nd
MgO
10%
25%
5% Ol
Cpx
Fig. 5. Trace element geochemistry of juvenile clasts, scoria and lava from Motukorea vs. MXRF. Vectors for crystallisation of olivine and Ti-augite are shown for Cr and Ni (references
throughout the eruptive sequence and show no correlation withMgO, and Al2O3 contents are higher in the magmatic units than inthe tuff sequence. Limited internal trends within individual unitsare observed; this is particularly striking in the tuff sequence wherethe data form a broad cluster with no trend in all the major elements.Within the scoria sequence and lava flows there is a slight correlationbetween MgO and FeO, and MgO and P2O5. Samples from themid-scoriaceous unit show more variation in the major elementsthan other units, particularly in SiO2 and Al2O3. As in SiO2 vs. total al-kalis, samples from the USU lie between samples from the tuff se-quence and those from the scoria mounds and lavas in all majorelements.
4.2. Trace elements
As with the major elements, within-unit correlations of trace ele-ments with MgO are minimal. However, the sample suite as a wholedisplays significant trends with MgO, caused by differences betweenthe bulk compositions of each eruptive unit. Samples from the tuff se-quence are lower in transition metals and higher in incompatible el-ements than the scoria mounds and lavas; samples from the USUspan the range between these units. All transition metals show
150
200
250
300
Ni
30
40
50
10 11 12 13 14
Nb/
U
MgO
500
700
900
1100
Sr
3.0
3.4
3.8
4.2
(La/
Sm
) N
10%
5%
gO. Trace elements in ppm, MgO in wt%. Symbols as in Fig. 4. Cr, Ni and Zr values fromas for Fig. 4).
1
10
100
1000
Rb Ba
Th U
NbTa
K La
CePr
SrNd
ZrHf
SmEu
GdTb
DyHo
ErTm
YbLu
Sam
ple/
Prim
itive
Man
tle
TuffMSUUSUScoria moundsLava
1
10
100
1000
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
Sam
ple/
Cho
ndrit
e
OIB
A
B
Fig. 6. (A) Rare earth element (REE) plot normalised to chondrite (after McDonoughand Sun, 1995). (B) Multi-element plot normalised to primitive mantle (afterMcDonough and Sun, 1995). Fields of samples are shown from the tuff sequence,mid-scoriaceous unit (MSU), upper scoriaceous unit (USU), scoria mounds and lavaflows. Units become progressively less enriched in LILE and LREE from the eruptionof tuff to the emission of lavas. Black line in (B) (OIB=Ocean Island Basalt) takenfrom Sun and McDonough, 1989.
368 L.E. McGee et al. / Lithos 155 (2012) 360–374
positive trends with MgO (Fig. 5); the trend through the sequence isextremely linear in Ni but more kinked in Cr and Sc (Sc not shown)showing a clearer separation between the phreatomagmatic (tuff)
15.45
15.55
15.65
15.75
38.0
38.5
39.0
39.5
40.0
18.0 18.5 19.0 19.5 20.0 20.5
207 P
b/20
4 Pb
HIMU
PacificMORB
206Pb/204Pb
208 P
b/20
4 Pb
Lower tuff MSUUpper tuff
USUScoriaLava
AVF
AVF
Lithospheric mantle
PacificMORB
HIMU
Lithospheric mantle
NHRL
NHRL
Fig. 7. 208Pb/204Pb, 207Pb/204Pb and 206Pb/204Pb for tuff bombs, scoria and lava from Motukolation by Stracke et al. (2003). NHRL=Northern Hemisphere Reference Line, calculated afterError bars for 206Pb/204Pb are similar to symbol size. Isotopic data from the AVF (McGee et alMarie Byrd Land, Antarctica (Storey et al., 1999) which are thought to represent lithospherical., 2006). A triangular pattern is observed in the Motukorea data which is particularly pro
and magmatic (scoria and lava) units. Incompatible elements all dis-play a negative trend with MgO. The data for the tuff sequence typi-cally form a cluster in all but the transition metals. Samples fromthe lava flows and scoria mounds typically show a more restrictedrange of values than samples from the USU and tuff sequence(Fig. 5); however, unlike the tuff sequence, the scoria and lava displayslight correlations with MgO. Variations in trace element ratios areseen through the sequence: (La/Sm)N is higher in the tuff sequence(4–4.2), whilst the lava and scoria mounds show more variation atlower (La/Sm)N (3.15–3.6), but higher Sm/Nd (0.20–0.21) comparedto the tuff (0.18–0.19). A positive trend from the tuff to the lava isseen in Nb/U with values ranging from 34.4 to 54.9.
