Laser Ablation Microprobe Research Articles

Recovery of 16S ribosomal RNA gene fragments from ancient halite.

During the last decade, sensitive techniques for detecting DNA have been successfully applied to archaeological and other samples that were a few hundred to a few thousand years old. Nevertheless, there is still controversy and doubt over claims of exceptionally ancient DNA. Additional accounts stretching back nearly a century suggest that microorganisms may survive over geological time in evaporite deposits. There is, however, often doubt over the age relationship between evaporite formation and the incorporation of microorganisms. Here, we have used petrographic and geochemical techniques (laser ablation microprobe inductively coupled plasma mass spectrometry) to verify the estimated geological age of halite (NaCl) evaporite samples. Fragments of 16S ribosomal RNA genes were detected by polymerase chain reaction amplification of DNA extracted from halite samples ranging in age from 11 to 425 Myr (millions of years). Haloarchaeal 16S rDNA amplicons were present in one sample (11 16 Myr), whereas other samples (65 425 Myr) yielded only bacterial 16S rDNA amplicons. Terminal restriction fragment length polymorphism analyses indicate complex and different populations of microorganisms or their free DNA in ancient halites of different ages.

Apatite as an indicator mineral for mineral exploration: trace-element compositions and their relationship to host rock type

Over 700 apatite grains from a range of rock types have been analysed by laser-ablation microprobe ICPMS for 28 trace elements, to investigate the potential usefulness of apatite as an indicator mineral in mineral exploration. Apatites derived from different rock types have distinctive absolute and relative abundances of many trace elements (including rare-earth elements (REE), Sr, Y, Mn, Th), and chondrite-normalised trace-element patterns. The slope of chondrite-normalised REE patterns varies systematically from ultramafic through mafic/intermediate to highly fractionated granitoid rock types. (Ce/Yb) cn is very high in apatites from carbonatites and mantle-derived lherzolites (over 100 and over 200, respectively), while (Ce/Yb) cn values in apatites from granitic pegmatites are generally less than 1, reflecting both HREE enrichment and LREE depletion. Within a large suite of apatites from granitoid rocks, chemical composition is closely related to both the degree of fractionation and the oxidation state of the magma, two important parameters in determining the mineral potential of the magmatic system. Apatite can accept high levels of transition and chalcophile elements and As, making it feasible to recognise apatite associated with specific types of mineralisation. Multivariate statistical analysis has provided a user-friendly scheme to distinguish apatites from different rock types, based on contents of Sr, Y, Mn and total REE, the degree of LREE enrichment and the size of the Eu anomaly. The scheme can be used for the recognition of apatites from specific rock types or styles of mineralisation, so that the provenance of apatite grains in heavy mineral concentrates can be determined and used in geochemical exploration.

In situ measurement of Re-Os isotopes in mantle sulfides by laser ablation multicollector-inductively coupled plasma mass spectrometry: analytical methods and preliminary results

A method has been developed for the in situ determination of Re-Os isotopes in single grains of sulfides in mantle-derived peridotites using a laser ablation microprobe attached to a multicollector-induced coupled plasma mass spectrometer (MC-ICPMS). High-precision Os isotope analysis by MC-ICPMS is demonstrated by the measurement of interlaboratory Os standards. Evaluation of mass bias correction procedures shows that the exponential law provides the best fit to the Os isotope data and that the ratio of the mass fractionation coefficients for Re and Os remains constant for the range of typical instrument operating conditions. This relationship enables the accurate and precise correction of the isobaric interference of 187Re on 187Os for 187Re/ 188Os values up to 1.6. Results are presented for single sulfide inclusions in olivine macrocryts from kimberlites in the Siberian and Slave Cratons, and sulfides enclosed in silicates and interstitial to silicates in peridotite xenoliths from the Slave Craton and Massif Central, France. Enclosed sulfides larger than 50 μm in diameter and with Os contents ≥40 ppm give 187Os/ 188Os ratios with a precision of 0.1% (2 SE), which is equivalent to N-TIMS whole-rock data. Interstitial sulfides typically have lower Os (10 to 30 ppm) and give analyses with lower precision (∼1 to 2%) but still provide valuable information on the movement of Os within the lithosphere. The sulfide inclusions in silicates preserve significantly less radiogenic Os isotopic compositions than interstitial sulfides and accordingly produce significantly older and more realistic Re-Os age information. Whole-rock Os isotope compositions reflect the proportions of different generations of enclosed and interstitial sulfides; this calls into question the significance of many published “depletion ages.”

