Abstract
Chromium isotopic data of physically separated components (chondrules,
CAI
s, variably magnetic size fractions) of the carbonaceous chondrites Allende and Murchison and bulk rock data of ...Allende, Ivuna, and Orgueil are reported to evaluate the origin of isotopic heterogeneity in these meteorites. Allende components show ε
53
Cr and ε
54
Cr from −0.23 ± 0.07 to 0.37 ± 0.05 and from −0.43 ± 0.08 to 3.7 ± 0.1, respectively. In components of Murchison, ε
53
Cr and ε
54
Cr vary from −0.06 ± 0.08 to 0.5 ± 0.1 and from 0.7 ± 0.2 to 1.7 ± 0.1, respectively. The non‐systematic variations of ε
53
Cr and
55
Mn/
52
Cr in the components of Allende and Murchison were likely caused by small‐scale, alteration‐related redistribution of Mn >20 Ma after formation of the solar system. Chondrule fractions show the lowest
55
Mn/
52
Cr and ε
54
Cr values of all components, consistent with evaporation of Mn and ε
54
Cr‐rich carrier phases from chondrule precursors. Components other than the chondrules show higher Mn/Cr and ε
54
Cr, suggestive of chemical and isotopic complementarity between chondrules and matrix‐rich fractions. Bulk rock compositions calculated based on weighted compositions of components agree with measured Cr isotope data of bulk rocks, in spite of the Cr isotopic heterogeneity reported by the present and previous studies. This indicates that on a sampling scale comprising several hundred milligrams, these meteorites sampled isotopically and chemically homogeneous nebular reservoirs. The linear correlation of
55
Mn/
52
Cr with ε
53
Cr in bulk rocks likely was caused by variable fractionation of Mn/Cr, subsequent mixing of phases in nebular domains, and radiogenic ingrowth of
53
Cr.
Fe-Ni metal-schreibersite-troilite intergrowths in Apollo 16 impact melt rocks and new highly siderophile element (HSE) and S abundance data indicate that millimeter-scale closed-system fractional ...crystallization processes during cooling of impactor-derived metal melt droplets in impact-melts are the main reason for compositional variations and strong differences in abundances and ratios of HSE in multiple aliquots from Apollo 16 impact melt rocks. Element ratios obtained from linear regression of such data are therefore prone to error, but weighted averages take into account full element budgets in the samples and thus represent a more accurate estimate of their impactor contributions. Modeling of solid metal-liquid metal partitioning in the Fe-Ni-S-P system and HSE patterns in impactites from different landing sites suggest that bulk compositions of ancient lunar impactites should be representative of impact melt compositions and that large-scale fractionation of the HSE by in situ segregation of solid metal or sulfide liquid in impact melt sheets most likely did not occur. The compositional record of lunar impactites indicates accretion of variable amounts of chondritic and non-chondritic impactor material and the mixing of these components during remelting of earlier ejecta deposits. The non-chondritic composition appears most prominently in some Apollo 16 impactites and is characterized by suprachondritic HSE/Ir ratios which increase from refractory to moderately volatile HSE and exhibit a characteristic enrichment of Ru relative to Pt. Large-scale fractional crystallization of solid metal from sulfur and phosphorous rich metallic melt with high P/S in planetesimal or embryo cores is currently the most likely process that may have produced these compositions. Similar materials or processes may have contributed to the HSE signature of the bulk silicate Earth (BSE).
Tellurium stable isotope compositions and abundances (δ128/126Te relative to SRM 3156) are reported for 43 ordinary, enstatite, and Rumuruti chondrites, which together with results from a companion ...study on carbonaceous chondrites are used to assess the origin of volatile element fractionations in chondrites. Whereas Te isotope variations among carbonaceous chondrites predominantly reflect mixing between isotopically light chondrules/chondrule precursors and CI-like matrix, Te isotope variations among non-carbonaceous chondrites mainly result from Te redistribution during parent body thermal metamorphism. The enstatite chondrites in particular display increasingly heavy Te isotopic compositions and decreasing Te concentrations with increasing degree of metamorphism, indicating migration of isotopically light Te from the strongly metamorphosed inner parts towards the cooler outer regions of the parent bodies. By contrast, ordinary and Rumuruti chondrites display less systematic Te isotope variations, implying more localized redistribution of Te during parent body thermal metamorphism.
We also report Te stable isotope data for 19 terrestrial mantle-derived rocks. Peridotites with Al2O3 contents close to those inferred for the bulk silicate Earth (BSE) exhibit uniform δ128/126Te values, which we interpret to represent the Te isotopic composition of the BSE. This composition overlaps with the Te isotope composition of some volatile-rich carbonaceous chondrites (most notably CM chondrites), but also with that of enstatite chondrites. Comparison of the Te results to Se isotopes and Se/Te ratios shows that due to uncertainties in the composition of the BSE and the isotopic composition of bulk chondrite parent bodies, neither Te isotopes alone nor the combined Se-Te elemental and isotopic systematics can distinguish between a carbonaceous and enstatite chondrite-like late veneer, which is the presumed source of Se and Te in the BSE. Together, the results of this study illustrate that the relative abundances and mass-dependent isotope compositions of volatile elements like Se and Te are modified by physical and chemical processes occurring after planetary accretion, which severely complicates their use as genetic tracers. A corollary of this is that contrary to prior proposals the Se-Te systematics are not contradicting an inner solar system origin of the late veneer, as it has been inferred using nucleosynthetic isotope anomalies of other elements.
