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8/6/2019 deJong_Numerical Modeling of Apparent Argon Loss Age Spectra Archean Hornblende_Geological Journal 2009
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Geosciences Journal
Vol. 13, No. 3, p. 317 329, September 2009DOI 10.1007/s12303-009-0030-4 The Association of Korean Geoscience Societiesand Springer 2009
Apparent partial loss 40Ar/39Ar age spectra of hornblende from the Palaeopro-
terozoic Lapland-Kola orogen (arctic European Russia): insights from
numerical modelling and multi-method in-situ micro-sampling geochronology
ABSTRACT: 4 0 Ar/3 9 Ar age spectra with progressively increasing
step ages are well known for metamorphic hornblende and have
been classically interpreted by partial loss of radiogenic argon bydiffusion processes during younger thermo-tectonic reworking.
Application of a number of numerical modelling tools based on
diffusion theory and that assume thermally activated loss of radio-
genic 4 0 Ar by solid-state volume diffusion suggests that staircase-
shaped age spectra of Neoarchaean tschermakitic hornblende from
the Lapland-Kola Orogenare due to argon losses of 4050% dur-ing reheating to 450 25 o C in Palaeoproterozoic time. However, in
hornblende samples that yielded staircase-type age spectra, biotite
occurs in the matrix, as well as intimately and abundantly inter-
grown with the amphibole along grain boundaries, cleavages, frac-
tures and other defects. Drilling of 1.5 mm diameter discs from
carefully selected hornblende grains in petrographic thin sections
permitted to minimise the effects of contaminant biotite inclusions
and/or compositional zoning of the amphibole.4 0
Ar/3 9
Ar laser probestep-heating of drilled biotite-free hornblende discs yielded flat age
spectra, suggesting absence of thermally activated radiogenic 4 0 Ar
loss. This would imply unrealistically contrasting temperature his-
tories for neighbouring grains. Apparent-loss age spectra, thus,
result from differential gas release of hornblende and an included,
earlier degassing minor contamination of much younger biotite,
which had apparently not been completely eliminated from the
amphibole separate, despite careful handpicking. This is confirmed
by the Ca/K ratio spectra a proxy for 3 7 ArCa/3 9 ArK of hornblende
that are flat for drilled biotite-free hornblende grains, but initially
low for hornblende separates. A drilled disc and a separate of horn-
blende from a biotite-free amphibolite did not yield apparent loss
spectra, but flat age and Ca/K ratio spectra, confirming the inter-
pretation of the role of biotite.
Key words: Archaean, argon loss spectra, geochronology, Kola pen-
insula, micro-sampling
1. INTRODUCTION
Time is a unique element in geosciences and the discov-
ery of radioactivity by Henri Becquerel in 1896 enabled
the development of radiogenic isotope geochemistry, open-
ing the way for measurements of the age of geological
materials and processes. This has enabled geoscientists to
place the evolution of the Earth and geological processes
within an absolute timeframe, which has fundamentally
changed thinking in the earth sciences.
Metamorphism is one of the prime geological processes.
Metamorphic rocks usually have undergone a complicated
tectono-metamorphic evolution. During this process they
have been affected by superimposed recrystallisation phases
under changing pressures and temperatures, as well as dif-
ferent fluid activities. Ages of such events in collision and
other tectonic zones have been recorded in micron-sized
domains of minerals, and must be interpreted in close rela-
tion to metamorphic mineral assemblages and the tectonic
fabrics of rocks. Such very small domains are hard to date
with the conventional geochronological techniques and
require the use of sophisticated micro-chronological meth-
ods. Used for the dating of Th- and U-bearing accessory
minerals are: SHRIMP (Sensitive High-Resolution Ion Micro-
Probe), different ion, electron and proton probe micro anal-
ysers applying the regression-based chemical ThUtotal
Pb isochron method (CHIME), and Laser Ablation Induc-
tively Coupled Plasma Mass Spectrometry (LA-ICPMS).
A number of40Ar/39Ar LASER (Light Amplification by Stim-
ulated Emission of Radiation) techniques have been used
for micro probe dating of small domains in K-bearing rock-
forming silicates. Instead of a laser probe, a microscope-
mounted drill can be used to obtain mm-scale discs of min-
erals from targeted sites in polished petrographic thin sec-
tions (Verschure, 1978). This micro-sampling techniquehas been applied to RbSr (Meffan-Main et al., 2004) and40Ar/39Ar laser step-heating (de Jong and Wijbrans, 2006)
dating.
In this study I use dating results that de Jong and Wijbrans
(2006) obtained on hornblende from the Lapland-Kola oro-
gen, which experienced a polyphase tectono-metamorphic
evolution of about 1 Ga from the late Neoarchaean to the
middle Palaeoproterozoic and is developed in the Kola
Peninsula, Arctic European Russia (Fig. 1, insert map). In
the present study I use numerical modelling of apparent
loss 40Ar/39Ar age spectra, i.e., spectra with progressively
rising apparent ages, they obtained on hornblende sepa-rates to shed light on the cause of apparent loss age spectra.