Trace element abundances in rutiles from eclogites and associated garnet mica schists

We present electron microprobe and laser ablation microprobe (LAM) data for a range of high field strength (Zr, Nb, Mo, Sn, Sb, Hf, Ta, W) and other trace elements (Al, Si, Ca, V, Cr, Mn, Fe, Pb, Th, U) in rutile from eclogites and garnet mica schists from Trescolmen, Central Alps. Most analysed rutiles are homogeneous (at least for Nb, Cr, W, Zr, V and Fe), both on a single grain scale and between grains from a single thin section. Concentrations of V, Zr, Nb, Sb and W determined by both electron and laser ablation microprobe techniques yield similar results and confirm the reliability of the analytical methods within estimated precision. Measurements of trace element contents of coexisting phases in eclogites and their modal abundances show that rutile is the dominant carrier (>90% of whole rock content) for Ti, Nb, Sb, Ta and W as well as an important carrier (5–45% of the whole rock content) for V, Cr, Mo and Sn. The crystallographic implications are that, for relatively rigid crystal sites such as in rutile, trace elements with a similar ionic radius are preferred over trace elements with the same charge but deviating size. Our results demonstrate the utility of rutile chemistry in the following applications: (1) By using a combination of the measured TiO 2 content of the whole rock and the trace element concentration of rutile, precise whole rock data on elements that are either difficult to analyze by conventional techniques such as XRF or solution ICP-MS (Nb, Sb, Ta, W) or may be susceptible to late stage alteration (Sb) can be estimated. (2) Trace element contents of detrital rutile grains are a potentially powerful tool for sedimentary provenance studies since they reflect key element ratios (e.g., Nb/TiO 2 and Cr/TiO 2) of their source rocks. In addition, measurements of trace elements in detrital rutiles might help distinguish possible source rocks, e.g., high-grade metamorphic rocks such as eclogites and high-pressure granulites from hydrothermal ore deposits and kimberlites. In view of the dominance of rutile in the Sb budget of subducting oceanic crust, and the enrichment of Sb in the slab component of subduction zones, additional experimental studies on Sb-partitioning between rutile and fluid are needed in order to understand the behaviour of Sb in subduction zones.

Excess argon evolution in HP–LT rocks: a UVLAMP study of phengite and K-free minerals, NW Turkey

Excess argon is a common problem in high-pressure metamorphic rocks. Here, we present 40Ar/ 39Ar data from metabasite, metachert and metapelite lithologies and a range of minerals. The results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high-pressure metamorphism. Samples were derived from blueschist and low-temperature eclogite facies rocks from the Tavsanli Zone of NW Turkey. Samples were analysed using the 40Ar/ 39Ar Ultra-Violet Laser Ablation Microprobe (UVLAMP) and yield ages ranging from 72±3 to 154±7 Ma. Detailed intra-grain profiling of phengites in situ, and analyses of sodic-amphibole, lawsonite, quartz and garnet, present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples, revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite. Whole rock XRF data indicates a scattered relationship between bulk K 2O content of the rock and the concentration of excess argon in phengite. We conclude that excess argon was derived in situ in a relatively closed, fluid-absent system, rather than introduced by fluids.

