63 research outputs found
A high field strength element perspective on early lunar differentiation
Lunar rocks are inferred to tap the different fossil cumulate layers formed during crystallisation of a lunar magma ocean (LMO). A coherent dataset, including Zr isotope data and high precision HFSE (W, Nb, Ta, Zr, Hf) and REE (Nd, Sm, Lu) data, all obtained by isotope dilution, can now provide new insights into the processes active during LMO crystallisation and during the petrogenesis of lunar magmas. Measured Zr-92 and Zr-91 abundances agree with the terrestrial value within 0.2 c-units. Incompatible-trace-element enriched rocks from the Procellarum KREEP Terrane (PKT) display Nb/Ta and Zr/Hf above the bulk lunar value (ca. 17), and mare basalts display lower ratios, generally confirming the presence of complementary enriched and depleted mantle reservoirs on the Moon. The full compositional spectrum of lunar basalts, however, also requires interaction with ilmenite-rich layers in the lunar mantle. Notably, the high-Ti mare basalts analysed display the lowest Nb/Ta and Zr/Hf of all lunar rocks, and also higher Sm/Nd at similar Lu/Hf than low-Ti basalts. The high-Ti basalts also exhibit higher and strongly correlated Ta/W (up to 25) and Hf/W (up to 140), at similar W contents, which is difficult to reconcile with ortho- and clinopyroxene-controlled melting. Altogether, these patterns can be explained via assimilation of up to ca. 20% of ilmenite- and clinopyroxene-rich LMO cumulates by more depleted melts from the lower lunar mantle. Direct melting of ilmenite-rich cumulates or the possible presence of residual metals in the lunar mantle both cannot easily account for the observed Ta/W and Hf/W patterns. Cumulate assimilation is also a viable mechanism that can partially buffer the Lu/Hf of mare basalts at relatively low values while generating variable Sm/Nd. Thus, the dichotomy between low Lu/Hf of lunar basalts and high time integrated source Lu/Hf as inferred from Hf isotope compositions can potentially be explained. The proposed assimilation model also has important implications for the short-lived nuclide chronology of the Earth-Moon system. The new Hf/W and Ta/W data, together with a compilation of existing W-Th-U data for lunar rocks, indicate that the terrestrial and lunar mantles are indistinguishable in their Hf/W. Virtually identical epsilon W and Hf/W in the terrestrial and lunar mantle suggest a strong link between final core-mantle equilibration on Earth and the Moon forming giant impact. Previously, linear arrays of lunar samples in W-182 vs. Hf/W and Nd-142 vs. Sm/Nd spaces have been interpreted as isochrons, arguing for LMO crystallisation as late as 250 Myrs after solar system formation. Based on the proposed assimilation model, the W-182 and Nd-142 in many lunar magmas can be shown to be decoupled from their ambient Hf/W and Sm/Nd source compositions. As a consequence, the W-182 vs. Hf/W and Nd-142 vs. Sm/Nd arrays would constitute mixing lines rather than isochrons. Hence, the lunar Hf-182-W-182 and Sm-146-Nd-142 data would be fully consistent with an early crystallisation age of the LMO, even as early as 50 Myrs after solar system formation when the Moon was probably formed. (C) 2010 Elsevier Ltd. All rights reserved
Evidence for tungsten mobility during oceanic crust alteration
Being a mobile element, the budget of W in the oceanic crust, an important geochemical reservoir, may be severely affected by hydrothermal alteration, but W data for altered oceanic crust (AOC) have not been available so far. In this study, we present the first high precision W concentration data obtained by isotope dilution and MC-ICPMS, along with high precision Th, U, Ta, Zr, Nb, Lu, and Hf data, for a complete section of altered upper oceanic crust (IODP-Borehole 1256D) down to the basal gabbros. As W, Th, U and Ta are similar melt incompatible, the latter were selected as framework to study W enrichment. In the studied section of altered oceanic crust, preserved alteration styles range from low temperature ( 400 degrees C) altered granoblastic dikes. Tungsten is systematically enriched over Th, U, and Ta in the whole depth succession. Uranium is also hydrothermally enriched locally, whereas Th and Ta both behaved entirely immobile over the entire depth succession. Based on U-series data, most alteration activity can be confined in age to >350 kyrs. The strongest W enrichments were found in contact zones between the different lithological units, at the sites of extreme fluid alteration. Generally, the elemental ratios of W/Th, Ta/VV and W/U show only minor overlap with pristine MORB ratios and may even exceed the values reported for global arc lavas with respect to W enrichment. The selective enrichment of W in many arc lavas may therefore be partially inherited from altered oceanic crust. Nevertheless, there are examples of arcs where subducted oceanic crust dominates the trace element budget, but the lavas do not show pronounced W enrichment. We propose three models to explain this conundrum, (a) retention of W in accessory phases, (b) selective enrichment of subduction zone fluids in U and Th relative to W, and (c) incomplete sampling of AOC by hole 1256D, leaving W depleted portions in the oceanic crust unsampled