Koen de Jong* School of Earth and Environmental Sciences, College of Natural Science, Seoul National University, Seoul151-747, Republic of Korea
*Corresponding author: [email protected]
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318 Koen de Jong
Modelling results of such staircase-type age spectra are
compared with laser step-heating age spectra of drilled
hornblende discs without included biotite, and that were
essentially flat. A sharply discordant RbSr age for a horn-
blendeplagioclase pair is also discussed making use of the
insights gained from micro-sampled hornblende.
2. MICRO-CHRONOLOGY AND POLYPHASE TEC-
TONO-METAMORPHIC EVOLUTION
In geodynamic environments characterised by superim-
posed tectono-metamorphic events, rocks had to adapt to
changing physical conditions by recrystallisation during
P-T specific mineral reactions. Mineral zoning indicates
that chemical-potential gradients were not eliminated by
intracrystalline diffusion (Fisher, 1977). The occurrence of
zoned metamorphic minerals, or the presence of relics,
thus implies that disequilibrium conditions existed during a
succession in time of different metamorphic facies. Fabric-
forming minerals record the changing physical conditions
along P-T-d-t paths. This is also the case for polygeneticTh- and U-bearing accessory minerals like monazite, xeno-
time, euxenitepolycrase, allanite and zircon (Crowley and
Ghent, 1999; Cocherie et al., 2005; Dahl et al., 2005; Dumond
et al., 2008; Li et al., 2008; Suzuki and Kato, 2008; Bosse
et al., 2009). Fluid-assisted recrystallisation that affected
ionic bonds in minerals, consequently, played a paramount
role during exchange or loss of radiogenic daughter iso-
topes (Wijbrans and McDougall, 1986; Miller et al., 1991;
Villa 1998; Kerrich and Ludden 2000; de Jong et al., 2001;
de Jong, 2003).A major geochronological challenge is to obtaining age
estimates for the timing of specific tectono-metamorphic
phases or events in orogenic belts and tectonic zones. Because
the host rocks have deformed and metamorphosed in a
number of phases, leading to chemical and isotopic zoning
of minerals, radiometric ages of these events too have been
recorded in small domains of minerals. Secondary Ion Mass
Spectrometry (SIMS) has been specifically designed for
single-grain and in-situ 207Pb/206Pb and UPb age determi-
nations in accessory minerals with a high spatial resolution
of 1530 m using SHRIMP, Cameca IMS 1270 or Nano-
SIMS NS50 instruments (Takahata et al., 2008). Electronand proton probe microanalyses in polished petrographic
Fig. 1. Tectonic sketch map of the Kola Peninsula, modified after Timmerman (1996) and Daly et al. (2006). Palaeozoic nepheline syen-ite intrusives omitted for clarity. The location of Figure 2 with position the samples is outlined.
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Hornblende apparent partial loss age spectra 319
thin sections using CHIME (Suzuki and Adachi, 1991;
Montel et al., 1996) offer a ten times higher spatial reso-
lution (Cocherie et al., 2005; Dahl et al., 2005;Dumond et al.,
2008;Kusiak and Lekki, 2008; Suzuki and Kato, 2008).For correct interpretation, in-situ microchronological data
have to be linked to detailed (micro) structural and petro-
logic data. But it is often very difficult to link detailed U
ThPb age data of accessory minerals, which typically occur
as sub-mm grains, to the evolution of assemblages of meta-
morphic minerals or to fabric-forming main phase silicates.
By linking REE patterns in zircon (e.g., Kelly and Harley
2005; Rubatto and Hermann 2007) and monazite (e.g.,
Pyle and Spear 2001;Foster et al., 2002) to growth phases
of specifically garnet but also K-Feldspar (Mahan et al.,
2006), compositional changes and correlated age informa-
tion contained in the accessory minerals can be linked to
changes in the rock-forming mineral assemblage, and thus
to the pressure-temperature the evolution.In-situ dating in
conjunction with microstructural studies enabled incre-
mental growth of monazite to be related to phases of super-
imposed deformation (e.g., Dahl et al., 2005;Dumond et
al., 2008). An often overlooked aspect when interpreting
ages of robust accessory minerals is that although Pb dif-
fusion at high temperature is extremely slow - assuring
conservation of earlier recorded age information - fluid-rock
interaction under greenschist-facies conditions may lead to
profound chemical and isotopic alteration of monazite
(Crowley and Ghent, 2000; Li et al., 2008; Bosse et al.,
2009) and zircon (Gebauer and Grnenfelder, 1976). This
is not always recognised. A regional 40Ar/39Ar study in the
Tianshan (NW China) revealed that an erroneous assump-
tion of a Triassic UPb SHRIMP age on zircon for the
UHP metamorphism must, in all likelihood, be due to syn-
exhumation fluid-mediated recrystallisation; see de Jong et
al. (2009), Li et al. (2008) and Wang et al. (2009), for dis-
cussion. In contrast to accessory mineral dating, 40Ar/39Ar
dating can be applied to a wide range of K-bearing, com-
mon rock-forming minerals, like micas, feldspars and amphib-
oles. This is a big advantage as their growth can be
straightforwardly correlated to major phases of the tectono-
metamorphic evolution of rocks.