Ar and K partitioning between clinopyroxene and silicate melt to 8 GPa

The relative incompatibility of Ar and K are fundamental parameters in understanding the degassing history of the mantle. Clinopyroxene is the main host for K in most of the upper mantle, playing an important role in controlling the K/Ar ratio of residual mantle and the subsequent time-integrated evolution of 40Ar/36Ar ratios. Clinopyroxene also contributes to the bulk Ar partition coefficient that controls the Ar degassing rate during mantle melting. The partitioning of Ar and K between clinopyroxene and quenched silicate melt has been experimentally determined from 1 to 8 GPa for the bulk compositions Ab80Di20 (80 mol% albite-20 mol% diopside) and Ab20Di80 with an ultraviolet laser ablation microprobe (UVLAMP) technique for Ar analysis and the ion microprobe for K. Data for Kr (UVLAMP) and Rb (ion probe) have also been determined to evaluate the role of crystal lattice sites in controlling partitioning. By excluding crystal analyses that show evidence of glass contamination, we find relatively constant Ar partition coefficients (DAr) of 2.6 × 10−4 to 3.9 × 10−4 for the Ab80Di20 system at pressures from 2 to 8 GPa. In the Ab20Di80 system, DAr shows similar low values of 7.0 × 10−5 and 3.0 × 10−4 at 1 to 3 GPa. All these values are several orders of magnitude lower than previous measurements on separated crystal-glass pairs.DK is 10 to 50 times greater than DRb for all experiments, and both elements follow parallel trends with increasing pressure, although these trends are significantly different in each system studied. The DK values for clinopyroxene are at least an order of magnitude greater than DAr under all conditions investigated here, but DAr appears to show more consistent behavior between the two systems than K or Rb. The partitioning behavior of K and Rb can be explained in terms of combined pressure, temperature, and crystal chemistry effects that result in changes for the size of the clinopyroxene M2 site. In the Ab20Di80 system, where clinopyroxene is diopside rich at all pressures, DK and DRb increase with pressure (and temperature) in an analogous fashion to the well-documented behavior of Na. For the Ab80Di20 system, the jadeite content of the clinopyroxene increases from 22 to 75 mol% with pressure resulting in a contraction of the M2 site. This has the effect of discriminating against the large K+ and Rb+ ions, thereby countering the effect of increasing pressure. As a consequence DK and DRb do not increase with pressure in this system.In contrast to the alkalis (Na, K, and Rb), DKr values are similar to DAr despite a large difference in atomic radius. This lack of discrimination (and the constant DAr over a range of crystal compositions) is also consistent with incorporation of these heavier noble gases at crystal lattice sites and a predicted consequence of their neutrality or “zero charge.” Combined with published DAr values for olivine, our results confirm that magma generation is an efficient mechanism for the removal of Ar from the uppermost 200 km of the mantle, and that K/Ar ratios in the residuum are controlled by the amount of clinopyroxene. Generally, Ar is more compatible than K during mantle melting because DAr for olivine is similar to DK for clinopyroxene. As a result, residual mantle that has experienced variable amounts of melt extraction may show considerable variability in time-integrated 36Ar/40Ar.

Authigenic Apatite in a Fluvial Sandstone Sequence: Evidence for Rare-Earth Element Mobility During Diagenesis and a Tool for Diagenetic Correlation

ABSTRACT Apatite grains in heavy-mineral separates of samples from the Statfjord Formation (Triassic-Lower Jurassic, North Sea), exhibit variably corroded detrital cores and euhedral diagenetic overgrowths. In the overlying marine deposits of the Nansen Formation, apatite grains show no evidence of either corrosion or overgrowths. Compositions of detrital apatite in both formations are indistinguishable, indicating that the source areas responsible for Statfjord and Nansen deposition included similar apatite-bearing lithologies. Apatite corrosion in the Statfjord Formation is considered to be a response to leaching by acidic groundwaters during sediment storage on the fluvial floodplain. Any variations in the degree of corrosion that may have been generated by differences in floodplain residence time have been homogenized by sediment reworking. Laser-ablation microprobe inductively coupled plasma mass spectrometry and wavelength-dispersive electron microprobe analysis indicate that the authigenic apatite contains high abundances (up to 3.5% total) of the rare earth elements (REE), implying a degree of REE mobilization during diagenesis. REE profiles (NASC normalized) for the authigenic apatites are convex-upwards with enrichment of the MREE. The REE are considered to have been derived from dissolution of other REE-bearing heavy minerals (e.g., garnet) during alluvial storage, with possible contributions from dissolved feldspars (which may account for slight positive Eu anomalies) and REEs adsorbed onto clay surfaces. REE mobilization was probably enhanced by fluoride and/or phosphate complexes, although organic complexing related to pedogenic processes is also a possibility. Through the Statfjord Formation, authigenic apatite displays subtle variations in major-element, minor-element, and trace-element chemistry, which may find application as a tool for diagenetic correlation.

