Harry Becker

Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones

The trace element compositions of eclogites, blueschists and mafic granulites from high-pressure terranes have been analysed to investigate element losses and fractionation that occur during dehydration of oceanic basalt in subduction zones. Abundances of elements that are suggested to be near-immobile (e.g., Nb, Zr, Ti), Sr-Nd isotopic compositions, and major element compositions indicate that most samples had altered MORB protoliths. The samples show only limited retrograde alteration, and cover a range in pressure-temperature conditions (1.2–4 GPa, 300–1000°C). In ratio diagrams, strong depletions (95–98%) of K, Rb and Ba relative to Nb and Th in most samples are obvious when compared with unaltered and altered MORB or ocean island basalts. The largest fraction of K, Rb and Ba appears to be lost at temperatures < 600–700°C. In contrast, elements such as Th, Nb, Ti, Zr, Nd, Sm and compatible elements show no evidence of significant losses (<10–20%). U and Pb also show losses, but these are significantly less than those for K and Ba. Eclogites retain nearly all Nb during dehydration. Consequently, the depleted nature of sub-arc mantle is the most likely cause for the low Nb abundances in arc lavas. The addition of U during sub-seafloor alteration and its restricted loss during subduction zone metamorphism substantially decreases Th/U and Nb/U in subducted altered MORB. The latter observation suggests that high U/Pb of many metabasaltic eclogites may have been caused by addition of U during sub-seafloor alteration. However, the correlation of U/Pb with Nd/Pb indicates that Pb loss during dehydration is the major cause of increased U/Pb in the eclogites. Model compositions of subducted altered oceanic crust have been established at 600°C and 900°Con the basis of the composition of the high-pressure rocks. Using these data, flux models indicate that Ba and Th in typical arc magmas must be mainly sediment-derived (fluids or melts from subducted sediment or shallow crustal contamination). In contrast, a large fraction of Rb and K and < 40% of the U in arc front magmas may be provided by fluids from subducted altered basalt. The models indicate that subducted altered oceanic basalt provides less than ca. 10% of the Pb and less than 5% of Sr to average arc front magma compositions. The low estimate for Sr confirms previous indications that contributions [page end] from average altered MORB cannot explain the Sr enrichment in arc lavas. Most of the Nd, heavy rare earth elements (REE), Y, high-field strength elements (HFSE) and compatible elements in primitive arc front magmas must be supplied by the depleted mantle wedge and a sedimentary component in arc lavas.

Comparative 187Re-187Os systematics of chondrites: Implications regarding early solar system processes

Abstract A suite of 47 carbonaceous, enstatite, and ordinary chondrites are examined for Re-Os isotopic systematics. There are significant differences in the 187Re/188Os and 187Os/188Os ratios of carbonaceous chondrites compared with ordinary and enstatite chondrites. The average 187Re/188Os for carbonaceous chondrites is 0.392 ± 0.015 (excluding the CK chondrite, Karoonda), compared with 0.422 ± 0.025 and 0.421 ± 0.013 for ordinary and enstatite chondrites (1σ standard deviations). These ratios, recast into elemental Re/Os ratios, are as follows: 0.0814 ± 0.0031, 0.0876 ± 0.0052 and 0.0874 ± 0.0027, respectively. Correspondingly, the 187Os/188Os ratios of carbonaceous chondrites average 0.1262 ± 0.0006 (excluding Karoonda), and ordinary and enstatite chondrites average 0.1283 ± 0.0017 and 0.1281 ± 0.0004, respectively (1σ standard deviations). The new results indicate that the Re/Os ratios of meteorites within each group are, in general, quite uniform. The minimal overlap between the isotopic compositions of ordinary and enstatite chondrites vs. carbonaceous chondrites indicates long-term differences in Re/Os for these materials, most likely reflecting chemical fractionation early in solar system history. A majority of the chondrites do not plot within analytical uncertainties of a 4.56-Ga reference isochron. Most of the deviations from the isochron are consistent with minor, relatively recent redistribution of Re and/or Os on a scale of millimeters to centimeters. Some instances of the redistribution may be attributed to terrestrial weathering; others are most likely the result of aqueous alteration or shock events on the parent body within the past 2 Ga. The 187Os/188Os ratio of Earth’s primitive upper mantle has been estimated to be 0.1296 ± 8. If this composition was set via addition of a late veneer of planetesimals after core formation, the composition suggests the veneer was dominated by materials that had Re/Os ratios most similar to ordinary and enstatite chondrites.