3. 40Ar/39Ar LASER PROBE AND MICRO-SAMPLING
TECHNIQUES
The 40Ar/39Ar technique, like the KAr method from which
it is derived, is based on the natural radioactive decay of40K in 40Ca and 40Ar* (radiogenic argon). In contrast to the
KAr method, in order to be able to be dated with the 40Ar/39Ar technique samples have to be irradiated with fast neu-
trons in a nuclear reactor, which leads to the production of39Ar from the 39K isotope in the mineral. Because all target
isotopes are gases they can be directly measured with a noblegas mass spectrometer, after being extracted from the sam-
ple in a high-vacuum system. This is done in a number of
steps at increasingly higher temperature with furnace sys-
tems or a defocussed laser beam on mineral concentrates or
entire grains, or by spot dating using the laser as micro-probe. Other isotopes that are produced during irradiation
are 38Ar and 37Ar from respectively the 37Cl and 40Ca isotopes
in the samples. These can be used to obtain information on
the chemistry of the dated materials, in terms of e.g., Ca/K
and Cl/K ratios - proxies for 37ArCa/39ArK and
38ArCl/39ArK,
respectively - especially when combined with electron probe
microanalyses (EPMA). This geochemical information is
useful to identify the effect of mineral zoning, the presence
of exsolution features and/or included contaminant miner-
als or fluids on the age of the target mineral, as possible
chemically different phases degas at different temperatures.
In general degassing of a heterogeneous phase leads to
trends in 40Ar/39Ar age and Ca/K ratio spectra, either sym-
pathetically or antipathetically, as is well established for
hornblende (e.g., Berger, 1975; Berry and McDougall, 1986;
Onstott and Peacock, 1987; Kelley and Turner, 1991; Lee,
1993; Rex et al., 1993; Wartho, 1995a; Villa et al., 2000;
de Jong and Wijbrans, 2006).
The 40Ar/39Ar method is not only a powerful tool to
establish isotopic ages of events on mineral concentrates
using furnace systems; it also enables spot dating of
micrometer scale domains within minerals in thin sections
using state-of-the-art laser probe techniques. 40Ar/39Ar laser
microprobe spot fusion techniques have revealed age gra-
dients in mica (Maluski and Moni, 1988; Phillips and
Onstott, 1988; Scaillet et al., 1990; de Jong et al., 1992)
and hornblende (Kelly and Turner, 1991). Focussed ultra-
violet lasers ablate and strip thin layers from the surface of
crystals rather than melt the sample as other laser tech-
niques; data can be obtained spatially resolved to 10 m
(Kelley et al., 1994; Kelley and Wartho, 2000; Mulch et al.,
2002). A microscope-mounted drill can be used to micro-
sample minerals in polished petrographic thin sections,
while targeting parts of grains without zoning or included
phases. Such 1.5 mm diameter drilled discs can subse-
quently be analysed by step-heating with a defocused laser
(de Jong and Wijbrans, 2006).40Ar/39Ar dating applying var-ious laser techniques thus enables to relate age information
to the microstructures and petrology of rocks.
4. LAPLAND-KOLA OROGEN
The Lapland - Kola orogen (LKO) is a high-pressure
Palaeoproterozoic collisional belt located in the northern-
most part of the Fennoscandian (Baltic) Shield, one of the
best-known Precambrian regions on Earth. The orogenic
belt occurs to the North of the Karelian Craton, a classic
Neoarchaean granitegreenstone province, and stretches
for ca. 700 km from the Caledonian front in northern Nor-way to the southeast, where it is covered by the Palaeozoic
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320 Koen de Jong
sedimentary rocks of the Russian Platform (Fig. 1). Bou-
guer anomalies (Poudjom Djomani et al., 1999) indicate
that the belt continues farther to the southeast. The LKO
was formed by the amalgamation of terranes of predomi-nantly Neoarchaean age and juvenile Palaeoproterozoic
crust, including accreted island arc material (Timmerman
and Daly, 1995; Timmerman, 1996; Daly et al., 2001, 2006).
The orogenic belt formed part of a Palaeoproterozoic large-
scale tectonic system of Grenville or Himalayan dimen-
sions, which is traceable across the Atlantic Ocean to Green-
land and Labrador (Bridgwater et al., 1992).
