N. Shimizu

ReOs, SmNd, and RbSr isotope evidence for thick Archaean lithospheric mantle beneath the Siberian craton modified by multistage metasomatism

A suite of peridotite xenoliths from kimberlites intruding the Siberian craton indicate the presence of lithospheric mantle over 150 km thick at 350 Ma. We report Sr-Nd isotope data for minerals from the peridotite xenoliths together with osmium isotopic compositions for whole-rocks and two olivine separates. Additionally, the osmium isotopic composition of a carbonatite from Fort Portal, Uganda, has been measured in order to evaluate the effect of carbonatite metasomatism on mantle ReOs systematics. Osmium isotope compositions of peridotite xenoliths from the Mir and Udachnaya kimberlites vary from those characteristic of the oceanic mantle, to considerably less radiogenic values (187Os1880s, 0.16469 to 0.10812), comparable to those previously found in other cratonic peridotites. In contrast, two eclogite xenoliths from Udachnaya have extremely radiogenic Os, 187Os188Os, up to 9.67. The lowest peridotite osmium isotopic compositions require Re depletion in the mid-Archaean (3.2 Ga) and this age is interpreted as the time of differentiation of the Siberian cratonic lithospheric mantle. Archaean depletion ages for spinel peridotites of relatively shallow origin and garnet peridotites and dunites containing diamond indicate that the depleted lithosphere reached from the Moho to 150 Km depth at this time and has been stable for 3 Ga. ReOs and SmNd model ages for two eclogite xenoliths are also in the range of 2.7 to 3.1 Ga and support an ancient origin for the Siberian lithosphere. The oldest peridotite depletion ages and the eclogite model ages correspond to the oldest crustal ages obtained from the Anabar Shield of the Siberian craton, and suggest that the initiation of major crust formation and stabilisation of a thick cratonic keel were coeval. In general, the Siberian low-temperature peridotites are not as enriched in incompatible elements as those from the Kaapvaal craton yet their diopsides possess similar, low SmNd. The low incompatible element concentrations but LREE/MREE enrichment seen in some Siberian lherzolites suggest they may be the products of disequilibrium melting. Neodymium and strontium isotopic compositions of minerals from the peridotites are extremely heterogeneous (ϵNd(350), −55.1 to 491; 87Sr86Sr, 0.70253 to 0.72235). Subcalcic garnets of diamond inclusion-like composition within megacrystalline peridotites have ϵNd(350 values varying from −55.1 to −12.1. Depleted mantle model Nd ages are as old as 3.2 Ga permitting an ancient, enriched origin similar to that suggested for diamond inclusions (Richardson et al., 1984). Alternatively, consideration of the complex garnet SmNd isotope systematics and the presence of unsupported radiogenic Sr together with marked trace element zonation (Shimizu et al., 1994) suggest that these subcalcic garnets crystallised recently (close to the time of kimberlite eruption) from ancient, LREE-enriched, high RbSr precursors. We propose that the isotope systematics of subcalcic garnet diamond inclusions can also be interpreted in terms of a recent origin.

Experimental partitioning of high field strength and rare earth elements between clinopyroxene and garnet in andesitic to tonalitic systems

Abstract Partition coefficients of the rare earth elements (REE) and the high field strength elements (HFSE: Ti, Zr, Hf, Nb, Ta) among clinopyroxenes, garnets, and andesitic to granodioritic melts were experimentally determined at pressures ranging from 1 to 3 GPa and temperatures between 900 and 1150°C. Natural rocks of quartz–dioritic and basaltic composition were used as starting materials. Melt compositions covered a range from dioritic to quartz–dioritic and granodioritic. Partition coefficients obtained from experiments with different doping levels at the same run condition indicate that Henry’s Law is fulfilled. Partition coefficients were investigated as a function of temperature and phase compositions. Apparent correlations between the clinopyroxene partition coefficients of the REE and degree of depolymerization expressed as nonbridging oxygens per tetrahedron (NBO/T) are due to variations in the Na contents of the clinopyroxenes which are controlled by pressure or NBO/T. Based on the model of Blundy and Wood (1994 ), which accounts for the strain associated with placing a cation on a particular crystallographic site when the radius of the cation differs from the optimal radius for that site, significant differences between Zr− and Hf partition coefficients as well as correlations between Ti and REE partition coefficients can be explained in terms of differences of their ionic radii. For garnets D La /D Yb ratios change as a function of temperature much more than for clinopyroxenes. This is also consistent with the Blundy and Wood (1994 ) model.

Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle

le Roux, P. J., le Roex, A. P., Schilling, J. G., Schimizu, N., Perkins, W. W., Pearce, N. J. G. (2002). Mantle heterogeneity beneath the southern Mid-Atlantic Ridge: trace element evidence for contamination of ambient asthenospheric mantle. Earth and Planetary Science Letters, 203 (1), 479-498. Article Number: PII S0012-821X(02)00832-4.

Metasomatism of the mantle lithosphere recorded by rare earth elements in garnets

Abstract Garnets derived from peridotite and kimberlite concentrates in southern African kimberlites show normal and sinusoidal REE patterns. Both types of REE patterns are interpreted to have resulted from the metasomatism of previously existing mantle garnets by LREE-enriched melts or fluids. In this model, metasomatism proceeded in accordance with garnet-liquid K d s which increase by orders of magnitude from the LREE and MREE to the HREE, thus fractionating the REE into three groups. The development of sinusoidal REE patterns resulted from the HREE lagging behind the LREE during metasomatic readjustment in composition. Sinusoidal REE patterns represent a state of disequilibrium during the re-equilibration process and are retained by garnets of relatively refractory compositions. Garnets from off-craton and on-craton regions show both types of REE patterns, suggesting that the presence of refractory garnets typical of cratonic regions is not restricted to the craton. Calculated liquid compositions for the metasomatic melts resemble kimberlite and lamproite, but were probably unrelated to the host kimberlites. Melts from off-craton regions are less LREE rich than melts from cratonic regions. These results suggest a different lithosphere in the off-craton region, possibly reflecting extensional tectonics and a higher degree of mantle upwelling.

EVOLUTION OF THE HOROMAN PERIDOTITE (HOKKAIDO, JAPAN) : IMPLICATIONS FROM PYROXENE COMPOSITIONS

Abstract Processes occurring in the Earths upper mantle are important in controlling evolution of the crust-mantle system. The effects of multiple igneous and metamorphic processes are recorded in upper-mantle peridotites, such as the Horoman Peridotite in Hokkaido, Japan. Geochemical studies of these peridotites and their minerals, combined with determination of the spatial geochemical variations, can be used to understand the sequence of processes that affected the perioditite. In this study we show that compositional zoning patterns of major and trace elements in clinopyroxene porphyroclasts reflect a sub-solidus, closed-system transition from garnet periodotite, equilibrated at 20–24 kbar and 1040–1160°C, to plagioclase periodotite equilibrated at ∼ 7 kbar and 850–950°C. The preservation of compositionally zoned pyroxenes indicates that this transition was a relatively recent process that probably occurred as the Horoman Peridotite was emplaced into the Hidaka metamorphic belt. The clinopyroxene compositions also show that: (1) harzburgites and lherzolites reacted with a fluid/melt that resulted in relative enrichment of highly incompatible elements, such as the light rare-earth elements; and (2) this enrichment process preceded the sub-solidus breakdown of garnet.