Hydrous high-pressure veins formed during dehydration of eclogites in two paleo-subduction zones (Trescolmen locality in the Adula nappe, central Alps and Münchberg 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 SiO
2 and Al
2O
3. 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.
The formation ages of lunar impact basins are critical to understanding the late accretion history of the inner solar system. Furthermore, the correct interpretation of the provenance and isotopic ...dates of basin-derived impact melt (‘basin melt’) is essential for the calibration of lunar chronology function. However, abundances of basin melt in the lunar near-surface are not well understood. Basin melt has been gardened by a long sequence of subsequent impact events, altering its abundance and size distribution. We developed a numerical model to investigate this process by means of the Monte Carlo method in a spatially resolved model. The fraction of melt in ejecta was tracked globally and at the Apollo 14–17 and Luna 20 sampling sites and was compared with K-Ar age distributions of lunar impact melt breccias. It was found that melt produced by the very large SPA basin as well as the relatively late-forming Imbrium basin should be dominant in the near-surface (top one meter). The simulation shows that the melt component at the Apollo 14–17 and Luna 20 sites is strongly affected by nearby mid- to late-forming basins. Imbrium melt should be abundant in Apollo 14–17 samples; Crisium melt is the most significant component of basin-sourced melt in Luna 20 samples; all the Apollo 14–17 and Luna 20 samples could include melt from Serenitatis and the SPA basin; Nectaris melt should occur in Apollo 16, Apollo 17 and Luna 20 samples; and Orientale melt has no significant mixing in the Apollo 14–17 and Luna 20 sampling sites. In general, besides a prominent age peak at 3.9 Ga (related to the Imbrium basin), the model predicts pronounced abundance peaks of older basin melt (>3.9 Ga) which tend to be absent from distributions of K-Ar ages of impactites from landing sites. The diffusion characteristics of basin melt suggest that for future sampling aimed at collecting early basin melt, the re-excavation zones of late impact craters larger than tens of kilometer in diameter inside basins may provide the highest abundances of melt from early basins.
•Impact gardening of basin melt modeled in 3D to estimate present surface abundance•Estimate relative abundances of differently-aged melt at Apollo/Luna sampling sites•Surface melt abundance strongly affected by nearby late-forming basins•Deficiency of ancient melt compared to aggregated K-Ar sample age distribution
Meteorite impact processes are ubiquitous on the surfaces of rocky and icy bodies in the Solar System, including the Moon. One of the most common accessory minerals, zircon, when shocked, produces ...specific micro-structures that may become indicative of the age and shock conditions of these impact processes. To better understand the shock mechanisms in zircon from Apollo 15 and 16 impact breccias, we applied transmission electron microscopy (TEM) and studied nano-structures in eight lunar zircons displaying four different morphologies from breccias 15455, 67915, and 67955. Our observations revealed a range of shock-related features in zircon: (1) planar and non-planar fractures, (2) “columnar” zircon rims around baddeleyite cores, (3) granular textured zircon, in most cases with sub-µm-size inclusions of monoclinic ZrO
2
(baddeleyite) and cubic ZrO
2
(zirconia), (4) silica-rich glass and metal inclusions of FeS and FeNi present at triple junctions in granular zircon and in baddeleyite, (5) inclusions of rutile in shocked baddeleyite, (6) amorphous domains, (7) recrystallized domains. In many grain aggregates, shock-related micro-structures overprint each other, indicating either different stages of a single impact process or multiple impact events. During shock, some zircons were transformed to diaplectic glass (6), and others (7) were completely decomposed into SiO
2
and Zr-oxide, evident from the observed round shapes of cubic zirconia and silica-rich glass filling triple junctions of zircon granules. Despite the highly variable effect on textures and Zr phases, shock-related features show no correlation with relatively homogeneous U–Pb or
207
Pb/
206
Pb ages of zircons. Either the shock events occurred very soon after the solidification or recrystallization of the different Zr phases, or the shock events were too brief to result in noticeable Pb loss during shock metamorphism.