The Neoarchaean terranes of the LKO mostly have pro-
toliths formed during a major 2.682.96 Ga old juvenile
crust-forming event (Timmerman and Daly, 1995; Tim-
merman, 1996). Soon after its creation, the juvenile Neoar-
chaean crust was the site of major crustal stretching
between 2.352.50 Ga, indicated by the initiation of rift
basins, widespread intrusion of mafic dyke swarms, man-
tle-derived large layered basicultrabasic intrusions and
gabbroanorthosite complexes, locally accompanied by
anorogenic potassic granites (Fig. 1; Balashov et al., 1993;
Mitrofanov et al., 1995; Amelin et al., 1996; Sharkov and
Smolkin, 1997; Balagansky et al., 2001; Daly et al., 2006).
Heaman (1997) observed similar rocks in various Archaean
cratons in the North Atlantic region and suggested that
they may have been part of large igneous province related
to the break-up of a Neoarchaean supercontinent. The Neoar-
chaean terranes were peneplaned in the earliest Palaeopro-
terozoic (Zagorodny, 1982; Sturt et al., 1994; Bridgwater et
al., 2001) following rift inversion accompanied by uplift,
erosion and weathering (Daly et al., 2006). Anorogenic 2.3
2.1 Ga potassic granitic sheets, ca. 2.1 Ga doleritic dyke
intrusions, and alkaline K-Na basalts imply a second exten-
sional phase (Sharkov and Smolkin, 1997; Daly et al.,
2006). This finally resulted in the opening of oceanic
basins in the 2.101.97 Ga period, represented by the
Pechenga, Imandra - Varzuga and Polmak-Pasvik volca-
nosedimentary belts that are characterised by tholeiite pil-
low lavas and other basalt flows (Melezhik and Sturt, 1994;
Fig. 1). Part of these volcanosedimentary belts formed as
back-arc basins (Melezhik and Sturt, 1994; Brewer andDaly, 1997; Sharkov and Smolkin, 1997), suggesting sub-
duction of oceanic crust, which also formed 2.121.91 Ga
juvenile island arcs, like the granulite-facies Lapland and
Umba Granulite terranes and the amphibolite-facies Tersk
Terrane (Fig. 1; Huhma and Merilinen, 1991; Timmerman,
1996; Glebovitsky et al., 2001; Daly et al., 2001, 2006).
The observed ca. 2.1 Ga K-metasomatism and migmatisa-
tion in lower crustal granulite-facies xenoliths (Kelley and
Wartho, 2000; Kempton et al., 2001) may be related to
hydrous fluids released during dehydration of subducting
oceanic lithosphere. Subsequent collision ofNeoarchaean
terranes as well as underthrusting and metamorphism ofPalaeoproterozoic juvenile material is well constrained by
1.911.93 Ga UPb zircon ages, whereas 1.901.87 Ga Sm
Nd, UPb and 40Ar/39Ar ages constrain cooling and exhu-
mation of the collision belt (Bibikova et al., 1993; Mitro-
fanov et al., 1995; Tuisku and Huhma 1998; de Jong et al.,1999; Bridgwater et al., 2001; Glebovitsky et al., 2001;
Daly et al., 2001, 2006). Ca. 1.81.7 Ga post-orogenic granite
intrusions (Fig. 1), micaceous pegmatites, lamproites and
lamprophyre dykes occur throughout the Kola Peninsula
(Mitrofanov, 1996; Daly et al., 2006). The ca. 1.76 Ga-old
composite granite plutons of the Litsa-Araguba suite and
associated porphyritic granite dykes (Vetrin et al., 2002)
stitch terrane boundaries (Figs. 1 and 2). This final Palae-
oproterozoic anorogenic-type magmatism heralds the end
of the Precambrian tectonic evolution of the LKO. Mus-
covite and biotite, when not affected by the occurrence of
excess and/or inherited argon, have 1.751.70 Ga 40Ar/39Ar
ages on a regional scale, from the northern foreland of the
LKO to large tracks of the southern footwall (Belomorian
Terrane) and the Archaean foreland of the Karelian Craton
(de Jong et al., 1999). Flat-lying, predominantly clastic
Neoproterozoic (0.91.4 Ga) sediments occur locally along
the northern and southern limits of the Kola Peninsula
(Mitrofanov, 1996; Figs. 1 and 2) and constrain the final
exhumation of the LKO.