Inter- and intra-crystal REE variations in apatite from the Bob Ingersoll pegmatite, Black Hills, South Dakota

Abstract Concentrations of rare earth elements (REE) have been measured on a suite of apatite crystals from an internally zoned granitic pegmatite enriched in Li, B, Be, F, Nb, Ta, Sn and U with a Cameca IMS 3f ion microprobe using energy filtering. An apatite specimen from the Tin Mountain pegmatite, analyzed previously by isotope dilution, was used as a standard. The chondrite-normalized pattern determined with the ion microprobe closely matches the pattern determined by isotope dilution, with maxima at Sm and Dy, and minima at Nd and Er. Apatite samples from the Bob Ingersoll pegmatite show a large range of REE patterns and concentrations. In one case, apatite crystals within millimeters show differences in REE concentrations and pattern shapes, including a switch from positive to negative Eu anomalies. Samples from several mineral assemblages show patchy individual crystal zoning with respect to the REE that is not mirrored by major element zoning. This indicates disequilibrium conditions on a small scale, consistent with rapid growth from a melt/fluid system that was either of locally heterogeneous melt structure, or in which the melt structure fluctuated rapidly as volatile rich minerals such as tourmaline crystallized. These effects may be coupled with non-ideal partitioning of REE in a heterogeneous mixture of melt, aqueous fluid and crystals. REE concentrations in apatite samples from the different pegmatite zones indicate a large variation in outer zones (10–500X chondrite), high concentrations (100–1000X chondrite) near the pegmatite core, and very low concentration in the core (2–20X chondrite). Patterns are flat to slightly inclined (Ce/Yb: 1 to 5), and most samples have positive Eu anomalies. The magnitude of positive Eu anomalies decreases with inward position in the pegmatite, possibly indicating a progressive increase in ƒ O 2 , and a sharp increase may be indicated by systematic Ce depletion in apatite from the pegmatite core. REE-specific volatile complexes may contribute to variations, including unusual kinks, observed in REE patterns of apatite from mineral assemblages in upper parts of the pegmatite.

The trace-element variations in clinopyroxenes from spinel peridotite xenoliths from southwest Poland

Abstract Major- and trace-element content, and Sr- and Nd-isotopic composition are reported for clinopyroxenes from anhydrous spinel peridotites (SW Poland). LREE enrichment which occurs in all samples is decoupled from relatively high 143 Nd 144 Nd ratios (0.51286–0.51291), indicating that mantle metasomatism took place relatively recently. REE and HFSE (high-field strength element) abundances show marked variation but, interestingly, correlate with each other: samples with U-shaped REE patterns have Ti Zr > 700 ; samples with a REE concave-downward pattern have Ti/Zr of 20–200 and samples with a REE concave-upward pattern are characterized by Ti/Zr of 2–40. We suggest that trace-element enrichment is the result of interaction between basaltic melt migrating through the veins and contiguous wall-rock peridotites. Modal composition and trace-element characteristics of samples with U-shaped REE patterns probably reflect interaction of depleted peridotite residue with carbonatite melt.

Exploring the lunar mantle with secondary ion mass spectrometry - A comparison of lunar picritic glass beads from the Apollo 14 and Apollo 17 sites

Abstract The major element characteristics of lunar picritic beads indicate that their composition approaches that of primary basaltic liquids and therefore may be our best geochemical-mineralogical probes of the lunar interior. The picritic magmas appear to represent partial melting of a variety of mantle mineral assemblages. Secondary ion mass spectrometry (SIMS) analysis of individual glass beads collected from the Apollo 14(A-14) and Apollo 17(A-17) landing sites shows that although there is substantial major, minor (TiO 2 , Al 2 O 3 ) and trace element variation at each site, the glasses from each site have distinguishable trace element signatures (Ba/Sr, LREE/HREE). The incompatible element characteristics of glasses of nearly identical major element chemistry are strikingly different between sites. The contrast between the A-14 and A-17 glasses appears to be the result of a higher KREEP component in all the A-14 glasses. The incorporation of evolved KREEP component into high-Mg magmas is not a result of either assimilation or magma mixing but incorporation during the partial melting episode that produced the picritic magmas. This is a more thermally efficient mechanism for incorporation of the KREEP component into a picritic magma. Calculated mantle chemistries based upon glass compositions, low degrees of partial melting and mineralogical components used in previous models suggest that the difference between A-14 and A-17 mantle sources is a 0.1–1.4% KREEP intercumulate melt. This difference will increase with an increase in the degree of partial melting in the model calculations. This intercumulate melt may interstitially reside in an evolved, high-Ti “cumulate” component. The juxtaposition of evolved (high-Ti cumulates +KREEP) and primitive (low-Ti cumulates) mantle components has been attributed to original mantle inhomogeneities, density contrasts during magma ocean crystallization and sinking of high-density evolved cumulates into less dense, primitive cumulates within the context of either magma-ocean- or serial magmatism-type models. Regardless of the model, the trace element signatures of glasses (and the basalts?) indicate that the character of the A-14 mantle source is intrinsically different from that of the A-17 mantle source. This indicates large-scale lateral (and vertical) inhomogeneities in the lunar mantle.

