Mineralogy,chemistry, and noble gas contents of Adzhi-Bogdo– an LL3–6 chondritic breccia with L-chondritic and granitoidal clasts

Abstract— Adzhi-Bogdo is an ordinary chondrite regolith breccia (LL3–6) which fell on 1949 October 30 in Gobi Altay, Mongolia. The rock consists of submm- to cm-sized fragments embedded in a fine-grained clastic matrix. The polymict breccia contains various types of clasts, some of which must be of foreign origin. Components of the breccia include chondrules, melt rock clasts (some of which are K-rich), highly recrystallized rock fragments (“granulites”), breccia clasts, pyroxene-rich fragments with achondritic textures, and alkali-granitoids. The composition of olivine in most fragments is in the range of LL-chondrites. However, olivine in some components has significantly lower fayalite contents, characteristic of L-chondrites. The bulk meteorite is very weakly shocked (S2). Based on the bulk chemical composition, Adzhi-Bogdo is an ordinary chondrite. The concentrations of Fe and Ni are somewhat intermediate between L- and LL-chondrites. The contents of solar gases indicate that Adzhi-Bogdo is a regolith breccia. Most of the solar He and probably a part of the solar Ne of Adzhi-Bogdo has been lost. It is suggested that Adzhi-Bogdo experienced an (impact-induced) thermal event early in its history, because most of the radiogenic ⁴⁰Ar is retained.     

Mineralogy and chemistry of Rumuruti: The first meteorite fall of the new R chondrite group

Abstract— The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine-rich chondritic breccia with light-dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa_～39) with about 70 vol%. Feldspar (～14 vol%; mainly plagioclase), Ca-pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low-Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni-rich and Ge-containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE, etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ¹⁷O (5.52 ‰) and δ¹⁸O (5.07 ‰). Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes-type chondrites and they comprise: Carlisle Lakes, ALH85151, Y-75302, Y-793575, Y-82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni-metals, occurrence of chalcopyrite); (c) exceptionally high Δ¹⁷O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (～70 vol%); (e) Ti-Fe³⁺?rich chromite (～5.5 wt% TiO₂); (f) occurrence of various noble metal-rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies. With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R chondrites and ordinary chondrites exclude formation of R chondrites from ordinary chondrites by oxidation. This implies a separate, independent R chondrite parent body.     

Hf-W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals

Application of ¹⁸²Hf-¹⁸²W chronometry to constrain the duration of early solar system processes requires the precise knowledge of the initial Hf and W isotope compositions of the solar system. To determine these values, we investigated the Hf-W isotopic systematics of bulk samples and mineral separates from several Ca,Al-rich inclusions (CAIs) from the CV3 chondrites Allende and NWA 2364. Most of the investigated CAIs have relative proportions of ¹⁸³W, ¹⁸⁴W, and ¹⁸⁶W that are indistinguishable from those of bulk chondrites and the terrestrial standard. In contrast, one of the investigated Allende CAIs has a lower ¹⁸⁴W/¹⁸³W ratio, most likely reflecting an overabundance of r-process relative to s-process isotopes of W. All other bulk CAIs have similar ¹⁸⁰Hf/¹⁸⁴W and ¹⁸²W/¹⁸⁴W ratios that are elevated relative to average carbonaceous chondrites, probably reflecting Hf-W fractionation in the solar nebula within the first ∼3 Myr. The limited spread in ¹⁸⁰Hf/¹⁸⁴W ratios among the bulk CAIs precludes determination of a CAI whole-rock isochron but the fassaites have high ¹⁸⁰Hf/¹⁸⁴W and radiogenic ¹⁸²W/¹⁸⁴W ratios up to ∼14 ε units higher than the bulk rock. This makes it possible to obtain precise internal Hf-W isochrons for CAIs. There is evidence of disturbed Hf-W systematics in one of the CAIs but all other investigated CAIs show no detectable effects of parent body processes such as alteration and thermal metamorphism. Except for two fractions from one Allende CAI, all fractions from the investigated CAIs plot on a single well-defined isochron, which defines the initial ε¹⁸²W = −3.28 ± 0.12 and ¹⁸²Hf/¹⁸⁰Hf = (9.72 ± 0.44) × 10⁻⁵ at the time of CAI formation. The initial ¹⁸²Hf/¹⁸⁰Hf and ²⁶Al/²⁷Al ratios of the angrites D’Orbigny and Sahara 99555 are consistent with the decay from initial abundances of ¹⁸²Hf and ²⁶Al as measured in CAIs, suggesting that these two nuclides were homogeneously distributed throughout the solar system. However, the uncertainties on the initial ¹⁸²Hf/¹⁸⁰Hf and ²⁶Al/²⁷Al ratios are too large to exclude that some ²⁶Al in CAIs was produced locally by particle irradiation close to an early active Sun. The initial ¹⁸²Hf/¹⁸⁰Hf of CAIs corresponds to an absolute age of 4568.3 ± 0.7 Ma, which may be defined as the age of the solar system. This age is 0.5-2 Myr older than the most precise ²⁰⁷Pb-²⁰⁶Pb age of Efremovka CAI 60, which does not seem to date CAI formation. Tungsten model ages for magmatic iron meteorites, calculated relative to the newly and more precisely defined initial ε¹⁸²W of CAIs, indicate that core formation in their parent bodies occurred in less than ∼1 Myr after CAI formation. This confirms earlier conclusions that the accretion of the parent bodies of magmatic iron meteorites predated chondrule formation and that their differentiation was triggered by heating from decay of abundant ²⁶Al. A more precise dating of core formation in iron meteorite parent bodies requires precise quantification of cosmic-ray effects on W isotopes but this has not been established yet.     