Rare earth element (REE) profiles (normalised to chondrite afterMcDonough and Sun, 1995) for the tuff sequence, MSU, USU, scoriamounds and lava all show a pattern of light-REE (LREE) enrichment(Fig. 6A); the tuff sequence shows the steepest profile with themid-scoriaceous unit lying in the middle of this field, and the lavathe flattest. The eruptive units are most similar in the heavy-REE(HREE), with the tuff and USU having very similar values in Er-Lu.The sequence shows a larger range of values in the LREE with a sepa-ration observed between the USU and the scoria mounds.
On a multi-element plot (normalised to primitive mantle afterMcDonough and Sun, 1995) all units show a similar profile peaking atNb and Ta followed by an overall decrease in enrichment (Fig. 6B), typ-ical of ocean island basalts (Sun andMcDonough, 1989). All units showa prominent negative K anomaly (deepest in the tuff, MSU and USUsamples), and less prominent negative Zr, Hf and U anomalies. Themid-scoriaceous unit shows both positive and negative Sr anomaliesand also a large range in Rb and Ba (due to outliers amongst thesesamples – see Supplementary Data). A slight positive Sr anomaly isalso seen in the upper scoriaceous unit.
4.3. Sr–Nd–Pb isotopes
There are six Sr–Nd isotope analyses available from Motukoreapublished by Huang et al. (1997), and these show very little variation:87Sr/86Sr ranges from 0.702790 to 0.702930, and 143Nd/144Nd from0.512970 to 0.51300 (not shown). Pb isotopes analysed for the current
38.80
38.85
38.90
38.95
19.15 19.20 19.25 19.30 19.35 19.40
15.59
15.60
15.6120
7 Pb/
204 P
b
206Pb/204Pb
208 P
b/20
4 Pb
NHRL
NHRL
2SD
rea. Pacific MORB and HIMU (Cook-Austral islands and St Helena) data from a compi-Hart (1984). 2SD error bar based on replicate analyses of JB-2 is shown for 207Pb/204Pb.., in review) are shown as a labelled pale grey field. Data from Group B and C dykes frommantle beneath New Zealand are plotted and labelled ‘Lithospheric mantle’ (Hoernle etminent in 207Pb/204Pb vs. 206Pb/204Pb space.
Fig. 8. Selected element and isotope plots vs. stratigraphic height of sample (in metres) showing change in chemistry up-sequence. MSU=Mid-scoriaceous unit, USU=upper sco-riaceous unit. The scoria mounds and lava are assigned an arbitrary stratigraphic position at the top of the sequence. In Mg#, La and Sm/Nd a gradual change in chemistry is seenthroughout the tuff sequence with a more dramatic change between tuff and magmatic phases (i.e. scoria and lava). The mid-scoriaceous unit (S17, see logs in Fig. 3) often shows alarge range in values for samples taken from this layer most likely due to post-depositional alteration. 208Pb/204Pb ratios up-sequence decrease throughout the tuff sequence, thenreturn to a similar starting composition in the magmatic phase before again decreasing; however, 207Pb/204Pb ratios decrease from the tuff to the magmatic phases. This is sugges-tive of early depletion of a radiogenic component and the presence of two melt batches. 2SD error bars are shown for Pb isotopes.
369L.E. McGee et al. / Lithos 155 (2012) 360–374
study display larger variations (Fig. 7): 206Pb/204Pb ratios range be-tween 19.158 and 19.360, 207Pb/204Pb between 15.589 and 15.605,and 208Pb/204Pb between 38.800 and 38.958. This places the databetween the Pacific MORB and HIMU fields, closest to Pacific MORB,similar to other data from the AVF (McGee et al., in review) (Fig. 7).As with the whole field data, samples from within the Motukorea se-quence occupy a triangular area in 206Pb/204Pb vs. 207Pb/204Pb and anessentially linear trend in 206Pb/204Pb vs. 208Pb/204Pb (Fig. 7). Thelava and scoria mounds are separated in Pb-isotopic space from thetuff and mid- and upper scoriaceous units, with the former havinglower 207Pb/204Pb and extending to lower 206Pb/204Pb and 208Pb/204Pb.