4.1. Murmansk Terrane: geology and age constraints
The Murmansk Terrane is one of the LKOs Neoar-
chaean terranes and separated from the other terranes by
the northwest-trending subvertical Murmansk Shear Zone
(Figs. 1 and 2). Equidimensional to low-aspect ratio mag-
matic bodies define its regional structure (Fig. 2), in contrast
to the other terranes of the LKO that have a clear structural
trend. The Murmansk Terrane predominantly comprises
amphibolite-facies, leuco- to mesocratic, tonalitictrondhjemitic
granodioritic orthogneisses (TTG gneisses) and intrusives,
some migmatite and only subordinate metasedimentary
material (Mitrofanov, 1996; Timmerman, 1996). The coarse-
grained TTG gneisses have a high-grade equilibrium micro-
structure. The often ill-defined foliation comprises a grain
shape fabric chiefly of oriented feldspar and quartz. A cen-timetre to decimetre-scale gneisses layering is defined by
variations in modal amounts of plagioclase (An 2530),
microcline, quartz, (blue) green amphibole or biotite. In some
TTG gneisses the layering is crosscut by veins of gneissic
tonalite, suggesting that deformation alternated with mag-
matism. Coarse-grained amphibolites occur as concordant
layers in the TTG gneisses or as decimetre-wide veins
cross-cutting the fabric of the host rocks. The tectonic lay-
ering is generally subvertically dipping; a lineation, when
present, is steeply plunging. The degree of preferred ori-
entation of the main constituents hornblende and plagio-
clase is variable. Because green hornblende may containrelics of clinopyroxene it is probably formed by retrograde
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Hornblende apparent partial loss age spectra 321
hydration. Also Na-plagioclase and minor quartz are probably
formed by retrogression. Hornblende is generally compo-
sitionally zoned with rims that are richer in actinolite. Gar-
net occurs occasionally in plagioclase. Hornblende in both
gneisses and amphibolites is locally replaced by
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322 Koen de Jong
clase RbSr age of biotite-free amphibolite MT-27 is com-
parable to the 40Ar/39Ar age of hornblende, whereas in
biotite-bearing amphibolite MT-11 it is comparable to the40Ar/39Ar age of biotite.
5. MODELLING OF AGE SPECTRA
Generally, isotopic ages in metamorphic rocks are inter-
preted within the framework of Dodsons 1973 diffusion
theory, which states that minerals in geological systems
start to accumulate isotopes formed by radioactive decay,
like radiogenic argon (40Ar*) and strontium (87Sr), in their
crystalline lattices if the temperature is below a critical
value. Above this so-called blocking (Jger, 1967) or clo-sure (Dodson, 1973) temperature 40Ar* is regarded to be
lost from minerals by volume diffusion. Age zoning of
grains, revealed by 40Ar/39Ar laser probe spot dating, show40Ar* gradients (Phillips and Onstott, 1989; Lee et al.,
1990; Scaillet et al., 1990; Kelley and Turner, 1991; de Jong
et al., 1992; Hames and Cheney, 1997). Already before the
advent of laser probe spot dating, furnace step-heating of
mineral separates that yielded 40Ar/39Ar age spectra with
progressively rising apparent ages had been interpreted as
caused by partial argon loss by diffusion processes from
so-called lower retentive lattice sites during younger thermo-
tectonic reworking or slow cooling (Turner, 1969). Thisprocess would result in younger apparent ages during the
early stages of degassing experiments in the laboratory.
Staircase-type hornblende age spectra, like those obtained
by de Jong and Wijbrans (2006) for hornblende separate
MT-10 (Fig. 3a), have been described widely from meta-
morphic terranes (e.g., Berger, 1975; Dallmeyer, 1975; Har-
rison and McDougall, 1980; Onstott and Peacock, 1987;
Wartho, 1995a) and have been classically interpreted as
pointing to partial resetting by thermally activated 40Ar* loss
by solid-state volume diffusion during younger thermo-tec-
tonic reworking. Spectra with very old apparent ages for
the first increments, similar to those for hornblende sepa-
rate MT-11 (Fig. 3b), have been explained by limited uptake
of excess argon that was superimposed upon partial loss of
argon (Harrison and McDougall, 1980; Berry and McDou-gall, 1986; Wijbrans and McDougall, 1987). In contrast,
thermally undisturbed minerals will, according to the dif-
fusion theory, show flat spectra, not unlike the spectra of
the hornblende drilled grain and separate of MT-27 (Fig.
3c). In order to test these assumptions and to gain insight
into the reasons behind the finding of flat age patterns for
drilled hornblende grains from samples that yielded dis-
turbed age spectra for the separate (MT-11; Fig. 3b), spec-
tra were modelled using the Double Pulse Program of the
University of Toronto (McMaster, 1987) and the MacAr-
gon programme (Lister and Baldwin, 1996). Such model
calculations are based on diffusion theory and comparableto Turners 1969 model and assume thermally activated
Fig. 3. 4 0 Ar/3 9 Ar age spectra (lower panels) and Ca/K ratio spectra (upper panels) of samples MT-10 (a), MT-11 (b) and MT-27 (c) fromthe Murmansk Terrane, acquired by step-heating of separates of 1020 milligram of hornblende (ac) and 59 milligram of biotite (a,b) with a resistance-heated furnace, and with a defocused laser probe on ca. 1.5 mm diameter drilled hornblende discs (b, c). Degassingtemperatures indicated in o C; those for biotite in italics. Data and analytical procedure are given in de Jong and Wijbrans (2006). RbSr ages of hornblende-plagioclase pairs (Cliff et al., 1997) in amphibolites MT-11 (b) and MT-27 (c) and their error are indicated by theobliquely ruled bar.