Landslide deposits as stratigraphical markers for a sequence‐based glacial stratigraphy: a case study of a Younger Dryas system in the Eastern Alps

Detailed ¹⁰Be and ¹⁴C dating and supporting pollen analysis of Alpine Lateglacial glacial and landslide deposits in the Hohen Tauern Mountains (Austria) constrain a sequence‐based stratigraphy comprising a major landslide (13.0±1.1 ka) overlain by till and termino‐lateral moraines of an advancing (12.6±1.0 ka) and retreating (11.3±0.8 ka) glacier in turn overlain by a minor landslide (10.8±1.1 ka). These results define glacier activity during the Younger Dryas age Egesen stadial bracketed by landslide activities during the Bølling‐Allerød interstadial and the Preboreal. In contrast to recent studies on Holocene glaciation in the Alps, no traces of any Holocene glacier advance bigger than during the Little Ice Age are documented. Furthermore, this study demonstrates the advantages of using an allostratigraphical approach based on unconformity‐bounded sedimentary units as a tool for glacial stratigraphy in formerly glaciated mountain regions, rather than a stratigraphy based on either isolated morphological features or lithostratigraphical characteristics.     

The formation of eucrites: Constraints from metal‐silicate partition coefficients

Abstract— This study explores the controls of oxygen fugacity and temperature on the solubilities of Fe, Ni, Co, Mo, and W in natural eucritic liquids to better constrain the formation of eucritic melts. The solubilities of all five elements in molten silicate in equilibrium with FeNiCo‐, FeMo‐, and FeW‐ alloys increase with increasingly oxidizing conditions and decrease with decreasing temperatures. In applying these data to formation scenarios of the eucrite parent body, we find that the siderophile element abundances in eucrites (meteoritic basalts) cannot be explained by a single‐step partialmelting process from a chondritic, metal‐containing source. The Ni content of the partial melt is too high, and the W and Mo contents are too low compared to the abundances in eucritic meteorites. But Fe, Ni, and Co concentrations in eucrites can be modeled by metal‐silicate equilibrium during more or less complete melting of the eucrite parent body with subsequent fractional crystallization of olivine and orthopyroxene. However, the computed values of Mo are still too low and those of W too high when compared with Mo and W abundances in eucritic meteorites. One possibility is that the Mo and W partition coefficients strongly depend on pressure, although the howardite‐eucrite‐diogenite (HED) parent body only had a minimal pressure gradient (maximum interior pressure = 0.1 GPa). Alternatively, sulfides may have played some role in establishing Mo abundances.     

Yamato 792947, 793408 and 82038: The most primitive H chondrites,with abundant refractory inclusions

Abstract— In this paper we report petrological and chemical data of the unusual chondritic meteorites Yamato (Y)‐792947, Y‐93408 and Y‐82038. The three meteorites are very similar in texture and chemical composition, suggesting that they are pieces of a single fall. The whole‐rock oxygen isotopes and the chemical compositions are indicative of H chondrites. In addition, the mineralogy, and the abundances of chondrule types, opaque minerals and matrices suggest that these meteorites are H3 chondrites. They were hardly affected by thermal and shock metamorphism. The degree of weathering is very low. We conclude that these are the most primitive H chondrites, H3.2–3.4 (S1), known to date. On the other hand, these chondrites contain extraordinarily high amounts of refractory inclusions, intermediate between those of ordinary and carbonaceous chondrites. The distribution of the inclusions may have been highly heterogeneous in the primitive solar nebula. The mineralogy, chemistry and oxygen isotopic compositions of inclusions studied here are similar to those in CO and E chondrites.     