4.4. Compositional variation with stratigraphic height
Thewell-constrained eruption sequence atMotukorea allows exam-ination of geochemical changes with the progression of eruption. InFig. 8 the scoria mounds and lava are placed on top of the tuff sequenceand upper scoriaceous unit and given an arbitrary height, as this iswhere they are known to occur temporally in the eruptive sequence(see Section 2). A gradual increase in Mg# (magnesium number, calcu-lated as mole percent Mg/Mg+Fe x100, with FeO calculated fromFe2O3
tot as FeO=Fe2O3tot/1.31134) is seen through the tuff sequence,
followed by amore dramatic increase in the upper scoriaceous unit, sco-ria mounds and lava (Fig. 8). A gradual decrease in incompatible ele-ments such as La (Fig. 8), Zr and Th (not shown) is observed, with amore dramatic decrease in the magmatic units. Some element ratiossuch as Sm/Nd (Fig. 8) and La/Sm (not shown) showvery little variationthrough the tuff sequence followed by a change in the scoria and lava.TheMSU typically shows a large range in values compared to the overallvariation in the tuff sequence; for example Mg# varies from 60 to 63 in
the lower and upper tuff, but themid-scoriaceous unit varies from62 to64 (Fig. 8). This unit also displays a larger range of variation in fluidmo-bile elements (Rb, Ba, Sr, U) compared to neighbouring fluid immobileelements (Th and Nb) on Fig. 6B. High variability of these elementshas been linked to post-eruptive alteration (Schiano et al., 1993). Thismay explain the variation seen in this particular layer, as it is highly po-rous compared to the upper and lower tuff sequences above and belowit. Systematic changes with stratigraphic height are also observed for Pbisotope data. 208Pb/204Pb (and 206Pb/204Pb, not shown) decrease fromthe start of the eruption until c.12 m height (i.e. in the upper tuff;note that the upper tuff sample which plots with the upper scoriaceousunit samples was taken from the layer directly below the latter). Theupper scoriaceous unit has a slightly more radiogenic Pb isotope com-position compared to the start of the tuff eruption, and this decreasesin the scoria mounds and lava (Fig. 8). A pronounced decreasein 207Pb/204Pb is observed through the eruption sequence, with similarvalues displayed through the tuff and most of the USU samples, anddistinctly lower values in the scoria mound and lava samples. Thecause of these systematic compositional differences and the decouplingof the 206Pb/204Pb and 208Pb/204Pb ratios from the 207Pb/204Pb ratiomerits further discussion.
5. Discussion
The separation of the samples from the tuff sequence and those fromthe scoriamounds and lavas inmajor and trace element and Pb-isotopiccompositions, together with the non-linear nature of the sequencein bivariate plots (Figs. 4 and 5), suggests that the dominantlyphreatomagmatic and dominantly magmatic phases of the eruptionwere the products of two different magma batches, with the upper
370 L.E. McGee et al. / Lithos 155 (2012) 360–374
scoriaceous unit having compositions between these two groupssuggesting mixing. The triangular pattern observed in Pb-isotopicspace implicates several sources in the magma genesis. In the followingdiscussion the nature and extent of the magmatic processes affectingboth magma batches, together with the characterisation of the sourcesinvolved, reveals a far more complex melting scenario than may havefirst been assumed from the island's monogenetic appearance ex-pressed in its physical volcanology (Section 2 and Fig. 3).
5.1. The limited effect of fractional crystallisation
Samples representing the eruptive products of Motukorea haveMg#s of 60–70 and have high MgO contents (9.5–13.5 wt.%) which in-dicate that they are near-primary melts. The lack of trends with MgOwithin eruptive units in the major elements and some of the trace ele-ments (Figs. 4 and 5), a feature that is particularly prominent in thetuff sequence, suggests that only minimal fractional crystallisation hastaken place during magma ascent. Olivine and clinopyroxene (Cpx –
titaniferous augite) are observed petrographically in Motukorea rocks;thus, fractional crystallisation modelling of these minerals was under-taken in order to see what effect this process has had on compositions(plotted as vectors on Figs. 4 and 5). Although small amounts of frac-tional crystallisation of olivine+Cpx in an approximately 50:50 ratiocould explain the trends seen in MgO vs. SiO2 and FeO for the wholesuite of samples, this cannot explain the trends in MgO vs. TiO2, Al2O3,Cr and Ni (Figs. 4 and 5). It is possible that the lava samples have beenaffected by very small (b3%) amounts of fractional crystallisation of ol-ivine; minor crystallisation in these samples is consistent with theirnear-primitive nature (Mg# 70). The low MgO nature of the tuff sam-ples cannot be attributed to fractionation from magmas that producedthe more primitive (higher MgO) scoria and lava samples, and this re-quires further explanation.