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Hornblende apparent partial loss age spectra 323
40Ar* loss by solid-state volume diffusion, zero argon con-
centration at grain surfaces and uniform grain size. Diffu-
sion theory stipulates that a thermal overprinting will result
in diffusion of
40
Ar* from the less retentive sites within thecrystal. This results in lower apparent ages during the early
stages of degassing in the laboratory. Despite the acicular
habit of hornblende a spherical diffusion geometry is gen-
erally assumed for the mineral (Kelley and Turner, 1991;
Lister and Baldwin, 1996). This agrees with the quite equi-
dimensional grains that were used for step-heating and the
extensive array of parting planes at high angles to the c-
axis of the mineral in thin section.
The Double Pulse Program allows for loss and/or gain of40Ar* during two superimposed events (t1 and t2) that are
separated in time. The input parameters include the number
of steps, their size and apparent ages, as well as the ages
of the main event and two later partial resetting phases t1
and t2, each with the percentage of inflicted 40Ar* loss and/
or gain. Based on these input parameters the program opti-
mises the fit by an iterative process. The output model
curve is modified so that individual apparent ages of steps
are modelled. The modelling suggests that hornblende in
separates MT-10 and MT-11 suffered argon losses of 40
50% in one or two pulses, with less than 5% of excess Ar
incorporation for the latter sample.
The MacArgon programme enables a number of differ-
ent thermal scenarios to be modeled. In our case, the most
successful models have a thermal history that is compara-
ble to the orogenic evolution of the LKO that is fairly well
constrained. The main input parameters were: a fast Neoar-
chaean cooling, followed by a peneplanation from 2350
Ma on (section 4), which was followed by underthrusting
to mid crustal levels during the 1.951.90 Ga Lapland-
Kola orogeny that is associated with a substantial reheating
(Fig. 4). The latter event is constrained by growth of stau-
rolite and cordierite in chlorite-muscovite schists, which
represent severely retrogressed Archaean rocks of the Cen-
tral Kola Terrane (Timmerman, 1996). The age spectra of
hornblende separates of MT-10 and MT-11 were success-
fully modelled by applying a Palaeoproterozoic reheating
to 450 25 oC, which is not necessarily followed byreheating to 400 oC at 1750 Ma, associated with the post-
tectonic magmatism of the Litsa-Araguba suite (Figs. 5a
and b). Stronger or less reheating could not fit the observed
age spectra of the hornblende separates. Applying a slow
cooling model approach resulted in a good fit as well. The
successful thermo-tectonic scenarios resulted in biotite
model ages as young as observed, as the mineral is above
the closure temperature for a long time. Modelling of the
flat age spectra of MT-27 suggest a thermally undisturbed
Neoarchaean isotope system and rapid cooling. These results
would imply that neighbouring samples experienced sharply
contrasting thermal histories. A key sample is hornblendeMT-11 that yielded an apparent partial loss spectrum of
progressively increasing apparent ages for the separate, not
shown by the age spectrum of drilled grain that shows
slightly decreasing apparent ages to ca. 2.64 Ga instead (Fig.
3b). This would imply that neighbouring grains within a
single sample would have experienced sharply contrasting
thermal histories. Consequently, modelling gives conflict-
ing and unrealistic results. This implies that phenomena
other that the thermal evolution of the samples alone, lie
behind the finding of apparent thermal loss age spectra for
some separates and flat spectra for the biotite-free drilled
grains.
6. Ca/K RATIO SPECTRA
Step-heating of hornblende MT-27 yielded fairly con-
stant Ca/K ratios of 1212.5 for the separate and 1314 for
the single grain during the entire 39Ar release (Fig. 3c); val-
ues that agree with Ca/K ratio of 1314 obtained by EPMA
(de Jong and Wijbrans, 2006). In contrast, separates MT-10
and MT-11 have progressively increasing Ca/K ratio spec-
tra (Figs. 3a and b). But, the drilled hornblende grain from
the latter sample has a fairly constant Ca/K ratio of about
12 that is comparable to the ratios of 1011.5 of the sep-
arate in the 9801270 oC range (Fig. 3b). Ca/K ratios of66.5 for the main degassing of hornblende separate MT-
Fig. 4. Three synoptic thermal scenario of the evolution of theMurmansk Terrane applied to model the age spectra that were ableto constrain the age spectra of hornblende separates of MT-10 andMT-11. The three models differ only in Palaeoproterozoic reheat-ing. The thin solid black line reheating to 475 o C; thick solid lightgrey line reheating to 425 o C at 1.951.90 Ga, both followed byreheating to 400 o C at 1750 Ma, associated with the post-tectonicmagmatism; thin dashed dark grey line, reheating to 475 o C at1.951.90 Ga, without late magmatism-related reheating.