186 Os^ 187 Os systematics of Gorgona Island komatiites: implications for early growth of the inner core

The presence of coupled enrichments in 186 Os/ 188 Os and 187 Os/ 188 Os in some mantle-derived materials reflects longterm elevation of Pt/Os and Re/Os relative to the primitive upper mantle. New Os data for the 89 Ma Gorgona Island, Colombia komatiites indicate that these lavas are also variably enriched in 186 Os and 187 Os, with 186 Os/ 188 Os ranging between 0.1198397 7 22 and 0.1198470 7 38, and with QOs correspondingly ranging from +0.15 to +4.4. These data define a linear trend that converges with the previously reported linear trend generated from data for modern Hawaiian picritic lavas and a sample from the ca. 251 Ma Siberian plume, to a common component with a 186 Os/ 188 Os of approximately 0.119870 and QOs of +17.5. The convergence of these data to this Os isotopic composition may imply a single ubiquitous source in the Earth’s interior that mixes with a variety of different mantle compositions distinguished by variations in QOs. The 187 Os- and 186 Os-enriched component may have been generated via early crystallization of the solid inner core and consequent increases in Pt/Os and Re/Os in the liquid outer core, with time leading to suprachondritic 186 Os/ 188 Os and QOs in the outer core. The presence of Os from the outer core in certain portions of the mantle would require a mechanism that could transfer Os from the outer core to the lower mantle, and thence to the surface. If this is the process that generated the isotopic enrichments in the mantle sources of these plume-derived systems, then the current understanding of solid metal^liquid metal partitioning of Pt, Re and Os requires that crystallization of the inner core began prior to 3.5 Ga. Thus, the Os isotopic data reported here provide a new source of data to better constrain the timing of inner core formation, complementing magnetic field paleointensity measurements as data sources that constrain models based on secular cooling of the Earth. Published by Elsevier Science B.V.

Garnet peridotite and eclogite Sm-Nd mineral ages from the Lepontine dome (Swiss Alps): New evidence for Eocene high-pressure metamorphism in the central Alps

In the central Alps the upper Pennine Cima Lunga nappe (Lepontine dome, Swiss Alps) underwent eclogite facies conditions up to 850-900 °C and 3.5-4.2 GPa. Sm-Nd dating of garnet peridotites and an eclogite from the Cima Lunga nappe yields consistent mineral ages of ca. 40 Ma. The mineral ages are probably cooling ages, but petrological arguments indicate that they approximate the time of eclogite facies metamorphism, implying that the latter is related to the collision of the Adriatic plate with Europe. Emplacement of the upper Pennine nappe stack with the hot Cima Lunga nappe at its base onto European basement (lower Pennine units) is suggested to be the primary cause for the middle Tertiary Barrovian-type metamorphism in the central Alps. This process accounts for the different metamorphic evolution of the Cima Lunga nappe compared to underlying lower Pennine units and explains the time difference between Sm-Nd mineral ages from the Cima Lunga nappe and U-Pb monazite ages from the lower Pennine units.

Re–Os fractionation in eclogites and blueschists and the implications for recycling of oceanic crust into the mantle