These findings are in contrast with the model of Smith et al. (2008)based on another well-exposed tuff-scoria-lava sequence in the AVF(Crater Hill – compositional field and progression of eruption areshown in Fig. 4, location marked on Fig. 1A). The entire volcanic se-quence at Crater Hill was sampled at high resolution and modelled asbeing controlled by deep-seated fractionation of high pressure (highAl2O3) Cpx. The positive trend in MgO vs. Al2O3 and the lack of trendin MgO vs. CaO observed in the Motukorea sequence contrasts withthose displayed by the Crater Hill samples (Fig. 4). A notable similarity,however, is that both eruptions beganwith the phreatomagmatic erup-tion of low MgO basalt and progressed to effusive eruption of moremafic lavas. The lack of a fractional crystallisation trend in the firstmagmas to be produced at Motukorea is significant as it suggests thatthey did not stall or pond en route and therefore erupted as virtuallyunmodified melts of their mantle source. Although the magmas givingrise to the lavas experienced minor amounts of shallow fractionation,this is minimal, suggesting all samples represent near-primary melts.
5.2. Source compositions and involvement during progression of melting
The triangular pattern observed in the Pb-isotope compositionsof samples from the Motukorea eruptive sequence (Fig. 7) revealsthe involvement of three source components in the petrogenesis ofthe magmas: a low 206Pb/204Pb component plotting to the left of theNorthern Hemisphere Reference Line (NHRL, after Hart, 1984) in the206Pb/204Pb vs. 208Pb/204Pb diagram (represented mostly by samplesfrom the upper tuff sequence), a high 207Pb/204Pb and 208Pb/204Pb com-ponent (represented by samples from the lower tuff sequence andsome upper scoriaceous unit samples) and a low 207Pb/204Pb compo-nent plotting close to the NHRL in both diagrams (represented by sam-ples from the scoria mounds and some upper scoriaceous unit samples)(Fig. 7). The composition of the low 206Pb/204Pb end-member hasalready been interpreted in other studies of New Zealand intraplatevolcanic fields as evidence for a subduction-modified lithospheric
component (Fig. 7) (Hoernle et al., 2006; Timm et al., 2010). This assim-ilation of melts has been identified as occurring to varying degrees onthe scale of the whole volcanic field (McGee et al., in review). BecausePb-isotopic compositions of both the tuff sequence and the scorias andlavas trend towards this region in Pb-isotopic space it is suggestedthat, as themagmas ascended, they assimilatedmelts from theoverlyingsubduction-metasomatised lithosphere.
The difference in Pb-isotopic composition between the beginningof the tuff eruption and the eruption of the scoria and lavas (mostnoticeable in stratigraphic height vs. 207Pb/204Pb, Fig. 8) implies a differ-ence in the source region for the melt that generated the tuff samplescompared to those that generated the scoria and lava flows. The higher207Pb/204Pb in the lower tuff samples and their relatively low Sm/Ndratios (Figs. 5 and 8) is suggestive ofmelting in the presence of recycledmaterial which is older, and more radiogenic, than the ambientasthenospheric mantle (DePaolo and Wasserburg, 1976). The meltingof garnet pyroxenite or eclogite veins (commonly related to ancientsubduction) is known to cause Pb-isotopic ratios to be more radiogenicdue to higher U/Pb ratios (e.g. Pilet et al., 2008; Thirlwall, 1997;Willbold and Stracke, 2006). It is likely that such material was incorpo-rated into the mantle in the Mesozoic subduction episode of the NorthIsland of New Zealand (e.g. Cook et al., 2005), explaining the somewhatelevated Pb-isotopic values, whilst not displaying truly HIMU values(Thirlwall, 1997; Vidal, 1992); this may explain the partial decouplingof 206Pb/204Pb and 208Pb/204Pb from 207Pb/204Pb (Fig. 8).
