The Rumuruti chondrite group

Since 1994, the Rumuruti (R) chondrites have been recognized as a new, well-established chondrite group differing from carbonaceous, ordinary, and enstatite chondrites. The first R chondrite, Carlisle Lakes, was found in Australia in 1977. Meanwhile, the number has increased to 107 (December, 2010). This group is named after the Rumuruti meteorite, the first and so far the only R chondrite fall. Most of the R chondrites are breccias containing a variety of different clasts embedded in a clastic matrix. Some textural and mineralogical characteristics can be summarized as follows: (a) the chondrule abundance in large fragments and in unbrecciated rocks is ∼35–50 vol%; (b) Ca,Al-rich inclusions are rare; (c) the olivine abundance is typically 65–78 vol%; (d) the mean chondrule diameter is ∼400 μm; (e) in unequilibrated R chondrites, low-Ca pyroxene is dominating, whereas in equilibrated R chondrites it is Ca-rich pyroxene; (f) the typical olivine in a metamorphosed lithology is ∼Fa_38–40; (g) matrix olivine in unequilibrated, type 3 fragments and rocks has much higher Fa (∼45–60 mol%) compared to matrix olivines in type 4–6 lithologies (∼Fa_38–41); (h) spinels have a high TiO₂ of ∼5 wt%; (i) abundant different noble metal-bearing phases (metals, sulfides, tellurides, arsenides) occur. The exception is the metamorphosed, type 5/6 R chondrite La Paz Icefield 04840 which contains hornblende, phlogopite, and Ca-poor pyroxene, the latter phase typically occurring in low-grade metamorphosed R chondrites only.In bulk composition, R chondrites have some affinity to ordinary chondrites: (a) the absence of significant depletions in Mn and Na in R chondrites and ordinary chondrites is an important feature to distinguish these groups from carbonaceous chondrites; (b) total Fe (∼24 wt%) of R chondrites is between those of H and L chondrites (27.1 and 21.6 wt%, respectively); (c) the average CI/Mg-normalized lithophile element abundances are ∼0.95 × CI, which is lower than those for carbonaceous chondrites (≥1.0 × CI) and slightly higher than those for ordinary chondrites (∼0.9 × CI); (d) trace element concentrations such as Zn (∼150 ppm) and Se (∼15 ppm) are much higher than in ordinary chondrites; (e) the whole rock Δ¹⁷O of ∼2.7 for R chondrites is the highest among all meteorite groups, and the mean oxygen isotope composition is δ¹⁷O = 5.36 ± 0.43, δ¹⁸O = 5.07 ± 0.86, Δ¹⁷O = +2.72 ± 0.31; (f) noble gas cosmic ray exposure ages of R chondrites range between ∼0.1 and ∼70 Ma. More than half of the R chondrites analyzed for noble gases contain implanted solar wind and, thus, are regolith breccias. The 43 R chondrites from Northern Africa analyzed so far for noble gases seem to represent at least 16 falls. Although the data base is still scarce, the data hint at a major collision event on the R chondrite parent body between 15 and 25 Ma ago.     

Mass Fractions of S,Cu, Se,Mo, Ag,Cd, In,Te, Ba,Sm, W,Tl and Bi in Geological Reference Materials and Selected Carbonaceous Chondrites Determined by Isotope Dilution ICP‐MS

Mass fractions of S, Cu, Se, Mo, Ag, Cd, In, Te, Ba, Sm, W and Tl were determined by isotope dilution sector field ICP‐MS in the same sample aliquot of reference materials using HF‐HNO₃ digestion in PFA beakers in pressure bombs and glassy carbon vessels in a high‐pressure asher (HPA‐S) for comparison. Additionally, Bi was determined by internal standardisation relative to Tl. Because isobaric and oxide interferences pose problems for many of these elements, efficient chromatographic separation methods in combination with an Aridus desolvator were employed to minimise interference effects. Repeated digestion and measurement of geological reference materials (BHVO‐1, BHVO‐2, SCo‐1, MAG‐1, MRG‐1 and UB‐N) gave results with < 5% relative intermediate precision (1s) for most elements, except Bi. Replicates of NIST SRM 612 glass digested on a hot plate were analysed by the same methods, and the results agree with reference values mostly within 2% relative deviation. Data for the carbonaceous chondrites Allende, Murchison, Orgueil and Ivuna are also reported. Digestion in a HPA‐S was as efficient as in pressure bombs, but some elements displayed higher blank levels following HPA‐S treatment. Pressure bomb digestion yielded precise data for volatile S, Se and Te, but may result in high blanks for W.     

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CB_b chondrite Hammadah al Hamra 237 and none were observed in the CB_a chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa_<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always ¹⁶O-rich (δ^17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly ¹⁶O-rich or ¹⁶O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either ¹⁶O-rich (δ^17,18O∼−40‰) or ¹⁶O-poor (δ^17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in ¹⁶O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in ¹⁶O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with ¹⁶O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in ¹⁶O-poor gaseous reservoirs or coated by ¹⁶O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.     