Abstract Metabasalts (eclogites, blueschists and mafic granulites) metamorphosed in paleosubduction zones show a range in rhenium (Re) abundances between 3 and 1689 parts per trillion (ppt), with a median value of 331 ppt Re. The median Re abundance of the metabasites corresponds to only ∼40% of the Re abundances expected for likely mafic protoliths. Osmium (Os) abundances in the metabasites (2–42 ppt, with one sample at 909 ppt) are comparable to abundances in mid-ocean ridge basalt (MORB). Re was lost from the protoliths either during dehydration, the most likely explanation, or during alteration of the protoliths near the ocean ridges. Radiogenic 187Os/188Os and 206Pb/204Pb of HIMU ocean island basalts (OIB) that are believed to contain a component of recycled altered MORB would require excessive amounts (>80–90%) of ∼0.5–1 Gyr old recycled altered MORB in the mantle source, if constraints on Re/Os and U/Pb from the metabasites are applied. The necessary amount of recycled component is only marginally lower (70%) if equal amounts of altered and unaltered 2 Gyr old MORB are present in the mantle source. Considering the major element abundances in alkaline OIB, these estimates appear unreasonably high. One possibility is that the simple bulk mixing models commonly employed are not applicable. Rapid melting during the formation of alkaline basalts and sluggish kinetics may prevent complete equilibration with Os-rich phases such as sulfides and alloys. Other possible explanations include the shielding of Os-rich phases in peridotite by silicates from reaction with partial melts of eclogite and the isolation of melt from peridotite by means of pyroxene-rich, Os-poor reaction zones along conduit walls. In the case of disequilibrium, the actual fraction of recycled component in OIB sources could be much lower than in equilibrium mixing models.

Effects of melt percolation on the Re–Os systematics of peridotites from a Paleozoic convergent plate margin

Abstract The effects of melt percolation on the Re–Os systematics of peridotites were studied in garnet- and spinel-bearing high-temperature peridotite bodies in the southern Bohemian massif (lower Austria). The mantle rocks occur in the high-grade core of a Carboniferous collision zone. Age constraints, occurrences of calc–alkaline rocks, and the trace element and isotopic composition of garnet pyroxenite layers in the peridotites indicate that the latter must have originated in the hanging wall mantle of a late Devonian–early Carboniferous subduction zone. The garnet pyroxenites are cumulates and their Eu anomalies, radiogenic initial 187 Os/ 188 Os ( γ Os i up to 450), 87 Sr/ 86 Sr (up to 0.709), and unradiogenic 143 Nd/ 144 Nd ( ϵ Nd i as low as −4.8) were probably inherited from melts that contained slab-derived components. Because the isotopic signatures of the melts were substantially different from normal mantle, these peridotites are particularly suitable for a study of the effects of melt percolation. Layered dunite–pyroxenite rocks resemble replacive dunites in ophiolites, and may have acted as high permeability channels for Mg-rich melts. The dunite–pyroxenite rocks are characterized by suprachondritic γ Os i (7.2 to 13.9), indicating addition of radiogenic Os from the melts. Assuming the dunites originally had a composition similar to harzburgites at the same outcrop ( γ Os i of −2.7 to −3.9), the change in γ Os i requires melt/rock ratios between 10 and 400. Osmium and Cr are depleted in the dunites by up to 50–70% compared to normal peridotites, and enriched in associated orthopyroxenites (up to 3.8 ppb Os and 5700 ppm Cr), indicating substantial transfer of peridotitic Os and Cr into melt. Preferred incorporation of Os and Cr into Mg-rich pyroxenites suggests a coupling of the chemical behavior of Os and Cr in Mg-rich igneous silicate systems. Rhenium abundances are uniformly low in these rocks, presumably because of low Re and sulfur contents in the melts. Melt percolation in depleted lherzolites and harzburgites in the vicinity of pyroxenite layers is indicated by Sr–Nd isotopic data and REE patterns. γ Os i in these rocks (−5.5 to +0.5) suggest no or only minor addition of radiogenic Os and low melt/rock ratios near 1. However, correlations of mildly incompatible elements such as Al with Re abundances and negative correlation of 143 Nd/ 144 Nd with Re in lherzolites suggest refertilization of previously depleted peridotites, with concurrent addition of significant amounts of Re. Metasomatic addition of radiogenic Os at high melt/rock ratios, and Re at low melt/rock ratios, produced large shifts in the Re–Os model ages of the peridotites.