The preferential melting of eclogite veins compared to a dominantlyperidotitic mantle at low degrees of partial melting has been suggestedas the cause formagmas to be silica-undersaturated (e.g. Hirschmann etal., 2003; Kogiso et al., 2003), resulting in the eruption of nephelinitecompositions, as is noted in the tuff sequence ofMotukorea (Fig. 4). Un-dersaturated magma compositions are a striking feature of small-volume basaltic systems (e.g. the South Auckland volcanic field (Cooket al., 2005), the Wudalianchi volcanic field (Hsu and Chen, 1998), theNewer Volcanic Province (Demidjuk et al., 2007), the Eifel volcanicfield, Germany (Haase et al., 2004) and the Harrat Ash Shaam volcanicfield, Jordan (Shaw et al., 2003)) suggesting that a discontinuous,eclogitic component may be a common feature of such fields. Thelower 207Pb/204Pb ratio of samples from the scoriamounds and lava sug-gests that they were produced from melting of the residual peridotiticmineralogy remaining after extraction of the silica-undersaturatedmagma.
The steepness of the REE plot for all eruptive units (Fig. 6A) is indic-ative of residual garnet in the source (e.g. Hoernle and Schminke, 1993).This is evidence for the magma being sourced from depths of ≥80 km(e.g. McKenzie and O'Nions, 1991). The 230Th-excess observed byHuang et al. (1997) in four samples from Motukorea from all partsof the sequence also supports melting in the garnet stability field,based on the incompatibility of Th in garnet (e.g. Elliott, 1997; Peateand Hawkesworth, 2005). The AVF contains some of the highest(230Th/232Th) ratios for continental basalts (up to 1.38, McGee et al.,2011, in review), suggesting that melting initiates at the bottom of along column (e.g. Peate andHawkesworth, 2005).We therefore suggestthat the sequence of deposits erupted at Motukorea involved twomagma batches originating from a heterogeneous garnet-bearing as-thenospheric source thatwas progressively depleted due to the exhaus-tion of an eclogitic component. These magmas then mixed with meltsfrom the overlying subduction-metasomatised lithosphere, althoughthe relatively un-radiogenic nature of 87Sr/86Sr ratios from Motukorea(see Section 4.3) and the similarity of the multi-element pattern forall samples suggest that this interaction was not extensive.
5.3. The nature and behaviour of the source beneath Motukorea
Using the points discussed above,wemodel themantle processes thatproduced the Motukorea sequence (Fig. 9). In (Gd/Yb)N vs. (La/Yb)Nspace samples plot in three distinct groups, each with a different trend:
Lower tuff
Upper tuff
USUScoriaLava
MSU
(Gd/Yb)N
1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
(La/
Yb)
N
0
5
10
15
20
25
30
35
A
CB
Eclogite (athenosphere)
Fertile peridotite (garnet-bearing)
Lithospheric source (spinel-bearing)
Mixing (% of lithospheric melt)
0.5%
1%
A
C
B
0.75%
1%
1.5%
2%
10%
30%
50%
70%
90%
20%
40%
2%
2.5%
1% Degree of partial melting
1%
60%
80%
Fig. 9. (Gd/Yb)N vs. (La/Yb)N (normalised to primitive mantle after McDonough and Sun, 1995) for the Motukorea sequence to illustrate melting and mixing processes which createthe observed chemical trends between the phreatomagmatic and magmatic phases of the eruption. Solid lines showmelting of three sources based on those modelled for the AVF inMcGee et al. (in review): Line A denotes melting of eclogite veins, Line B denotes melting of a fertile asthenospheric garnet peridotite source and Line C denotes melting of moredepleted lithospheric spinel-bearing peridotitic source. Italicised numbers in all models denote degrees of partial melting. Bold numbers on mixing lines show percentage of lith-ospheric melt. See Section 5.3 for discussion of model and Table 2 for melt proportions, source modes, starting compositions and partition coefficients.