Evidence for fractional condensation and reprocessing at high temperatures in CH chondrites

Abstract— We performed a detailed study of silica‐rich components (SRC) in the paired CH chondrites Acfer 182 and 207. These SRCs appear either as chondrules or fragments, and they contribute <0.1 vol% to the bulk meteorite. They usually contain a silica and a silicate portion. Both portions are, in most cases, cryptocrystalline and have bulk SiO₂‐concentrations between 65 and 85 wt%. The silicate generally has a pyroxene normative composition. The silica often appears as blebs within the silicate matrix or vice versa. If there are no blebs, silica and silicate still form rounded interfaces. The SRCs are depleted in refractory elements like Ca, Al, and Ti relative to CI. A few SRC‐like objects are extremely rich in Mn and show no depletion in refractory elements. We conducted micro‐Raman studies on the silica portions of the SRCs to determine their structure, and we identified several silica phases: α‐quartz, cristobalite, glass, and a yet unidentified polymorph. The silicate portion is glass when the silica is glass and crystalline when the silica is crystalline. The low contents of Al and Ca make an igneous origin of the SRCs very unlikely, and the absence of metal excludes the formation by reduction of pyroxene. We suggest, instead, a fractional condensation origin of the SRCs from a Si‐enriched gas after removal of gaseous Mg by forsterite condensation. Additional evidence for fractional condensation is provided by a unique layered object with olivine in the core, pyroxene and metal at the rim, and silica at the outermost border; these layers record the condensation sequence. Two chondrules were found with several percent of Mn and high Cr, Na, and K contents, providing further evidence for condensation from a fractionated gas. The texture of the SRCs and the occurrence of cristobalite and silica glass, however, require formation by liquid immiscibility at high temperatures, above 1968 K, and subsequent fast cooling. Therefore, we propose a 2‐stage model for the formation of SRCs in CH chondrites: 1) fractional condensation of forsterite, enstatite, and SiO₂‐rich phases; and 2) reheating of SiO₂‐rich components to temperatures above 1968 K followed by rapid cooling. All other phases identified in CH chondrites can be understood within the framework of this model. Thus, the extremely unequilibrated CH chondrites provide a wealth of evidence for fractional condensation processes in the early solar nebula, in metals (Meibom et al. 1999), and in silicates.     

10Be surface exposure dating reveals unexpected high deformation rates in the central Andean wedge interior

Understanding deformation associated with active thrust wedges is essential to evaluate seismic hazards. In this study, we investigate the spatial distribution, timing, and rates of deformation across the central Andean backarc of Argentina, where deformation and shortening have been assumed to occur within a narrow wedge‐front zone. The combined results of our geomorphic mapping, topographic surveying, and ¹⁰Be exposure dating demonstrate that fault activity related to the growth of the Andean orogenic wedge is not only limited to a narrow thrust front zone but also occurs in the Andean wedge interior. There, internal structures with deformation rates of ca. 1.3 mm/year have been active during the last ~140 ka. Widely distributed deformation implies that seismic hazards may have been underestimated in the internal part of the Andean orogen.     

Tungsten isotopes in ferroan anorthosites: Implications for the age of the Moon and lifetime of its magma ocean

New W isotope data for ferroan anorthosites 60025 and 62255 and low-Ti mare basalt 15555 show that these samples, contrary to previous reports [Lee, D.C., et al., 1997. Science 278, 1098-1103; Lee, D.C., et al., 2002. Earth Planet. Sci. Lett. 198, 267-274], have a W isotope composition that is indistinguishable from KREEP and other mare basalts. This requires crust extraction on the Moon later than ∼60 Myr after CAI formation, consistent with ¹⁴⁷Sm-¹⁴³Nd ages for ferroan anorthosites. The absence of ¹⁸²Hf-induced ¹⁸²W variations in the Moon is consistent with formation of the Moon at after CAI formation that has been inferred based on the indistinguishable ¹⁸²W/¹⁸⁴W ratios of the bulk Moon and the bulk silicate Earth. The uncertainties on the age of the Moon and the age of the oldest lunar samples currently hamper a precise determination of the duration of magma ocean solidification and are consistent with both an almost immediate crystallization and a more protracted timescale of ∼100 Myr.     

Energy dependence of Ti/Fe ratio in the Galactic cosmic rays measured by the ATIC-2 experiment

Titanium is a rare, secondary nucleus among Galactic cosmic rays. Using the Silicon matrix in the ATIC experiment, Titanium has been separated. The energy dependence of the Ti to Fe flux ratio in the energy region from 5 GeV per nucleon to about 500 GeV per nucleon is presented. The article was translated by the authors.