Rayleigh fractionation of heavy rare earths and yttrium during metamorphic garnet growth

We present laser-ablation microprobe inductively coupled–mass spectrometry analyses of Y, Yb, Er, Dy, and Gd in garnet crystals from high-grade metamorphic and migmatitic rocks from the Sierras Pampeanas of Argentina. These rocks contain large garnet porphyroblasts (≥4 mm) that are compositionally zoned. Rare earth element concentrations in the rims are typically one order of magnitude lower than those in the cores. A notable feature of this zonation is the inversion of Yb/Er and Yb/Dy ratios, from >1 in the cores to <1 in the rims. We show quantitatively that the spatial distribution of these trace elements in garnet can be most simply and effectively explained as arising from Rayleigh fractionation during garnet growth. We also analyzed a small garnet crystal (<2 mm) representative of a garnet population that is associated with migmatitic leucosomes. This crystal displays uniformly low concentrations of all trace elements that are virtually identical in both absolute and relative magnitudes to those in the rims of the larger garnet porphyroblasts. The small crystals, and perhaps part of the porphyroblast rims, are likely to be peritectic products of incongruent melting reactions. We argue that melts formed from garnet-bearing sources may generally be unable to equilibrate with metamorphic garnet cores and may thus be more strongly depleted in heavy rare earth elements and Y than modeling based on bulk source-rock abundances of these elements would indicate.

Partial melting in Archean subduction zones: constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonalitic melts under upper mantle conditions

The partitioning of an extensive suite of trace elements between garnet, clinopyroxene, and hydrous tonalitic melts has been studied experimentally at 1.8 GPa and 1000–1040 °C. The knowledge of trace element partitioning between minerals and coexisting tonalitic melts is essential for geochemical models in order to explain the genesis of Archean tonalite-trondhjemite-granodiorite suites (TTGs). Our experiments were performed on a natural tonalite doped with two different trace element mixtures and with TiO 2 to stabilize titanates. Trace element concentrations of crystals and coexisting melts were measured by laser ablation microprobe. The partition coefficients ( D's) are independent of the trace element concentration over the concentration ranges used. The partition coefficients reported in this paper are a considerable extension of the few available data sets in intermediate to silicic systems. The D-values mostly agree well with previously published data and with predictive models. The garnet partition coefficients show steeper patterns with higher D's for heavy REE, lower D's for highly incompatible elements and lower La/Yb-ratios than previously published data for basaltic systems, whereas clinopyroxene partition coefficients show similar patterns but higher values than for basaltic systems. A set of calculated partition coefficients based on best-fit parabolas was used to model trace element fractionation in melts in equilibrium with rutile-bearing eclogitic residues. The calculated melts reproduce the overall trace element patterns of Archean TTGs, i.e. the melts show strongly fractionated REE patterns, depletions in Nb and heavy REE, and high Zr and Hf concentrations relative to Sm on normalized plots. The calculated residues have depleted trace element patterns with positive Nb anomalies and flat heavy rare earth element patterns similar to rutile-bearing xenolithic eclogites reported from Siberia and West Africa. Restites from eclogite melting are complementary to Archean TTGs and may be an important reservoir of Nb and Ta.

Experimentally determined partition coefficients for minor and trace elements in peridotite minerals and carbonatitic melt, and their relevance to natural carbonatites

A laser ablation microprobe coupled to an ICP-MS was used to analyse concentrations of K, Rb, Cs, Sr, Ba, Ti, Zr, Hf, Nb, La, Ce, Sm, Ho, Yb and Lu in experimentally produced garnet, clinopyroxene, amphibole and magnesiocarbonatitic melt. The experiment was conducted at 1050 °C and 2.5 GPa on a synthetic composition designed to simulate natural peridotite melting. Crystal/melt partition coefficients determined from the experiment were used to calculate minor and trace element concentrations in a hypothetical carbonatitic melt equilibrated with a garnet-bearing amphibole lherzolite of primitive mantle composition. The melt is strongly enriched in alkaline earths and rare earths relative to high-field-strength elements and alkalis, and plots within the compositional range of natural carbonatites. However, the fractionation of alkaline earths and REE from alkalis and HFSE is usually larger in natural carbonatites than in the calculated composition. The absolute concentrations of many incompatible elements are also much larger in some natural carbonatites. Because of these features, processes in addition to peridotite melting (such as crystal fractionation, wall rock reactions, and variations in source chemistry) are needed to fully account for the minor and trace element characteristics of carbonatites.