371L.E. McGee et al. / Lithos 155 (2012) 360–374
1: the tuff sequence, 2: the scoria and lava samples, and 3: samples frommainly the upper scoriaceous unit (USU). These trends correspond tothree distinct mixing events involving the three sources identified inPb-isotopic space. (Gd/Yb)N vs. (La/Yb)N is used as a proxy for both de-gree of melting as well as the nature and proportion of the aluminousmineral phase in the mantle source (i.e. spinel vs. garnet). As we have
Table 2Parameters used in melting model (Fig. 9). Melt compositions were calculated using thebatch melting equation taking into account the melting proportion of each mineral:Cm=1/X x(1-((1-(P xX/D)) ^ (1/P)))xCo where Cm=composition of the melt, X=degree of partial melting, P=bulk melting proportion, D=bulk partition coefficient andCo=source composition. Depleted and fertile peridotites are based on primitive mantle(Hofmann, 1988), and eclogite composition is based on xenolith K91-11 from Barth etal. (2001). Melting proportions for peridotites are taken from Thirlwall et al. (1994),and based on these for eclogite. Partition coefficients are from aAdam and Green(2006) and bGreen et al. (2000); all others are from McKenzie and O'Nions (1991).
Degree of melting (X) 0.5% 1–2% 0.75%Partitioncoefficients
La Gd YbOlivine 0.0004 0.0015 0.0015Clinopyroxene 0.03a 0.3 0.43a
Orthopyroxene 0.0006a 0.034b 0.077a
Spinel 0.0005a 0.498 4.54a
Garnet 0.01 0.01 0.01
shown that themagmas have not been significantly affected by fractionalcrystallisation processes en route to the surface (see Section 5.1) thecompositions can be viewed as essentially unmodified, and thereforeappropriate for source composition and melting modelling.
Three source components related to those identified in Pb-isotopicspace (Fig. 7) are modelled in Fig. 9 using starting compositions andmineral modes based on the three components identified on the scaleof the whole volcanic field in McGee et al. (in review), and calculatedusing the batch melting equation taking into account melting propor-tions of each mineral (after Thirlwall et al., 1994). Parameters of themodel are given in Table 2. We model the melting of eclogite veins(source A, Fig. 9) within a fertile garnet peridotite mantle (B) (both as-thenospheric) and a spinel-bearing lithospheric source (C). Source Acontains a high proportion of clinopyroxene and garnet to reflect a typ-ical eclogitic mineralogy, source B contains 5% garnet, has a mineralogytypical of a fertile peridotite (Table 2) and has a composition slightlymore enriched than primitive mantle, and source C contains 4% spineland has a compositionmore depleted than primitivemantle to simulatea less fertile peridotite. Source B is intended to reflect the mantle sur-rounding the eclogite domains once melting has exhausted the latter.Mixing lines are plotted between the lithospheric source (C) and bothasthenospheric sources (A and B) to simulate the incorporation of lith-ospheric melts during ascent of asthenospheric magmas.
A first-order observation in the trace element data is that there is atwo-fold decrease in concentration from the tuff samples to the lavaand scoria samples in the incompatible elements (e.g. Zr, Sr, Th Fig. 5,also the LREEs (Fig. 6A) and Nb, not shown). As the effect of fractionalcrystallisation has been shown to be very minimal (Section 5.1), it canbe assumed that this decrease is caused by approximately doublingthedegree of partialmelting of the sources involved. Generally small de-grees ofmelting are also evident from the LILE and LREE enriched natureof the samples fromMotukorea. At very small degrees of partial melting(b1%) amantle source containing veins of pyroxenitic or eclogitic mate-rial can generate silica-undersaturatedmelts due to the higher fusibilityof pyroxenite compared to peridotite (e.g. Hirschmann et al., 2003; Pilet
372 L.E. McGee et al. / Lithos 155 (2012) 360–374
et al., 2008). We model that as this melt ascends through the litho-sphere, a very small degree of melting is generated (0.25% of source C)and a small percentage of these melts (b25%) is incorporated into therising nephelinitic magmas (A–C mixing line in Fig. 9). The veins nowexhausted, the asthenospheric source is of a peridotitic composition(source B), and there is a two-fold increase in degree of melting of thissource (c. 2%). On ascent thesemelts also incorporate a small percentageof lithospheric melt (mixing line B-C).