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324 Koen de Jong
10 (Fig. 3a; 9701085 oC range) are comparable to values
of 6.17.7 obtained by EPMA (de Jong and Wijbrans,
2006). This shows that the age information contained in
the spectra of drilled grains and separates for the degassingabove 970 oC refers essentially to hornblende. However,
the low Ca/K ratios of hornblende separates MT-10 and
MT-11 for gas release below 970 oC (Figs. 3a and b) point
to degassing of included K-rich phases, like the observed
tiny biotite crystals that occur intimately intergrown in the
amphibole. Also the main degassing of biotite from the
matrix of these samples took place at temperatures below
1000 oC (Figs. 3a and b). In contrast, hornblende grain MT-
11 from which biotite inclusions could be avoided by well
targeted drilling yielded spectra with fairly constant appar-
ent ages and Ca/K ratios for (Fig. 3b). This shows that such
typical partial-loss age spectra stem from the differential
degassing of hornblende and small amounts of included
biotite, and are thus the result of a binary mixing (Berger,
1975; Rex et al., 1993; Ahn and Cho, 1998; de Jong and
Wijbrans, 2006). This interpretation implies that the low
retentive hornblende sites in numerical models may in fact
be an artifact related to inclusions of earlier degassing
biotite in natural hornblende.
This interpretation similarly questions the classic inter-
pretation of flat age spectra and age plateaux as simply
reflecting a homogeneous distribution of Ar in the crystal
lattices of minerals not been affected by thermally induced
Ar loss. There is a lot of evidence that Ar release during
step-heating of hydrous minerals like amphiboles under
vacuum occurs as a consequence of chemical and struc-
tural changes within the crystals, rather than by volume dif-
fusion (Gaber et al., 1988; Lee et al., 1991; Wartho et al.,
1991; Wartho, 1995b). A major consequence of this is that39ArK and
40Ar* may be released simultaneously from cores
and rims of crystals, leading to homogenisation of age gra-
dients (Lee et al., 1990; Kelley and Turner, 1991; Lee, 1993).
7. GEOLOGICAL SIGNIFICANCE
The transformation of hornblende into biotite indicates
hydration and an increased activity of K-ions. Biotite growthis confined to grain boundaries, cleavage planes and lattice
imperfections in hornblende implying localised ingress of
an aqueous fluid. This can be understood from the obser-
vations that grain and sub grain boundaries are disorgan-
ised and rich in vacancies due to mismatch of mineral
lattices (Lomer and Nye 1952), and that grain-boundary
diffusion is generally faster than intracrystalline diffusion
(Fisher, 1977). Consequently, K would be expected to be
mobile on grain boundaries and lattice imperfections.
Under hydrous conditions these areas will hence be coated
by a fluid film and may become the site of concentrated
fluid ingress. As biotite-bearing quartz-feldspar gneissessurrounding the amphibolites are richer in K, the localised
hydration implies an open system on at least the scale of
several dm to metres. The first increment of MT-10 has an
apparent age that is comparable to the youngest biotites
from the matrix of gneisses and amphibolites of the Mur-mansk Terrane, like MT-11 (1800 6 Ma). The integrated
age of 1988 8 Ma of matrix biotite MT-10 is likely to be
due to excess and/or inherited argon as this date is far older
than the regionally occurring 1.75 1.70 Ga 40Ar/39Ar min-
imum mica ages (de Jong et al., 1999). These data suggest
that fluid ingress leading to biotite growth occurred during
the middle Palaeoproterozoic.
The apparent age of 2645 Ma of the final degassing of
hornblende separate MT-11, which is the least affected by
included younger biotite, is concordant with the apparent
age of 2650 Ma for the flat part of spectrum of biotite-free
drilled hornblende grain from this sample (Fig. 3b). These
ages are concordant with the weighted mean ages of ca.
2.64 Ga for both hornblende step heating experiments on
MT-27 (Fig. 3c). The apparent age of the final degassing of
hornblende separate MT-10 that is least affected by included
younger biotite is 2548 Ma (Fig. 3a), suggesting that horn-
blende in the tonalitic gneiss is younger than in amphib-
olites MT-11 and MT-27 that occur as bands in tonalitic
gneisses. Because amphibolite MT-11 forms a band in tonal-
itic gneiss MT-10, differences in thermal history cannot be
the reason of the observed age difference between the
hornblendes in both samples. In slowly cooled rocks, age
gradients occur at the scale of hornblende grains (Kelley
and Turner, 1991). Consequently, neighbouring grains with
different diameters can in principle yield different ages, as
Dodsons (1973) diffusion theory predicts. The typical grain
size of hornblende crystals measured in cross section per-
pendicular to c-axes in thin section is much smaller for MT-10
than MT-11, viz. 2500 m, respectively. These
two samples thus display an age-grain size relationship in
line with diffusion theory, hornblende MT-11 with the larg-
est grain size being the oldest. However, as hornblende in
MT-27 (>3500 m in cross section) is still larger but not
older than in MT-11, it is unlikely that grain size is the gov-
erning factor behind the observed difference in age between
hornblende in the two amphibolites and the tonaliticgneiss. As the amphibolites form sub-concordant, but bou-
dinaged layers in the gneisses the younger age of horn-
blende MT-10 might be due to a longer lasting tectono-
metamorphic recrystallisation of the gneisses. However,
because the last three steps for hornblende MT-10 yielded
progressively older ages and higher Ca/K ratios than the
previous steps, pointing to degassing of a more Ca-rich con-
taminant phase, like clinopyroxene (de Jong and Wijbrans,
2006), a full understand of the meaning of the 2548 Ma
age is difficult.