Inter- and intra-group compositional variations in Apollo 15 pyroclastic green glass - An electron- and ion-microprobe study

Abstract Representative samples of the five compositionally distinct groups (A-E) of Apollo 15 pyroclastic green glass were systematically analyzed for major- and trace-element abundances by electron-and ion-microprobe techniques. These primary magma compositions define a high-Co trend (groups A and D) and a low-Co trend (groups B and C) on Sr, Zr, Ba, and Nd versus Co variation diagrams. Group E glasses are significantly enriched in Sr relative to the glass groups of the two trends. Pearce element ratio analyses and Sr/Ca-Ba/Ca systematics indicate that olivine, orthopyroxene, and clinopyroxene controlled green glass compositions. Simple crystal-liquid igneous fractionation processes involving such an assemblage, however, cannot explain the observed inter- and intra-group trace-element variation trends. The magmas represented by the high- and low-Co trends are interpreted to have been erupted from two distinct but compositionally variable source regions. A source mixing model involving differentiated cumulate, trapped residual liquid, and extremely Fe-rich components can successfully account for the compositional ranges of the two sources required to produce the observed chemical variations upon partial melting. Group E glasses appear to represent a magma erupted from a source with a more limited chemical variability. The Apollo 15 green glasses represent multiple eruptive events from three chemically distinct but compositionally variable source regions assuming that this mixing model is correct.

In-situ Pb isotopic analysis of sulfides in abyssal peridotites: New insights into heterogeneity and evolution of the oceanic upper mantle

Abyssal peridotites and mid-oceanic ridge basalts (MORBs) represent complementary residue-liquid products of melting and melt migration in the oceanic mantle. Because MORBs are mixtures of melts from different mantle depths, their isotopic signature does not directly describe the isotopic composition of the mantle source, but instead describes the local average composition of different parts of the mantle. In contrast, abyssal peridotites, the residues of fractional melting and melt-rock reaction, should shed more light on the distribution of isotopic heterogeneities. We analyzed Pb isotopic compositions in sulfide grains from the Southwest Indian Ridge and the Gakkel Ridge (Arctic Ocean) using the high-resolution Cameca 1280 ion microprobe. Sulfide Pb isotope ratios show very large variations, with 16 grains from 1 sample covering ∼25% of the entire range observed in the oceanic mantle. Pb isotopes in sulfides preserve a record of mantle compositions not seen in whole-rock MORBs from the same area. Sulfides from the Atlantis II Fracture Zone (Southwest Indian Ridge) confirm the presence of ancient refractory material scatter in the oceanic upper mantle. Gakkel Ridge sulfides define a high degree of isotopic variability, suggesting that oceanic mantle, not subcontinental lithospheric mantle, is the main source of such heterogeneity. Our results confirm that the source of MORBs, as represented by abyssal peridotites, is very heterogeneous and that other mantle end-member components are intimately mixed in. In-situ sulfide analysis is a powerful tool to detect the isotopic diversity of the MORB mantle source.