Larger degrees of melting (2%) of source Awould eliminate the needfor a peridotitic source (B), as this would produce a similar trend withlithospheric melts. However, this would require that the second, largerdegree asthenospheric melt mixes in a 50:50 ratio with a lithosphericmelt. This large addition of a subduction-metasomatised melt wouldbe expected to have an effect on the trace element and isotopic charac-teristics of the resultant magmas. Volcanic centres in the AVF whichhave been modelled as incorporating a large proportion of lithosphericmelt display an obvious positive Sr anomaly on a multi-elementplot, and also lie towards lower 206Pb/204Pb and higher 207Pb/204Pb(McGee et al., in review). Neither of these features is evident in samplesfrom the scoria mounds and lavas at Motukorea (Figs. 6B and 7). Notethat although samples from the upper scoriaceous unit lie along the py-roxenitic asthenosphere-lithosphere mixing line (A–C), we attributetheir chemistry to mixing between the tuff-producing magma andmelts of the peridotitic source, instead of incorporation of c. 50% litho-spheric melts. Samples from this unit consistently plot as a transitionbetween the tuff and scoria and lava compositions (Figs. 4, 5, 8, and 9)showing that they are produced by mixing of the two magma batches.In addition to this, the compositions of samples from this unit show
Groundwater
Crust
Asthenosphere
Mixing of melts
Deposition of USU
Termination ofphreatomagmatic
phase
Recharge ofprimitive magma,
larger degree of melting
Deposition of lower tuff unit
Ascent through subduction-
metasomatisedlithosphere
Silica-undersaturatedsmall degree melts
(=Nephelinites)
Gt/Sptransition
Depof
USU
Forsco
(4)
eclogiteveins
Exhaustion ofeclogite veins
(1)
Fig. 10. Schematic model of the eruption of Motukorea illustrating the volcanological featuaceous unit. Gt/Sp transition=garnet/spinel transition. (1)–(3) depict the melting of eclogitemagma batch with the second, produced by larger degrees of melting leading to the deposimagma batch and resultant formation of the scoria cone, emission of lava flows and rafting
no evidence of having incorporated a substantial proportion of litho-spheric melt, as discussed above. We therefore favour the hypothesisthat the progressive melting of an eclogitic source leads to exhaustionof this component and eruption of nephelinitic compositions, followedby production of a second melt batch by larger degrees of melting of aperidotitic source which produced basanite and alkalic basalt composi-tions.Mixing between these twomelt batches produced the transitionalcompositions of the upper scoriaceous deposit. Depletion of the sourceregion with the progression of melting and consequent eruption is alogical concept (e.g. Reiners, 1998; Reiners and Nelson, 1998), and wedemonstrate that this can actually be observed as a result of high reso-lution sampling through a continuous eruption sequence.
5.4. Linking composition and volcanic stratigraphy
A schematic of themodel for Motukorea is shown in Fig. 10 illustrat-ing both the compositional and volcanological progression of the erup-tion. The upper and lower tuff sequences are shown as being producedfrom the same source and with the samemelting conditions, explainingthe general similarity in composition throughout the tuff sequence (1).The juvenile samples in these deposits are nephelinitic in compositiondue to the preferential melting of eclogitic veins. As suggested byBryner (1991), the mid-scoriaceous unit (MSU) is likely to have formeddue to the temporary exhaustion of local groundwater (2), with thephreatomagmatic eruption continuing after deposition of this juvenile-clast-dominated layer presumably due to recharge of the water supply(3). The phreatomagmatic stage of the eruption is then terminated bythe arrival at the surface of larger volume and larger degree melts
MSU
osition MSU Drying out of
groundwater
Deposition ofupper tuff unit
Recharge ofgroundwater
Completed tuff ring
mation ofria cone
Rafting of scoriaby lava flows
More primitive,less enriched
alkalic basalts and basanites
Most primitivelavas erupted
(5) (6)
Continued building of scoria cone
Emission of lava from
scoria cone vent
(2) (3)
res and geochemical trends observed. MSU=mid-scoriaceous unit, USU=upper scori-veins resulting in the deposition of the tuff sequence, (4) depicts the mixing of the firsttion of the transitional USU. (5)–(6) illustrate the ascent of the second, possibly largerof scoriaceous material. See Section 5.4 for further discussion.