Subduction zone dynamics in the SW Pacific plate boundary region constrained from high-precision Pb isotope data
This study presents high-precision Pb isotope data obtained by MC-ICP-MS for lavas from the Solomon island arc that covers ca. 1000 km of the SW Pacific plate boundary. Following a reversal of subduction polarity, the presence of the old subducted Pacific oceanic crust, and the presently subducting Indian-Australian plate beneath the entire island arc is confirmed by geophysical and geochemical evidence. New high-precision Pb isotope data now permit to distinguish more clearly between Indian and Pacific-type subduction components in this complex plate tectonic constellation. The values range from Pb-206/Pb-204 = 18.345 to 18.845, Pb-207/Pb-204 = 15498 to 15.545, and Pb-208/Pb-204 = 38.120 to 38.372, indicating the absence of pelagic sediments in the sub-arc mantle. These data reveal a predominance of subduction components derived from ancient subduction of the Pacific plate. In contrast, lavas with Indian-type Pb isotope signatures are confined to the central New Georgia Group of the Solomon Islands, where the young Woodlark Ridge spreading centre is currently subducting. All other Solomon arc segments with a lower geothermal gradient have apparently not yet been overprinted by younger subduction components over the past 6 Myrs. Moreover, trace element and Pb isotope data for Woodlark Ridge basalts show that subduction components from the fossil Pacific slab have locally modified the source of Woodlark Basin lavas. Altogether, our study highlights the potential of high-resolution Pb isotope measurements to distinguish between Indian and Pacific-type subduction components along the SW Pacific plate boundary. (C) 2011 Elsevier B.V. All rights reserved
La-Ce isotope measurements by multicollector-ICPMS
The La-138-Ce-138 decay system (half-life 1.02 x 10(11) years) is a potentially highly useful tool to unravel information about the timing of geological processes and about the interaction of geological reservoirs on earth, complementing information from the more popular Sm-147-Nd-143 and Lu-176-Hf-176 isotope systems. Previously published analytical protocols were limited to TIMS. Here we present for the first time an analytical protocol that employs MC-ICPMS, with an improved precision and sensitivity. To perform sufficiently accurate La-Ce measurements, an efficient ion-chromatographic procedure is required to separate Ce from the other rare earth elements (REE) and Ba quantitatively. This study presents an improved ion-chromatographic procedure that separates La and Ce from rock samples using a three-step column separation. After REE separation by cation exchange, Ce is separated employing an Ln Spec column and selective oxidation. In the last step, a cation clean-up chemistry is performed to remove all remaining interferences. Our MC-ICPMS measurement protocol includes all stable Ce isotopes (Ce-136, Ce-138, Ce-140 and Ce-142), by employing a 10(10) ohm amplifier for the most abundant isotope 140Ce. An external reproducibility of +/-0.253-units (2 r. s. d) has been routinely achieved for Ce-138 measurements for as little as 150-600 ng Ce, depending on the sample-skimmer cone combinations being used. Because the traditionally used JMC-304 Ce reference material is not commercially available anymore, a new reference material was prepared from AMES laboratory Ce metal (Cologne-AMES). In order to compare the new material with the previously reported isotopic composition of AMES material prepared at Mainz (Mainz-AMES), Cologne-AMES and JMC-304 were measured relative to each other in the same analytical session, demonstrating isotope heterogeneity between the two AMES and different JMC-304 batches used in the literature. To enable sufficiently precise age correction of radiogenic Ce-138 and to perform isochron dating, a protocol was developed where La and Ce concentrations are determined by isotope dilution (ID), using an isotope tracer enriched in La-138 and Ce-142. The new protocols were applied to determine the variations of Ce isotope compositions and La-Ce concentrations of certified geochemical reference materials (CRMs): BCR-2, BCR-1, BHVO-2, JR-1, JA-2, JB-3, JG-1, JR-1, JB-1b, AGV-1 and one in-house La Palma standard
Accurate stable tungsten isotope measurements of natural samples using a W-180-W-183 double-spike