Origin of Halogens and Nitrogen in Enstatite Chondrites

The EH and EL enstatite chondrites are the most reduced chondrite groups, having formed in nebular regions where the gas may have had high C/O and/or pH₂/pH₂O ratios. Enstatite chondrites (particularly EH) have higher CI- and Mg-normalized abundances of halogens (especially F and Cl) and nitrogen than ordinary chondrites and most groups of carbonaceous chondrites. Even relative to CI chondrites, EH and EL chondrites are enriched in F. We have found that literature values for the halogen abundance ratios in EH and EL chondrites are strongly correlated with the electronegativities of the individual halogens. We suggest that the most reactive halogens were the most efficient at forming compounds (e.g., halides) that were incorporated into EH-chondrite precursor materials. It seems plausible that, under the more-oxidizing conditions pertaining to the other chondrite groups, a larger fraction of the halogens remained in the gas. Nitrogen may have been incorporated into the enstatite chondrites as simple nitrides that did not condense under the more-oxidizing conditions in the regions where other chondrite groups formed. Literature data show that unequilibrated enstatite chondrites have light bulk N (δ ¹⁵N ≈ −20‰) compared to most ordinary (−5 to +20‰) and carbonaceous (+20 to +190‰) chondrites; this may reflect the contribution in enstatite chondrites of nitride condensates with δ¹⁵ N values close to the proposed nebular mean (~−400‰). In contrast, N in carbonaceous chondrites is mainly contained within ¹⁵N-rich organic matter. The major carrier of N in ordinary chondrites is unknown.     

Mineralogy,petrography, and oxygen isotopic compositions of ultrarefractory inclusions from carbonaceous chondrites

We report on the mineralogy, petrography, and in situ oxygen isotopic composition of twenty-five ultrarefractory calcium-aluminum-rich inclusions (UR CAIs) in CM2, CR2, CH3.0, CV3.1–3.6, CO3.0–3.6, MAC 88107 (CO3.1-like), and Acfer 094 (C3.0 ungrouped) carbonaceous chondrites. The UR CAIs studied are typically small, < 100 μm in size, and contain, sometimes dominated by, Zr-, Sc-, and Y-rich minerals, including allendeite (Sc₄Zr₃O₁₂), and an unnamed ((Ti,Mg,Sc,Al)₃O₅) mineral, davisite (CaScAlSiO₆), eringaite (Ca₃(Sc,Y,Ti)₂Si₃O₁₂), kangite ((Sc,Ti,Al,Zr,Mg,Ca,□)₂O₃), lakargiite (CaZrO₃), warkite (Ca₂Sc₆Al₆O₂₀), panguite ((Ti,Al,Sc,Mg,Zr,Ca)_1.8O₃), Y-rich perovskite ((Ca,Y)TiO₃), tazheranite ((Zr,Ti,Ca)O_2−x), thortveitite (Sc₂Si₂O₇), zirconolite (orthorhombic CaZrTi₂O₇), and zirkelite (cubic CaZrTi₂O₇). These minerals are often associated with 50–200 nm-sized nuggets of platinum group elements. The UR CAIs occur as: (i) individual irregularly-shaped, nodular-like inclusions; (ii) constituents of unmelted refractory inclusions – amoeboid olivine aggregates (AOAs) and Fluffy Type A CAIs; (iii) relict inclusions in coarse-grained igneous CAIs (forsterite-bearing Type Bs and compact Type As); and (iv) relict inclusions in chondrules. Most UR CAIs, except for relict inclusions, are surrounded by single or multilayered Wark-Lovering rims composed of Sc-rich clinopyroxene, ±eringaite, Al-diopside, and ±forsterite. Most of UR CAIs in carbonaceous chondrites of petrologic types 2–3.0 are uniformly ¹⁶O-rich (Δ¹⁷O ∼ −23‰), except for one CH UR CAI, which is uniformly ¹⁶O-depleted (Δ ¹⁷O ∼ −5‰). Two UR CAIs in Murchison have heterogeneous Δ¹⁷O. These include: an intergrowth of corundum (∼ ‒24‰) and (Ti,Mg,Sc,Al)₃O₅ (∼ 0‰), and a thortveitite-bearing CAI (∼ −20 to ∼ ‒5‰); the latter apparently experienced incomplete melting during chondrule formation. In contrast, most UR CAIs in metamorphosed chondrites are isotopically heterogeneous (Δ¹⁷O ranges from ∼ −23‰ to ∼ −2‰), with Zr- and Sc-rich oxides and silicates, melilite and perovskite being ¹⁶O-depleted to various degrees relative to uniformly ¹⁶O-rich (Δ¹⁷O ∼ −23‰) hibonite, spinel, Al-diopside, and forsterite. We conclude that UR CAIs formed by evaporation/condensation, aggregation and, in some cases, melting processes in a ¹⁶O-rich gas of approximately solar composition in the CAI-forming region(s), most likely near the protoSun, and were subsequently dispersed throughout the protoplanetary disk. One of the CH UR CAIs formed in an ¹⁶O-depleted gaseous reservoir providing an evidence for large variations in Δ¹⁷O of the nebular gas in the CH CAIs-forming region. Subsequently some UR CAIs experienced oxygen isotopic exchange during melting in ¹⁶O-depleted regions of the disk, most likely during the epoch of chondrule formation. In addition, UR CAIs in metamorphosed CO and CV chondrites, and, possibly, the corundum-(Ti,Mg,Sc,Al)₃O₅ intergrowth in Murchison experienced O-isotope exchange with aqueous fluids on the CO, CV, and CM chondrite parent asteroids. Thus, both nebular and planetary exchange with ¹⁶O-depleted reservoirs occurred.     