373L.E. McGee et al. / Lithos 155 (2012) 360–374
which overwhelm the environment by increasing the magma:waterratio, leading to the onset of magmatic activity with the eruption ofthe upper scoriaceous unit (USU); within-conduit mixing explains thetransitional nature of this deposit's composition. This coincides withthe exhaustion of the eclogite source component (4). A scoria conebegins to develop, as the eruption style becomes purely strombolian(5); material produced is basanitic to basaltic in composition. As theeruption progresses, lava is emitted from the main vent and rafts someof the scoriaceous material away from the cone to form the scoriamounds, whilst the cone continues to form (6).
It is possible that the magma flux was higher in the second magmabatch; the evidence for this is: 1) the two-fold increase in degree ofmelt-ing in this secondmagma batch (Section 5.3, Fig. 9), 2) the larger volumeof the resultant eruptive products (scoria cone and lava flows)(Section 2) and 3) the abrupt change fromphreatomagmatic tomagmat-ic volcanismwhich is indicative of a highermagma:water ratio. Our find-ings support the hypothesis of Strong andWolff (2003) that eruptions atmonogenetic volcanoesmay occur in pairs, with the first magma being a‘path-finder’ for a second, larger (in the case of Motukorea) magmabatch.
The mixing of the first and second melt batch, inferred from thetransitional nature of the upper scoriaceous unit, is significant inthat it shows that the tail of the small-degree nephelinitic magmawas followed immediately by the head of the second more primitivemelt (panel 4, Fig. 10), with no hiatus in the volcanic record or in thechemical trends observed (Figs. 3–5, 8, and 9). This is evidence that aseemingly monogenetic eruption can be composed of more than onemagma batch, i.e. be’compositionally polygenetic’ whilst remaining‘superficially monogenetic’, and also suggests that the conduit re-mains open between the two melt batches despite the very small vol-umes involved.
6. Conclusions and implications for monogenetic volcanism
Major and trace element and Pb-isotopic compositions ofstratigraphically controlled samples have revealed details about thesources and melting conditions of a complete monogenetic sequence.Systematic chemical changes through the phreatomagmatic phase ofthe eruption to the purelymagmatic phase indicates a farmore complexmelting scenario than the term ‘monogenetic’ implies. We have shownthat the magma represented by the early tuff sequence and later scoriaexperienced no fractional crystallisation, whilst that represented by thelava experienced only very minor crystallisation of olivine. Modellingshows that juvenile material in the tuff ring was produced from verysmall degrees of melting of eclogite veins within the asthenosphere,and that this component was exhausted during melting. A secondmagma batch was produced from the remaining garnet peridotitesource, at degrees of melting twice as large as in the initial magmabatch. A compositionally transitional deposit shows that there was nohiatus in the ascent of these melt batches. The magma recharge eventmay have been accompanied by an increase in magma flux, evidencedby the transition to magmatic eruptions and the larger volume of erup-tive products in the magmatic phase.
Our model is in good agreement with the findings of past studiesthat have illustrated thatmonogenetic eruptions can show considerablecompositional differences through sequences and between phases ofthe eruption (Brenna et al., 2010, 2011; Erlund et al., 2010; Siebe etal., 2004; Strong and Wolff, 2003). Although the term ‘monogenetic’can be applied to the volcanology of such sequences (in that they definea continuous eruptive episode), this label does not adequately describethe complex melting and mixing processes within mantle sourceswhich are often reflected in the geochemistry. The detailed analysis ofa complete small-scale volcanic sequence such as at Motukorea allowsus to document the variations in melting conditions and changes thatthe magmatic system undergoes during the progression of a monoge-netic eruption, from start to finish, and has also shown that mantle
components identifiable on the scale of a volcanic field can also be ob-served on the scale of a single eruption.
Acknowledgements
We thank the New Zealand Department of Conservation for accessand transportation to Motukorea. Lucas Hogan, Aleksandra Zawalna-Geer, Gabor Kereszturi and Javier Agustin-Flores are thanked for field-work assistance and we thank Gabor Kereszturi for the volume esti-mates. We benefitted from field discussions with Bruce Hayward, whowe also thank for providing the aerial photograph of Motukorea. Wethank John Wilmshurst for XRF analyses and Charlotte Allen at theAustralian National University for assistance with LA-ICP-MS analyses.Steve Blake and Terry Plank are thanked for constructive reviews ofthis work. This project was funded by the Determining Volcanic Risk inAuckland (DEVORA) project as part of LEM's PhD thesis. JML gratefullyacknowledges support from the New Zealand Earthquake Commission.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2012.09.012.
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