Carsten Münker

Rapid accretion and early core formation on asteroids and the terrestrial planets from Hf-W chronometry

The timescales and mechanisms for the formation and chemical differentiation of the planets can be quantified using the radioactive decay of short-lived isotopes. Of these, the 182Hf-to-182W decay is ideally suited for dating core formation in planetary bodies. In an earlier study, the W isotope composition of the Earths mantle was used to infer that core formation was late (≥60 million years after the beginning of the Solar System) and that accretion was a protracted process. The correct interpretation of Hf–W data depends, however, on accurate knowledge of the initial abundance of 182Hf in the Solar System and the W isotope composition of chondritic meteorites. Here we report Hf–W data for carbonaceous and H chondrite meteorites that lead to timescales of accretion and core formation significantly different from those calculated previously. The revised ages for Vesta, Mars and Earth indicate rapid accretion, and show that the timescale for core formation decreases with decreasing size of the planet. We conclude that core formation in the terrestrial planets and the formation of the Moon must have occurred during the first ∼30 million years of the life of the Solar System.

Stable isotope compositions of cadmium in geological materials and meteorites determined by multiple-collector ICPMS

A new technique for the precise and accurate determination of Cd stable isotope compositions has been developed and applied to geological materials and meteorites. The Cd isotope analyses are performed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) using external normalization to Ag for mass bias correction. The accuracy of the new procedure was ascertained by the comparison of data for meteorites with published results acquired by thermal ionization mass spectrometry and double spiking. Some results were also confirmed by measurements using external normalization to Sb on a different MC-ICPMS instrument. A long-term reproducibility of ± 1.1 eCd/amu (2 sd) was obtained for separate dissolutions and multiple analyses of several rock and meteorite samples (eCd/amu represents the deviation of a Cd isotope ratio of a sample relative to the JMC Cd standard in parts per 104, normalized to a mass difference of 1 amu). As little as 5–20 ng of Cd are sufficient for the acquisition of precise and accurate data. Terrestrial rock and mineral samples display little variations in Cd isotope compositions (eCd/amu between −1 and +1.2), except for a tektite sample that was found to be enriched in the heavy Cd isotopes by +7.6 eCd/amu. The carbonaceous chondrites Orgueil, Murchison and Allende have Cd isotope ratios that are unfractionated relative to the JMC Cd standard and terrestrial rocks. The ordinary chondrites analyzed in this study and a Rumuruti chondrite display Cd isotope fractionations, ranging from −19 to +36 eCd/amu. These results suggest that substantial (inorganic) natural Cd isotope fractionations are generated only by evaporation and/or condensation processes. The lack of resolvable Cd isotope variations between the different carbonaceous chondrites, despite large differences in Cd concentrations, implies that the primary depletion of Cd in the early solar system did not involve Rayleigh evaporation. The Cd isotope fractionation in ordinary and Rumuruti chondrites is probably due to the redistribution of Cd by evaporation and condensation processes during thermal metamorphism on the parent bodies. Models that explain the enrichments of highly volatile elements in unequilibrated ordinary chondrites by primary equilibrium condensation appear to be inconsistent with the Cd isotope data.

Melt percolation monitored by Os isotopes and HSE abundances: a case study from the mantle section of the Troodos Ophiolite