Tungsten is a moderately siderophile element and, thus, enriched in the Earth's core. Moreover, W behaves incompatibly during partial melting, causing relative enrichment in the Earth's crust compared to the mantle. However, little is known about the geochemical cycle of the redox-sensitive element W in the crust-mantle system and in modern to ancient low-temperature environments. High resolution stable W isotope measurements of rock samples from different geochemical reservoirs might be a powerful tool to better constrain this cycle. So far, low relative mass differences between the different W isotopes and analytical challenges hampered such high-resolution measurements. Notably, some pioneering studies on the stable W isotope composition of geological reference material show inconsistent results, calling for further verification of the true compositions of these materials. This study presents an analytical protocol for stable W isotope measurements including the calibration of a W-180-W-183 double-spike as well as W isotope and W concentration data of several geological reference materials (BHVO-2, AGV-2, SDC-1, W-2a, ScO-2, NOD-A-1, NOD-P-1). The reproducibility of stable W isotope measurements (+/- 0.018% in delta(186)/W-184; 2 s.d.) is significantly improved compared to previous studies, which allows resolving between the stable W isotope compositions of various rock reservoirs on Earth. Relative to the NIST SRM 3163 standard, the highest delta W-186/184 value was observed for the Pacific Mn crust NOD-P-1 (+ 0.154 +/- 0.013%; 2 s.d.; n = 6), which is significantly different from the delta(186)/W-184 value of the Atlantic Mn crust NOD-A-1 (+ 0.029 +/- 0.014%; 2 s.d.; n = 6). Considering equilibrium fractionation between seawater WO42- and slowly growing Mn oxides, this indicates an isotopically heterogeneous distribution of W in the modern oceans. Igneous rocks also show a resolvable range in delta(186)/W-184 values. Magmatic reference materials range in d(186)/W-184 between + 0.016 +/- 0.032% (andesite AGV-2; 2 s.d.; n = 5) and + 0.082 +/- 0.010% (basalt BHVO-2; 2 s.d.; n = 5) showing relative enrichment of light isotopes in more evolved magmatic rocks. These isotopic differences might result from isotope fractionation during magmatic differentiation. Alternatively, the mobilization of W by hydrothermal and/or magmatic fluids might be accompanied by isotope fractionation
Redox-dependent fractionation of iron isotopes in suspensions of a groundwater-influenced soil
Redox conditions control the release of iron (Fe) into soil pore waters. A fluctuating groundwater table in soils results in significant changes in redox conditions with both time and depth. The effects of short-term differences in redox conditions on the stable isotope inventory of dissolved Fe in such soils have not yet been studied. Bulk Fe isotope compositions of a Gleysol yielded delta Fe-57 values from + 0.3 parts per thousand (humic topsoil, Ah horizon) to -0.2 parts per thousand (Fe-enriched subsoil, CrBg horizon). In microcosm experiments, soil suspensions of the Ah and CrBg horizons were subjected to controlled redox conditions ranging from high redox potential (E-H) (>430 mV, pH 5.1 to 5.6), moderate E-H (similar to 330 mV, pH 4.9 to 5.9), to low E-H (<= 170 mV, pH 5.2 to 6.7). Membrane-filtered (0.45 mu m) solutions taken from the suspensions were analysed for their Fe concentrations (Fe2+, and total Fe: Fe-tot) and isotopic compositions. The microcosm experiments demonstrated that the ferrihydrite-and organic-rich Ah horizon is a highly dynamic and rapidly responding reservoir with respect to Fe mobilization and isotopic fractionation at low E-H. Iron concentrations and isotope ratios of the solutions from the Ah horizon varied depending on E-H with negative delta Fe-57 values (-0.4 parts per thousand) and Fe-tot (-1.6 mg L-1) at moderate E-H, and even lower delta Fe-57 values (-1.1 parts per thousand) but high Fe-tot (similar to 7.8 mg L-1) at low E-H. At high E-H, delta Fe-57 values slowly increased from + 0.3 parts per thousand to + 1.0 parts per thousand and Fetot decreased to similar to 0.2 mg L-1 within six weeks. The goethite-rich CrBg horizon constitutes a stable redox-insensitive pool with very low amounts of mobilized Fe and a small degree of isotopic fractionation, even after exposure to low E-H over several weeks. In a natural open system, removal of Fe from the dynamic Ah horizon will result in progressively higher soil delta Fe-57 values due to preferential release of Fe-54. Vertical movement of a low-delta Fe-57 solution from the topsoil may result with time in the formation of a subsoil with delta Fe-57 values that are lower than the topsoil after repeated low and high E-H cycles. At high E-H, Fe mobility in the Ah horizon is much lowered, but release of Fe with high delta Fe-57 values is in agreement with earlier studies for the formation of Fe pools (Fe oxides, colloids, organic complexes) with high delta Fe-57 signatures. (C) 2014 Elsevier B.V. All rights reserved
Earth's volatile element depletion pattern inherited from a carbonaceous chondrite-like source