The discrepancy between the RbSr age of 1881 23
Ma for the hornblende-plagioclase pair of MT-11 and theNeoarchaean integrated 40Ar/39Ar age has been explained
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Hornblende apparent partial loss age spectra 325
by substantial excess argon incorporation by amphibole
(Cliff et al., 1997). However, both the drilled hornblende
grain and the mineral separate of sample MT-11 are only
marginally affected by excess argon uptake. This is revealedby excessively old ages for the first steps and by the
slightly older total gas age of the drilled grain with respect
to the grain separate (Table 1). However, the 2650 Ma appar-
ent ages of the final parts of the spectra are concordant to
weighted mean ages of ca.2.64 Ga of hornblende MT-27
(Figs. 3b and c). Excess argon is often incorporated het-
erogeneously, leading to important age differences between
neighbouring samples and even between grains in a single
sample, or parts of grains (de Jong et al., 2001), making the
interpretation by Cliff et al. (1997) unlikely. The horn-
blende-plagioclase RbSr age of 1881 23 Ma in the biotite-
bearing amphibolite is comparable to the 1988 8 Ma inte-
grated 40Ar/39Ar age of biotite (Fig. 3b) in this rock. The
ingress of an aqueous fluid enriched in K implied by the
localised growth of biotite points to recrystallisation of the
amphibolite, suggesting that plagioclase in this rock might
have been affected by recrystallisation and isotope exchange
too.
Cliff et al. (1997) calculated the two-point RbSr min-
eral-isochron for the hornblende-plagioclase pair on the
assumption that they grew and recrystallised in equilibrium
with a fluid phase in the rock, which also is the initial Sr
reservoir. Such fluid no longer being present, the whole-
rock or a Rb-poor and Sr-rich mineral is taken as a proxy
for the initial 87Sr/86Sr of the fluid, like plagioclase (Free-
man et al., 1997) or apatite (Meffan-Main et al., 2004).
Implicit in this calculation is that the analysed minerals,
viz. hornblende and plagioclase, sampled the same Sr res-
ervoir during (re)crystallisation. If this is not the case the
result is a geologically meaningless mixed age. Especially
the different abilities of the minerals used for the calcula-
tion to interact with circulating (late-stage) fluids may have
a strong influence on the age result (Andriessen, 1991).
The RbSr age of a mineral will depend on the relative
rates of loss or gain of Rb, Sr and 87Sr by a mineral during
cooling (Jenkin et al., 2001). Assuming that plagioclase in
MT-11 was above its closure temperature the mineral could(continue to) exchange strontium and attain equilibrium
with the (Palaeoproterozoic) metamorphic fluid. Biotite
growth in hornblende MT-11 and the implied K-influx
show open system behaviour, at least on the scale of sev-
eral dm to metres. As the gneisses surrounding the amphib-
olite are Neoarchaean their recrystallisation may have
released 87Sr too that was added to the fluid that penetrated
the amphibolite. If a given plagioclase was in equilibrium
with a fluid that has a raised 87Sr/86Sr ratio and above its
closure temperature for Sr exchange, 87Sr could in principle
have diffused inward along a concentration gradient (Jen-
kin et al., 1995, 2001), and so altering the isotopic signa-ture of the feldspar. Such a process would render the
assumption that plagioclase is a proxy for the initial 87Sr/86Sr
ratio invalid. Significant Sr isotope exchange could result
in a much younger age for the plagioclase-hornblende pair,
not Neoarchaean, but an age comparable to the ca.
1900Ma RbSr age of MT-11. Pettke and Diamond (1997)
described a very rare and extreme case where circulating
fluids carried radiogenic strontium leached from a base-
ment, thereby making the whole-rock point meaningless.
In contrast to hornblendes MT-10 and MT-11, biotite is
absent from the matrix of amphibolite MT-27 and from the
hornblende itself. Hornblende MT-27 seems thus not affected
by Palaeoproterozoic fluid infiltration. MT-27 yielded a
RbSr age of 2680 19 Ma for a hornblende-plagioclase pair,
which is concordant to the 2657 10 Ma integrated 40Ar/39Ar hornblende age and only marginally elevated above
the ca. 2640 Ma apparent ages of the main increments ofboth step-heating experiments on this sample. The use of
Fig. 5. Modelled age spectra of MT-10 (a) and MT-11 (b) on thebasis of the thermal scenario of Figure 4, using the MacArgonsoftware.
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Hornblende apparent partial loss age spectra 327
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Manuscript received June 4, 2009
Manuscript accepted September 9, 2009