Combined siderophile and lithophile element systematics in mantle rocks can be used to monitor melt percolation processes in the Earth’s mantle. Here we present a coherent dataset from a single melt channel from the mantle section of the Troodos Ophiolite Complex on Cyprus. The melt channel is composed of a dunite vein that is surrounded by harzburgite. Dunite and harzburgite both have refractory Cr-spinel (Cr/(Cr+Al) of 0.58–0.60). Likewise, clinopyroxenes in both the dunites and harzburgites have strongly depleted REE patterns with (Gd/Yb)N values varying from 0.03 to 0.07. Such consistent lithophile element patterns suggest that the harzburgite and dunite interacted with the same melt during the melt percolation process. The distribution of the highly siderophile elements (HSEs) (Os, Ir, Ru, Pt, Pd and Re) in the melt channel cannot be explained by conventional partial melting models, but can be explained by melt-peridotite reaction. The harzburgites have slightly suprachondritic Os isotope ratios (187Os/188Ost=90 Ma=0.1288–0.1311) compared to the 187Os/188Ost=90 Ma of the carbonaceous chondrite reference (0.1264), and their HSE concentrations overlap with the range observed for lherzolites and harzburgites world-wide. In contrast, the dunites are significantly enriched in 187Os (187Os/188Os90 Ma=0.1335–0.1374), like volcanic rocks from island arcs world-wide. HSE patterns in the dunites are also typical for mantle melts, in that they are enriched in Pd, Pt and Re relative to Ir, Os and Ru, which are lower than in the primitive mantle. Hence, the harzburgites and dunites have complementary HSE concentrations and ratios. In addition, HSE ratios such as Ir/Os, Re/Os, systematically increase from the harzburgite towards the dunite ((Ir/Os)N: 0.36–1.8; (Re/Os)N: 0.14–9.5). This implies that Ir, Os and Ru behave incompatibly and become fractionated from each other during the melt percolation process. These features are interpreted to reflect the progressive reaction of a mantle melt with spinel–lherzolite to form harzburgite and eventually dunite. We suggest that an upper mantle peridotite was infiltrated by a radiogenic mantle melt typical for subduction-related volcanism. At low melt/rock ratios a harzburgite residue is left behind and its HSE distribution and the REE pattern of cpx can be explained by open-system melting if one assumes the HSEs to behave incompatibly. Continued melt percolation eventually produces dunites, and all mantle sulfides are removed from the peridotite. Thus, the sulfides and the HSE distribution in the dunites are not of residual origin but are dominated by sulfides that segregated from a sulfide-saturated melt with a radiogenic Os signature. The HSE variation in harzburgites and dunites from the melt channel can be interpreted as a mixing line that has HSE-bearing sulfides from the melt and from the residual mantle as end members. We conclude that HSEs become significantly mobilized and fractionated during melt percolation processes, thus providing useful proxies for melting and enrichment processes in the Earth’s mantle.

Determination of ultra-low Nb, Ta, Zr and Hf concentrations and the chondritic Zr/Hf and Nb/Ta ratios by isotope dilution analyses with multiple collector ICP-MS

This study presents a new technique for the determination of precise and accurate concentrations of the high field strength elements (HFSE) Zr, Hf, Nb and Ta. The Ta concentration was determined for the first time by the isotope dilution (ID) technique using an isotopic tracer enriched in 180Ta. Zirconium and hafnium concentrations were also determined by ID, whereas the concentration of the mono-isotopic Nb was measured relative to Zr, after quantitative separation of the HFSE from the matrix. The analyses were performed on a Micromass Isoprobe multiple collector (MC) inductively coupled plasma source mass spectrometer (ICP-MS). Only about 0.5 ng of Zr, Hf and Ta are necessary to perform an ID analysis with an external reproducibility of better than 1% on the MC-ICP-MS using Faraday collectors. This new technique enables the precise and accurate determination of the HFSE concentrations even in ultra-depleted rocks like peridotites. The absolute uncertainties for ultra-depleted rocks, particular for Ta concentrations at the sub-ng level are limited by blanks and sample heterogeneities and not by the precision of the measurement. New and more precise Zr, Hf, Nb and Ta concentration data for the geological standard reference materials BHVO-2, BCR-2, BE-N, BIR-1 and the ultra-depleted standards PCC-1 and DTS-1 are presented. External reproducibilities of the concentration measurements are 0.4–5% for basalts and 2–10% for depleted peridotite samples (2 RSD), depending on element and concentration. The Zr/Hf and Nb/Ta ratio of the solar system was determined based on new data for two chondrites and six achondrites. The chondritic Nb/Ta of 17.6±1.0 determined in this study agrees with previous predicted values from the literature. However, the chondritic Zr/Hf of 34.2±0.3 determined in this study differs from previous literature values.