Ratios of S, Se and Te in the silicate Earth require a volatile-rich late veneer

The excess of highly siderophile (iron-loving) elements (HSEs) and the chondritic ratios of most HSEs in the bulk silicate Earth (BSE) may reflect the accretion of a chondritic ‘late veneer’ of about 0.5 per cent of Earth’s mass after core formation. The amount of volatiles contained in the late veneer is a key constraint on the budget and the origin of the volatiles in Earth. At high pressures and temperatures, the moderately volatile chalcogen elements sulphur (S), selenium (Se) and tellurium (Te) are moderately to highly siderophile; thus, if depleted by core formation their mantle abundances should reflect the volatile composition of the late veneer. Here we report ratios and abundances of S, Se and Te in the mantle determined from new isotope dilution data for post-Archaean mantle peridotites. The mean S/Se and Se/Te ratios of mantle lherzolites overlap with CI (Ivuna-type) carbonaceous chondrite values. The Se/Te ratios of ordinary and enstatite chondrites are significantly different. The chalcogen/HSE ratio of the BSE is similar to that of CM (Mighei-type) carbonaceous chondrites, consistent with the view that the HSE signature of the BSE reflects a predominance of slightly volatile-depleted, carbonaceous-chondrite-like material, possibly with a minor proportion of non-chondritic material. Depending on the estimates for the abundances of water and carbon in the BSE, the late veneer may have supplied 20 to 100 per cent of the budget of hydrogen and carbon in the BSE.

Constraints from high-pressure veins in eclogites on the composition of hydrous fluids in subduction zones

Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Munchberg Gneiss Massif, Variscan fold belt, Germany) constrain the major and trace element composition of solutes in fluids liberated during dehydration of eclogites. Similar initial isotopic compositions of veins and host eclogites at the time of metamorphism indicate that the fluids were derived predominantly from the host rocks. Quartz, kyanite, paragonite, phengite, zoisite and omphacite are the dominant minerals in the veins. The major element compositions of the veins are in agreement with experimental evidence indicating that the composition of solutes in such fluids is dominated by SiO2 and Al2O3. Relative to N-MORB, the veins show enrichments of Cs, Rb, Ba, Pb, and K, comparable or slightly lower abundances of Sr, U, and Th, and very low abundances of Nd, Sm, Zr, Nb, Ti and Y. The differential fractionation of highly incompatible elements such as K, U and Th in the veins, as well as the presence of hydrous minerals in the eclogites rule out partial melting as a cause for vein formation. These results confirm previous suggestions that fluids derived from subducted basalt may have low abundances of high field strength elements, rare earth elements and Y. Variable vein-eclogite enrichment factors of incompatible alkalis and to a lesser extent Pb appear to reflect mineralogical controls (phengite, epidote-group minerals) on partitioning of these elements during dehydration of eclogite in subduction zones. However, abundance variations of incompatible elements in minerals from eclogites suggest that the composition of fluids released from eclogites at temperatures <700°C may not reflect true equilibrium partitioning during dehydration. Simple models for the trace elements U and Th indicate the relative importance of the basaltic and sedimentary portions of subducted oceanic crust in producing the characteristic chemical signatures of these elements in convergent plate margin volcanism.

Isotopic constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle xenoliths, Washington, USA