Earth's volatile element abundances (for example, sulfur, zinc, indium and lead) provide constraints on fundamental processes, such as planetary accretion, differentiation and the delivery of volatile species, like water, which contributed to Earth becoming a habitable planet. The composition of the silicate Earth suggests a chemical affinity but isotopic disparity to carbonaceous chondrites-meteorites that record the early element fractionations in the protoplanetary disk. However, the volatile element depletion pattern of the silicate Earth is obscured by core formation. Another key problem is the overabundance of indium, which could not be reconciled with any known chondrite group. Here we complement recently published volatile element abundances for carbonaceous chondrites with high-precision sulfur, selenium and tellurium data. We show that both Earth and carbonaceous chondrites exhibit a unique hockey stick volatile element depletion pattern in which volatile elements with low condensation temperatures (750-500 K) are unfractionated from each other. This abundance plateau accounts for the apparent overabundance of indium in the silicate Earth without the need of exotic building materials or vaporization from precursors or during the Moon-forming impact and suggests the accretion of 10-15 wt% CI-like material before core formation ceased. Finally, more accurate estimates of volatile element abundances in the core and bulk Earth can now be provided
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 measurement 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
Decoupled Hf and Nd isotopes in suspended particles and in the dissolved load of Late Archean seawater
It is generally agreed that decoupling of the Hf and Nd isotope systems in modern aqueous systems is a result of incongruent release of Hf during terrestrial weathering of the continental crust, although the mechanism(s) behind this process are not yet fully understood. We here present Hf-Nd isotope data for the Neoarchean Krivoy Rog Banded Iron Formation (BIFs), Ukraine, and combine observations on modern aqueous environments with those of the early Earth to further evaluate the mechanism(s) behind Hf-Nd isotope decoupling in aqueous systems. The pure Late Archean Krivoy Rog chemical sediment endmember, representing the dissolved pool of ancient seawater, shows decoupled epsilon Nd2.60Ga-epsilon Hf2.60Ga values of -2.3 and + 9.48, respectively, and suggests that decoupled Hf-Nd isotopes had been a global rather than a local phenomenon in Neoarchean seawater. This further reveals that incongruent Hf release via terrestrial weathering and erosion of emerged and evolved continental landmasses were widespread geological processes by Late Archean time. Impure Krivoy Rog BIF samples, composed of a mixture of seawater-derived and detrital Nd and Hf, show systematically more positive epsilon Nd-2.60Ga values, but still reveal decoupled eNd-eHf values relative to an associated schist that plots slightly below the terrestrial array. This suggests that mineral sorting between a zirconbearing sand-sized fraction and fine-grained sediment particles occurred on/in Late Archean continents, rivers and oceans, and had significant impact on the chemical compositions of the suspended and dissolved element loads of Late Archean seawater. Less radiogenic Hf isotope compositions in the Krivoy Rog seawater relative to detritus-contaminated BIFs further suggest a pathway for high-temperature hydrothermal Hf into anoxic Archean seawater, that diluted the even more radiogenic Hf isotopic composition of continental run-off, created by the mineralogical composition of the continental hinterland and the zircon effect. Alternatively, the less pronounced decoupling of eHf-eNd in Late Archean seawater may be related to a shorter residence time of Hf relative to Nd. Furthermore, systematically more positive initial eNd values in detritus-contaminated Archean BIFs relative to respective dissolved seawater loads suggest that weathered and eroded material of (ultra) mafic rock suites had significant impact on the suspended and dissolved fractions in Archean seawater
The combined Zr and Hf isotope inventory of bulk rock and sequentially leached chondrite samples
The stepwise acid digestion of primitive chondritic meteorites allows the identification of nucleosynthetic isotope anomalies that are otherwise hidden on the bulk rock scale. Here, we present for the first time combined isotope data for acid leachates, residues, and bulk rock aliquots of several primitive chondrites for the geo- and cosmochemically similar elements Zr and Hf. Our analyses reveal significant Zr and Hf isotope anomalies that (i) are complementary between acid leachates and residues and (ii) well-correlated with each other. The observed Zr and Hf anomalies strongly suggest variable contributions of common s-process carrier phases to the different leachates and residues. Ratios of r- (and p-process) Zr and Hf isotopes appear to be uniform in leachates and residues. In contrast to the well-correlated anomalies found in our leaching experiments, nucleosynthetic Zr and Hf isotope signatures seem to be decoupled on the bulk rock scale. This contrast may result from the heterogeneous distribution of neutron-rich Zr material devoid Hf, or alternatively be caused by the presence of anomalous CAI material which overprinted s-process deficits that were initially correlated. In contrast to a previous study, we find no direct evidence for the presence of a third isotopically distinct nucleosynthetic Zr component. (C) 2019 Elsevier Ltd. All rights reserved
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