Lunar soil is a fine mixture of local rocks and exotic components. The bulk-rock chemical composition of the newly returned Chang’E-5 (CE-5) lunar soil was studied to understand its chemical ...homogeneity, exotic additions, and origin of landing site basalts. Concentrations of 48 major and trace elements, including many low-concentration volatile and siderophile elements, of two batches of the scooped CE-5 soil samples were simultaneously obtained by inductively coupled plasma mass spectrometry (ICP-MS) with minimal sample consumption. Their major and trace elemental compositions (except for Ni) are uniform at milligram levels (2–4 mg), matching measured compositions of basaltic glasses and estimates based on mineral modal abundances of basaltic fragments. This result indicates that the exotic highland and KREEP (K, rare earth elements, and P-rich) materials are very low (<5%) and the bulk chemical composition (except for Ni) of the CE-5 soil can be used to represent the underlying mare basalt. The elevated Ni concentrations reflect the addition of about 1 wt% meteoritic materials, which would not influence the other bulk composition except for some highly siderophile trace elements such as Ir. The CE-5 soil, which is overall the same as the underlying basalt in composition, displays low Mg# (34), high FeO (22.7 wt%), intermediate TiO2 (5.12 wt%), and high Th (5.14 µg/g) concentrations. The composition is distinct from basalts and soils returned by the Apollo and Luna missions, however, the depletion of volatile or siderophile elements such as K, Rb, Mo, and W in their mantle sources is comparable. The incompatible lithophile trace element concentrations (e.g., Ba, Rb, Th, U, Nb, Ta, Zr, Hf, and REE) of the CE-5 basalts are moderately high and their pattern mimics high-K KREEP. The pattern of these trace elements with K, Th, U, Nb, and Ta anomalies of the CE-5 basalts cannot be explained by the partial melting and crystallization of olivine, pyroxene, and plagioclase. Thus, the mantle source of the CE-5 landing site mare basalt could have contained KREEP components, likely as trapped interstitial melts. To reconcile these observations with the initial unradiogenic Sr and radiogenic Nd isotopic compositions of the CE-5 basalts, clinopyroxene characterized by low Rb/Sr and high Sm/Nd ratios could be one of the main minerals in the KREEP-bearing mantle source. Consequently, we propose that the CE-5 landing site mare basalts very likely originated from partial melting of a shallow and clinopyroxene-rich (relative to olivine and orthopyroxene) upper mantle cumulate with a small fraction (about 1–1.5 %) of KREEP-like materials.
The New Caledonia Ophiolite hosts one of the largest obducted mantle sections worldwide, offering a unique opportunity to investigate key mantle processes. The ophiolite comprises refractory ...harzburgites, locally overlain by mafic-ultramafic cumulates, and minor lherzolites. Previous geochemical studies indicated that the lherzolites are akin to abyssal-type peridotites, while the harzburgites underwent multiple melting episodes in MOR and supra-subduction zone environments, followed by late stage metasomatism.
In this work, Os isotopes, highly siderophile (HSE) and chalcophile element data are reported for the New Caledonia peridotites, in order to constrain the behaviour of these elements in abyssal-type and fore-arc mantle.
The variably serpentinised lherzolites (LOI = 6.4–10.7%) yield slightly subchondritic to suprachondritic initial Os isotopic compositions (187Os/188Osi = 0.1273–0.1329) and subchondritic to chondritic Re/Os ratios (0.04–0.11). The gently sloping HSE patterns with increasing depletion towards Au show concentrations in the range of other lherzolites from MOR or continental setting. Sulphur contents are high and variable (202–1268 ppm), and were likely increased during serpentinisation. By contrast, Se/Te ratios and concentrations are within the range of primitive mantle (PM) values, meaning that these elements were not significantly mobilised during serpentinisation.
Although displaying homogenous petrographic and geochemical features, the harzburgites are characterised by extremely heterogeneous ReOs and HSE compositions.
Type-A harzburgites exhibit subchondritic 187Os/188Osi (0.1203–0.1266) and low Re/Os ratios (0.01–0.04). The strong IPGE-PPGE fractionations (PdN/IrN = 0.21–0.56), coupled with positive Pt anomalies and S-Se-Te abundances often below the detection limit, suggest high melt extraction rates, resulting in sulphide consumption and OsRu metal alloy stabilisation.
Type-B harzburgites possess strongly fractionated, Os-Ir-Pt poor (Os = 0.003–0.072 ng/g, Ir = 0.0015–0.079 ng/g) and PdRe enriched patterns, associated with chondritic to suprachondritic measured 187Os/188Os (0.127–0.153). These characters are uncommon for highly depleted mantle residues. Interaction with an oxidised component does not appear as a viable mechanism to account for the IPGE-depleted patterns of type-B harzburgites, as calculated oxygen fugacities are close to the FMQ buffer (Log ΔFMQ = 0.35 to 0.65).
The strikingly uniform mineralogical and geochemical features displayed by both harzburgite sub-types suggest that the different HSE patterns are not linked to their recent evolution, implying that subduction-related processes were superimposed on geochemical heterogeneous mantle domains, which exerted an important control on HSE behaviour during melt extraction and post melting metasomatism.
We propose that the HSE characters of the studied peridotites reflect the presence of a highly heterogeneous mantle source with a long term (>1 Ga) evolution, possibly linked to the Zealandia formation.
•We present a comprehensive ReOs and HSE study for the New Caledonia peridotites.•Melt percolation and low-moderate melt extraction are inferred for the lherzolites•Based on HSE and Os isotopes, two groups of harzburgites (A-B) can be identified.•The signature of type-A harzburgites results from high degrees of melt extraction.•Type-B harzburgites own atypical HSE patterns, unrelated to their recent evolution
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