Abstract Spinel harzburgite and websterite mantle xenoliths from Simcoe volcano in southern Washington represent fragments of mantle lithosphere from the back-arc side of the Cascade arc front. Previous studies have shown that metasomatism by either silica-rich fluids or hydrous melts crystallized phlogopite, imparted high oxygen fugacities (0.3 to 1.4 log units above QFM), and more radiogenic Os isotopic compositions on these peridotites. These features are consistent with part or all of the metasomatic agent being derived from the Juan de Fuca slab. New Re–Os, Sm–Nd, Sr, and U–Th–Pb isotopic data shed further light on the origin and composition of the metasomatic agent. The clinopyroxenes from the xenoliths have correlated Pb isotopic compositions ( 206 Pb / 204 Pb =18.63–19.55, 207 Pb / 204 Pb =15.56–15.63, 208 Pb / 204 Pb =38.22–38.87). The most radiogenic Pb isotopic compositions extend beyond the most radiogenic Pb isotopic compositions for the Cascade arc lavas and display a shallower trend. Mixtures between Juan de Fuca basalts and pelagic or terrigenous sediments would result in Pb isotopic compositions that are not radiogenic enough in 207 Pb / 204 Pb and 208 Pb / 204 Pb at the high 206 Pb / 204 Pb end of this array. Therefore, models for rapid transfer of components from the slab to the mantle lithosphere are not viable in this case. Instead, a multi-stage model is preferred. In the first stage, the slab component is transferred via fluid or melt into, and reacts with the hanging wall mantle. This results in a residual slab depleted in Pb relative to U and Th, and consequent high U/Pb and Th/Pb. Additional dehydration or melting of the slab imparts this chemical signature to the peridotite in the hanging wall. In the second stage, the hybridized hanging wall peridotite evolves for tens of million years until corner flow drags it down to deeper levels in the mantle wedge where melting occurs in response to higher temperatures. In the third stage, this melt migrates upward where it metasomatizes the mantle lithosphere represented by the Simcoe xenoliths. Trace element compositions of the clinopyroxenes, and the presence of high alkali glasses in the xenoliths, are consistent with the metasomatic agent derived from the hybridized hanging wall being alkali-rich, and possibly similar to potassic-rich lavas found in arc and back-arc settings. These data therefore demonstrate the importance of the hybridized hanging wall mantle above slabs as a source for melts which can be metasomatic agents in the upper mantle, and as a site for storage of material derived from the slab for periods of at least tens of million years.

GEOCHEMISTRY OF GARNET PERIDOTITE MASSIFS FROM LOWER AUSTRIA AND THE COMPOSITION OF DEEP LITHOSPHERE BENEATH A PALAEOZOIC CONVERGENT PLATE MARGIN

Abstract In the southern Bohemian massif garnet-bearing high-temperature peridotite massifs have been exhumed during Carboniferous collision of the Baltic plate and rifted fragments of Gondwana. Geological and geochronological data suggest that the peridotites represent fragments of lithosphere derived from a destructive plate margin. Depleted garnet lherzolites, spinel and spinel-garnet harzburgites from lower Austria show major and trace element variations with MgO and HREE abundances which indicate that they underwent moderate (∼ 5%) to high (> 20%) degrees of partial melting in the spinel peridotite facies. Peridotites sampled away from pyroxenite layers have isotopic compositions typical of depleted mantle ( 87 Sr 86 Sr = 0.7021 to 0.7033 , ϵNd (335 Ma) = 8 to 12). Peridotites that occur close to LREE-enriched pyroxenites with negative Eu anomalies yield evidence for a multi-stage history, i.e. they have ( Tb Yb ) n and ( La Sm ) n > 1 . Re-enrichment in incompatible elements, e.g., high ( La Sm ) n in harzburgites and depleted lherzolites, and the relatively high SiO2 contents of some of these rocks, can be explained by percolation of the LREE-enriched melts which were parental to nearby pyroxenite layers. Refertilization, bulk REE enrichment and isotope disequilibrium of minerals in some peridotites are related to late physical mixing with garnet pyroxenites during the exhumation process. A second group of peridotites comprises garnet lherzolites with ( La Sm ) n . These LREE-depleted garnet lherzolites contain thin pyroxenite layers with flat to LREE-enriched REE patterns without Eu anomalies. Both, these peridotites and pyroxenites ( 87 Sr 86 Sr up to 0.7039, ϵNd (335 Ma) from 11 to −1.1) also appear to be isotopically distinct from the pyroxenites with negative Eu anomalies. A third group of lherzolites has high 87 Sr 86 Sr (up to 0.7049) and moderately high Nd isotopic compositions (ϵNd (335 Ma) from 7.9 to 4.4), and were probably also modified by the interaction with melts. The variability of isotopic compositions in the peridotites may indicate the possible variability in hangingwall lithospheric mantle beneath convergent plate margins. The average major and trace element composition of the peridotites from lower Austria is more depleted than the average of continental spinel peridotite xenoliths, but less depleted compared with garnet peridotite xenoliths from cratonic settings.