Laser-ablation microprobe (LAM)-ICPMS unravels the highly siderophile element geochemistry of the oceanic mantle

The highly siderophile element (HSE) contents of base-metal sulphides have been determined by laser-ablation microprobe (LAM)-ICPMS in abyssal peridotites from the Mid-Atlantic and South West Indian ridges. (Pd/Ir)N (0.007–505, N: CI-chondrite-normalised), (Pt/Ir)N (0.001–0.77) and (Rh/Ir)N (0.159–273) vary significantly between both grains and samples, irrespective of indicators of melt removal, but in line with bulk-rock platinum-group element (PGE) ratios and sulphide modal abundances. Positive deviations of PGE abundance ratios in whole-rock analyses are due to late-precipitated Cu–Ni-rich magmatic sulphides from incompletely extracted partial melts. These results contradict explanations of the HSE systematics of the oceanic mantle as reflecting global scale processes such as core–mantle exchange.

Age and provenance of early Precambrian metasedimentary rocks in the Lapland–Kola Belt, Russia: evidence from Pb and Nd isotopic data

207Pb/206Pb ages are presented for detrital zircons (Laser Ablation Microprobe ICP‐MS) and whole‐rock Nd isotopic determinations (TIMS) from samples of Neoarchean and Palaeoproterozoic metasedimentary rocks from the Umba granulite terrane and the Keivy domain of the Central Kola composite terrane, Kola Peninsula, north‐western Russia. Three are samples of rocks from the Umba granulite terrane that were deposited ≈ 2.20–1.90 Ga; they contain Archaean detritus, much of it older than 3.0 Gyr, as well as abundant 2.20–1.95‐Gyr‐old material. Deposition may have occurred on the margin of an Archaean craton with an exposed Palaeoproterozoic magmatic arc source, possibly during orogenesis. Two samples from the Keivy domain have remarkably similar, dominantly Archaean detrital zircon age spectra. One was deposited pre‐2.4 Ga, whereas the other was probably deposited post‐2.01 Ga. Both had similar sources, compatible with the proximal country rocks, and possible shallow‐water (?) cratonic margin depositional settings.

Trace element partitioning between coexisting biotite and muscovite from metamorphic rocks, Western Labrador: structural, compositional and thermal controls

Coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western Labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to ICP-MS (LAM-ICP-MS) for selected trace elements—Li, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf and Ta. The samples have experienced a single prograde Grenvillian metamorphism ranging from 490 to 680°C and from 7 to 12 kbar. The trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure, major element composition and temperature on the partitioning of each element between biotite and muscovite. Overall, trace element distributions are systematic across the range of metamorphic grade and bulk composition, suggesting that chemical equilibrium was approached. Most distribution coefficients (biotite/muscovite) show good agreement with published data. However, distribution coefficients for Co and Sr are significantly different from previous determinations, probably because of contamination associated with older data obtained by bulk analysis techniques. The sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas. In particular, distribution coefficients exhibit the sequence Cr 3+ (0.615 Å) > V 3+ (0.64 Å) > Sc 3+ (0.745 Å) in VI-sites, and Ba 2+ (1.61 Å) > Sr 2+ (1.44 Å) and Cs + (1.88 Å) > K + (1.64 Å) > Rb + (1.72 Å) > Na + (1.39 Å) in XII-sites. The distributions of Li, Sc, Sr and Ba appear to be thermally sensitive but are also controlled by major element compositions of micas. V and Zr partitioning is dependent on T and may be used to cross-check thermometry calculations where the latter suffer from retrograde re-equilibration and/or high concentrations of Fe 3+. The ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling.