Nb/Ta fractionation in a Cambrian arc/back arc system, New Zealand : source constraints and application of refined ICPMS techniques

Abstract This study presents evidence for systematic Nb/Ta variations in an Cambrian island arc system of the Early Paleozoic Takaka Terrane, New Zealand. These volcanic rocks comprise an unusual variety of compositions ranging from back arc basin basalts through boninites to calc-alkaline basalts and andesites. They have been analyzed by ICPMS and display Nb contents between 0.15 and 30 ppm and Ta contents between 0.02 and 2 ppm. Nb/Ta ratios vary from 8 to 25. To monitor accuracy of the analytical procedure, ICPMS results for Nb and Ta in 20 geostandards are presented. Prior to analysis, HCl was added to the sample solutions and measurements were run within 24 h of dissolution to avoid precipitation of both elements. The constancy of Nb/Ta in samples of single volcanic suites at different H2O and CO2 contents and at different Mg# and Cr contents shows, that neither alteration processes nor crystal fractionation affected the primary Nb/Ta ratio of the samples. Nb/Ta in relatively primitive samples (Mg# 50–75) is positively correlated with Nb, Ta, La/Yb and Th/Yb below the chondritic Nb/Ta ratio (Low Nb/Ta trend). An inverse correlation is observed at higher ratios (High Nb/Ta trend). No correlations exist with ratios of moderately incompatible elements such as Ti/Zr and Zr/Hf, suggesting the Nb/Ta ratios not to be controlled by the depletion of the mantle wedge. Conversely, these systematics strongly suggest an important role of a residual mineral phase, most likely rutile, in mineral/melt (high Nb/Ta trend) and mineral/fluid systems (low Nb/Ta trend), respectively. The overall Nb–Ta depletion in the magmas, however, is best explained by extremely low partitioning of HFSE into slab equilibrated fluids as suggested by recent experimental work. The low Nb/Ta ratios in rocks that are extremely depleted in incompatible elements are due to rutile/fluid interaction. High Nb/Ta ratios in the medium- to high-K volcanic rocks reflect the interaction between rutile and siliceous magma produced by low degree melting of the slab. The subducting slab can be demonstrated here as the most likely locality for both Nb–Ta depletion and Nb/Ta fractionation, implying a small scale transfer of Nb and Ta from the slab to the mantle wedge by melts and fluids.

Nb/Ta, Zr/Hf and REE in the depleted mantle: implications for the differentiation history of the crust-mantle system

Abstract High-precision Nb, Ta, Zr, Hf, Sm, Nd and Lu concentration data of depleted mantle rocks from the Balmuccia peridotite complex (Ivrea Zone, Italian Alps) were determined by isotope dilution using multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) and thermal ionisation mass spectrometry (TIMS). The Zr/Hf ratios of all investigated samples from the Balmuccia peridotite complex are significantly lower than the chondritic value of 34.2, and the most depleted samples have Zr/Hf ratios as low as 10. Correlated Zr/Hf ratios and Zr abundances of the lherzolites preserve the trend of a mantle residue that has been depleted by fractional melting. This trend confirms experimental studies that predict Hf to behave more compatibly than Zr during mantle melting. Experimentally determined partition coefficients imply that the major Zr and Hf depletion most likely occurred in the spinel stability field, with (DZr/DHf)cpx≈0.5, and not in the garnet stability field, where (DZr/DHf)grt is probably close to one. However, minor amounts of melting must have also occurred in a garnet facies mantle, as indicated by low Sm/Lu ratios in the Balmuccia peridotites. The Nb/Ta ratios of most lherzolites are subchondritic and vary only from 7 to 10, with the exception of three samples that have higher Nb/Ta ratios (18–24). The overall low Nb/Ta ratios of most depleted mantle rocks confirm a higher compatibility of Ta in the mantle. The uniform Nb/Ta ratios in most samples imply that even in ‘depleted’ mantle domains the budget of the highly incompatible Nb and Ta is controlled by enrichment processes. Such a model is supported by the positive correlation of Zr/Nb with the Zr concentration. However, the overall enrichment was weak and did barely affect the moderately incompatible elements Zr and Hf. The new constraints from the partitioning behaviour of Zr–Hf and Nb–Ta provide important insights into processes that formed the Earth’s major silicate reservoirs. The correlation of Zr/Hf and Sm/Nd in depleted MORB can be assigned to previous melting events in the MORB source. However, such trends were unlikely produced during continental crust formation processes, where Sm/Nd and Zr/Hf are decoupled. The different fractionation behaviour of Zr/Hf and Sm/Nd in the depleted mantle (correlated) and the crust (decoupled) indicates that crustal growth by a simple partial melting process in the mantle has little effect on the mass budget of LREE and HFSE between crust and mantle. A more complex source composition, similar to that of modern subduction rocks, is needed to fractionate the LREE, but not Zr/Hf and the HREE.

Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central orogenic belt, North China craton

The 2.55–2.50 Ga Zunhua and Wutaishan belts within the central orogenic belt of the North China craton contain variably metamorphosed and deformed tectonic blocks of peridotites and amphibolites that occur in a sheared metasedimentary matrix. In the Zunhua belt, dunites comprise podiform chromitites with high and uniform Cr-numbers (88). Peridotites and associated picritic amphibolites are characterized by light rare earth element (LREE)–enriched patterns and negative high field strength element (HFSE: Nb, Zr, and Ti) anomalies. They have positive initial ϵ Hf values (+7.9 to +10.4), which are consistent with an extremely depleted mantle composition. Mass-balance calculations indicate that the composition of the 2.55 Ga mantle beneath the Zunhua belt was enriched in SiO 2 and FeO T compared to modern abyssal peridotites. These geochemical signatures are consistent with a suprasubduction zone geodynamic setting. Metasomatism of the subarc mantle by slab-derived hydrous melts and/or fluids at ca. 2.55 Ga is likely to have been the cause of the subduction zone geochemical signatures in peridotites of the Zunhua belt. In the Wutaishan belt, chromitite-hosting harzburgites and dunites display U-shaped rare earth element (REE) patterns and have high Mg-numbers (91.1–94.5). These geochemical characteristics are similar to those of Phanerozoic forearc peridotites. The dunites might have formed by dissolution of orthopyroxene in reactive melt channels, similar to those in modern ophiolites. However, they differ in detail, and they might be residues of Archean komatiites. Following the initiation of an intra-oceanic subduction zone, they were trapped as a forearc mantle wedge between the subducting slab and magmatic arc. Slab-derived hydrous melts infiltrating through the mantle wedge metasomatized the depleted mantle residue, resulting in U-shaped rare earth element (REE) patterns.

Contrasting geochemical patterns in the 3.7-3.8 Ga pillow basalt cores and rims, Isua greenstone belt, Southwest Greenland : implications for postmagmatic alteration processes

Abstract Pillow basalts from the early Archean (3.7 to 3.8 Ga) Isua greenstone belt, West Greenland, are characterized by well-preserved rims and concentric core structures. The pillow rims and cores have different mineral assemblages, and chemical and isotopic compositions. The rims have systematically higher contents of Fe2O3, MgO, MnO, K2O, Rb, Ba, Ga, Y, and transition metals than the cores. In contrast, the cores possess higher concentrations of SiO2, Na2O, P2O5, Sr, Pb, U, Nb, and the light rare earth elements (REEs than the rims). These compositional variations in the rims and cores are likely to reflect the mobility of these elements during posteruption alteration. Variations of many major and trace element concentrations between the rims and cores of the Isua pillow basalts are comparable to those of modern pillow basalts undergoing seafloor hydrothermal alteration. Al2O3, TiO2, Th, Zr, and the heavy REEs display similar values in both rims and cores, suggesting that these elements were relatively immobile during postemplacement alteration. In addition, the rims and cores have distinctive Sm-Nd and Rb-Sr isotopic compositions in that the rims are characterized by higher 143Nd/144Nd and 87Sr/86Sr ratios than the cores. The pillow basalts yield 2569 ± 170 Ma and 1604 ± 170 Ma errorchron ages on 143Nd/144Nd vs. 147Sm/144Nd and 87Sr/86Sr vs. 87Rb/86Sr diagrams, respectively. The Sm-Nd errorchron age may correspond, within errors, to a late Archean tectonothermal metamorphic event recorded in the region. The Sm-Nd errorchron may have resulted from a combination of isotopic homogenization and preferential loss of Nd, relative to Sm, during late Archean metamorphism. Although the Rb-Sr errorchron age overlaps with the timing of an early to mid-Proterozoic tectonothermal metamorphic event recorded in the region, because of a considerably large mean square of weighted deviates value and scatter in 86Sr/87Sr and 87Rb/86Sr ratios, this age may not have a precise geological significance. The 1.6 Ga Rb-Sr errorchron is likely to have resulted from the loss of radiogenic 87Sr. Collectively, the Sm-Nd and Rb-Sr data obtained from the 3.7–3.8 Ga Isua pillow basalt rims and cores are consistent with disturbances of the Sm-Nd and Rb-Sr systems by tectonothermal metamorphic events long after their eruption. In contrast to the Sm-Nd and Rb-Sr systems, the Lu-Hf system appears to be largely undisturbed by metamorphism. Five core samples and three rim samples yield a 3935 ± 350 Ma age, within error of the approximate age of eruption (3.7 to 3.8 Ga). Two rim samples that have gained Lu give an age of 1707 ± 140 Ma, within error of the Rb-Sr errorchron age. Initial 176Hf/177Hf ratios of the undisturbed samples at 3.75 Ga lie within ±1 e-unit of the chondritic value, suggesting no long-term depletion in the mantle source of the basalts.