Experimental approach to constrain second critical end points in fluid/silicate systems: Near-solidus fluids and melts in the system albite-H2O

Experimental investigations in the system albite-water have been carried out to evaluate the accuracy of a new procedure to determine major-element solubilities in fluid phases and water solubilities in melts. The system albite-water had been examined previously and some consensus has been achieved with respect to phase relations, solidi, and water solubilities in the melts. Above 15 kbar the system approaches a second critical end point, where a distinction between fluid and melt no longer can be made and the term solidus has to be reconsidered. All experimental runs were carried out at near-solidus conditions at 5–17 kbar and 625–775 °C. Experimental charges contained albite, water, and a layer of diamond crystals (grain size 50 μm). The pore space between the diamonds is preserved during the run indicating that the fluid was able to circulate throughout the entire capsule. During quenching, the material dissolved in the fluid precipitated between the diamond crystals and, as a result, could so be separated from the solid residue. The recovered capsules were directly embedded in epoxy and the diamond layers were analysed by laser ablation microprobe (LAM-ICP-MS). The new method allows the determination of a broad range of silicate/water ratios in aqueous fluids and hydrous melts. In some experimental runs at 15 and 17 kbar, fluids with approximately 40 wt% solute could be trapped leading to the conclusion that supercritical conditions were reached. Results furthermore indicate that albite dissolves non-stoichiometrically at 5 kbar producing a higher Na/Al and Si/Al in the solute than in the melt. Dissolution and melting at 10 kbar and higher pressures appears to be congruent within the limits of this method.

The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites

Zircon megacrysts represent a late stage in the crystallisation of the magmas that produced the low-Cr megacryst suite (Ol+Opx+Cpx+Gnt+Ilm+Phl+Zir) found in many kimberlites, and may carry information on the sources of the parent magmas and the interaction of these magmas with the cratonic lithosphere. The isotopic composition of Hf has been measured in 124 mantle-derived zircon megacrysts from African, Siberian and Australian kimberlites, using a laser-ablation microprobe (LAM) and a multi-collector (MC) ICPMS. The zircons range in age from 90 Ma to ca 2500 Ma, allowing indirect analysis of mantle-derived Hf over a long time span. Most values of ε Hf fall between 0 and +10, but zircon suites from several kimberlites range down to ε Hf = −16. Combined with published Nd data on the silicate members of the low-Cr megacryst suite, these data indicate crystallisation of zircon from magmas lying well below the terrestrial ε Hf-ε Nd array. LAM-ICPMS analyses of garnets and clinopyroxenes from mantle-derived peridotite xenoliths suggest that cratonic lithospheric mantle has Hf/Nd (0.3–0.5) greater than estimated Bulk Silicate Earth. The depleted and metasomatised lherzolites and harzburgites that make up much of the Archean lithospheric mantle have Lu/Hf ratios (≤0.15) low enough to account for the lowest ε Hf observed in the zircons, over time spans of 1–3.5 Ga. We therefore suggest that the magmas from which the kimberlitic zircons crystallised were derived from Depleted Mantle or OIB-type sources, and developed negative ε Hf through reaction with the subcontinental lithospheric mantle.

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Direct measurement of Ar diffusion profiles in a gem-quality Madagascar K-feldspar using the ultra-violet laser ablation microprobe (UVLAMP)

Controversy surrounding the mechanisms and controls on argon diffusion in K-feldspar has led us to undertake direct diffusion measurements on a crystal with simple microtextures, over a range of temperatures. Measurements of argon diffusion profiles in a gem-quality iron-rich orthoclase heated in a cold seal apparatus, have been undertaken in situ using an ultra-violet laser ablation microprobe (UVLAMP) technique. The results agree very closely with the previously determined bulk values for Benson Mines orthoclase (activation energy ( E)=43.8±1 kcal mol −1) and vacuum furnace cycle-heating studies of K-feldspars ( E=46±6 kcal mol −1). However, instead of defining a single activation energy ( E) and diffusion coefficient ( D o), the data yield two sets of parameters: a low-temperature (550–720°C) array with an E of 47.2±2.5 kcal mol −1 (198.2±10.5 kJ mol −1) and a D o of 0.0374 +0.1123 −0.0281 cm 2 s −1, and a high-temperature (725–1019°C) array with an E of 63.8±3.4 kcal mol −1 (268.0±14.3 kJ mol −1) and a D o of 55.0 +225.5 −44.2 cm 2 s −1. The new results closely reproduce two sets of apparent activation energies previously measured in cycle-heating studies of Madagascar K-feldspar (40±3 and 57±3 kcal mol −1). Previous interpretations of the two arrays have included multiple domains with variable activation energies and fast track diffusion. However, the UV depth profile analyses indicate simple diffusion to the grain surface and importantly, diffusion radii calculated by combining the UVLAMP and cycle-heating data, are the same as the physical grain sizes used in the experiments, around 1 mm. Vacuum furnace stepped heating experiments on slowly cooled K-feldspars have been interpreted as showing diffusion radii of around 6 μm and indicate complex populations of sub-grains. This study indicates that Madagascar K-feldspar and thus probably all gem-quality K-feldspars act as single diffusion domains and that short-circuit (or pipe) diffusion was not an important loss mechanism. An apparent diffusion compensation relationship in the stepped heating data for Madagascar K-feldspar implies that similar relationships seen in other K-feldspars are a result of a range of diffusion mechanisms.