Generation of Eoarchean tonalite-trondhjemite-granodiorite series from thickened mafic arc crust

The earliest compounds forming Earth9s first continental crust were magmatic rocks with tonalitic-trondhjemitic-granodioritic composition (TTGs). TTGs are widely seen as originating from melting of hydrated oceanic crust in subduction zones. Alternative models argue that they may have formed by melting within thickened mafic oceanic protocrust. To simulate formation of Eoarchean TTGs in different tectonic regimes, we combine for the first time the thermodynamic calculation of residual assemblages with subsequent modeling of trace element contents in TTGs. We compare water-absent partial melting of two hydrated starting compositions, a modern mid-oceanic-ridge basalt (MORB) and a typical Eoarchean arc tholeiite from the Isua Supracrustal Belt that represents the country rock of Earth9s oldest TTGs in southern West Greenland. At 10 kbar, partial melting of MORB-like residues results in modeled TTG compositions that are very different from natural ones. Melting at higher pressures (14 and 18 kbar) leads to a better match, but several key trace element parameters in TTGs are still amiss. A perfect fit for trace element compositions is achieved by melting of Eoarchean arc tholeiites at 10 and 14 kbar. These protoliths contain less Al and Na and more Fe and Mg as compared to present-day MORB and form amphibole-rich and plagioclase-free residues even at low pressures. Formation of Earth9s oldest continental crust is therefore best explained by melting within tectonically thickened mafic island-arc crust.

Provenance of late Palaeozoic metasediments of the SW South American Gondwana margin: a combined U–Pb and Hf-isotope study of single detrital zircons

Combined U–Pb and Lu–Hf isotope measurements of single detrital zircon grains in Carboniferous metasediments from Patagonia delineate the source areas of the sediments. The detritus, represented by four metasandstone samples, was deposited prior to onset of subduction in Late Carboniferous time along the south Patagonian proto-Pacific Gondwana margin. A broad series of detrital zircon age peaks (0.35–0.7 Ga, 0.9–1.5 Ga) and a large spread (0.3–3.5 Ga) in the age spectra require numerous sources. A fifth metasediment was deposited after the onset of subduction. This syncollisional sample shows two distinct U–Pb age peaks at c. 290 Ma and 305 Ma. This points to a few sources only (Patagonia, West Antarctica). Initial Hf-isotope compositions of selected U–Pb dated zircons from the Carboniferous metasediments reveal zircon protoliths originating from both recycled crust and juvenile sources (εHf(T=0.4–3.5Ga)=−14 to +12). A comparison with crustal compositions of possible source areas indicates that the detritus mainly originated from the interior of Gondwana (Extra-Andean Patagonia, the Argentine Sierra de la Ventana, southernmost Africa, East Antarctica), as well as northern Chile and northwestern Argentina. The sediment transportation paths are consistent with an autochthonous palaeogeographical position of Patagonia with respect to Gondwana in Carboniferous time.