The non-destructive determination of REE in fossilized bone using synchrotron radiation induced K-line X-ray microfluorescence analysis

The sensitivity and applicability of the synchrotron radiation induced X-ray microfluorescence (μ-SRXRF) spectrometer at the Hamburg synchrotron laboratory Hasylab for the determination of the distribution of trace concentrations of rare-earth elements (REE) in fossilized bone are discussed and critically compared to those of other trace analytical methods such as instrumental neutron activation analysis (INAA) and LAMP-ICPMS (laser ablation microprobe inductively-coupled plasma mass spectrometry). Measurements were carried out on two bone samples from contrasting terrestrial depositional environments at Olduvai Gorge (Tanzania). Results indicate that the microdistribution of the REE in these biological materials is not homogeneous and that the relative abundance of these elements can provide information on the palaeoenvironment during the fossilization process. The heterogeneous distribution of the REE can be determined in a quantitative and completely non-destructive manner provided the concentrations of individual REE are above 10 μg/g.

Cryptic metasomatism in the upper mantle beneath Southeastern Austria: A laser ablation microprobe-ICP-MS study

Upper mantle xenoliths from the classical location, Kapfenstein, Styria, as well as from Fehring, Styria, and Tobaj, Burgenland, have been analyzed by laser ablation microprobe inductively-coupled plasma mass spectrometry (LAM-ICP-MS). At all locations spinel lherzolite is the predominant xenolith type and thus our sample contains nine spinel lherzolites and only one harzburgite (from the richest location, Kapfenstein, sample Ka 167). All the rocks have protogranular to protogranular — porphyroclastic transitional textures. Mineral compositions are typical for fertile upper mantle rocks with 0.89 〈mg〉 0.92 for silicates and 0.10 〈cr〉 0.12 for spinel (0.18 for the harzburgite). The minerals are equilibrated with respect to major, minor and trace elements, except for clinopyroxenes in the sample from Tobaj (To 100) and one sample from Fehring (Feh 002) which have variable incompatible trace element contents.

U–Pb geochronology of the eastern Hall Peninsula, southern Baffin Island, Canada: a northern link between the Archean of West Greenland and the Paleoproterozoic Torngat Orogen of northern Labrador

A corridor east of Iqaluit that crosses several regionally significant lithologic boundaries has been mapped and sampled for geochronology. Zircon, monazite and titanite were analysed by the isotope dilution U–Pb method to characterize igneous and metamorphic events, and Pb–Pb ages of detrital zircons were determined by laser ablation microprobe ICP-MS. Tonalitic gneisses at the eastern end of the corridor are Archean, with emplacement ages of 2920+8/−6, 2848±3, 2844+6/−5 and 2797+27/−15 Ma; these samples record variable tectonothermal overprints at ca 2.77 Ga, and between 1844 and 1736 Ma, and may be related to similar rocks in West Greenland. A sequence of quartzites and semipelites associated with the Archean gneisses contains exclusively Archean detrital zircons, and was metamorphosed at 1.88 Ga. In the central part of the corridor, semipelitic and psammitic rocks contain almost exclusively Paleoproterozoic detritus deposited after 1.93±0.05 Ga, and are interpreted as the northern continuation of the Tasiuyak gneiss of the Torngat Orogen. At the western end of the corridor, monzogranites of the Cumberland batholith intruded at 1869+9/−3, 1857+5/−3 and 1850+3/−2 Ma. The present data suggest that the north–south Nagssugtoqidian collision (<1.89 Ga) in West Greenland may predate the east–west Torngat collision (<1.87 Ga) in northern Labrador.