Clast sizes of ejecta from explosive eruptions on asteroids: implications for the fate of the basaltic products of differentiation

We use a simple model of the formation, growth, coalescence and migration of veins of basaltic melt generated by partial melting in chondritic asteroids to deduce the sizes of, and pressures within, the fluid-filled dikes reaching the surfaces of such bodies. The gas contents ( 1000 ppm of mainly CO and N₂) of the asteroids were high enough that bubbles of free gas trapped in the melt veins gave the basaltic melts significant buoyancy; expansion of these gases as a dike opened to the vacuum at the surface led to fragmentation of the melts into liquid droplets which were transported upwards by the accelerating gases to the surface. The sizes of these droplets and, hence, of the pyroclastic glass beads into which they cooled, are calculated to lie in the range 30 μm to 4 mm; this range is essentially independent of the size or gas content of the asteroid parent and only weakly dependent on the internal pressure of the erupting fluid. The fate of the pyroclasts, however, does depend on all of these factors. At very low internal pressures, significant separation of the gas and liquid in a rising dike may take place and not all of the liquid will be expelled from the dike when it opens to the surface. For relatively large ( 100 km radius) asteroids with relatively low ( 300 ppm) gas contents, the larger clasts are too heavy to be lifted from the level at which magma fragmentation takes place by the gas flow and so would also remain behind to form basaltic veins. The apparent absence of basaltic veins in meteorites then implies both that internal pressures in near-surface dikes were generally greater than 0.3 MPa and that low gas contents were not common. Finally, as long as pyroclasts are lofted from the magma fragmentation level, they will be accelerated to at least 90% of the final gas speed. If this speed exceeds the escape speed from the asteroid (as happens readily for high gas contents and small asteroids), the pyroclasts will be expelled into space and lost from the meteorite record. Otherwise (low gas contents or large asteroids), they will eventually fall back to be incorporated into the surface regolith, modifying the chemical and physical properties of meteorites subsequently derived from it.     

Compositions of four unusual CM or CM-related Antarctic chondrites

To identify chemical group affinities and infer the occurrence of thermal metamorphism or aqueous alteration in their histories, we quantified 43 trace elements in the CM or CM-related Antarctic carbonaceous chondrites EET 96010, LAP 02277, MET 01070, and WIS 91600. We also analyzed LAP 02206, a CV chondrite, to add to our comparison database. We present whole-rock oxygen isotope data for LAP 02206, LAP 02277, and MET 01070 to complement our trace element results. With these data, we confirm the CV classification of LAP 02206 and CM or CM-like classification for the other four chondrites in this study. On the basis of moderately volatile element content, our results show that EET 96010 experienced open-system heating, while any heating LAP 02277 and MET 01070 may have experienced was in a chemically closed-system. WIS 91600, on a trace element basis, appears to be CM-like material. Our analyses support the idea that CM material has experienced a wide variety of post-accretionary processing.     

Identification of stable remanence carriers through a magneto-impedance scanning magnetic microscope

To identify the stable remanence carrier in rock samples, we conducted magnetic microscopic observations combined with conventional stepwise demagnetization experiments. The instrument, which employs an amorphous wire-based magneto-impedance sensor (30 μm diameter, 5 mm length), can document magnetic anomalies (vertical component) of the millimeter to sub-millimeter-thick rock samples with a resolution of 500 μm. Our new technique allows identification of the sources of both stable and unstable remanence components in meteorite and shocked granite samples. However, stray magnetic fields from the sensor magnetize the magnetic minerals in the sample and makes serious artifacts on the magnetic images. Although the artifacts of the induced magnetization should be solved, this new corroborative technique leads to a microscopic discrimination of stable paleomagnetic records from terrestrial and extraterrestrial materials.     

A unified model for hydrogen in the Earth and Moon: No one expects the Theia contribution

The Moon is thought to have formed after a planetary embryo, known as Theia, collided with the proto-Earth 4.5 billion years ago. This so-called Giant Impact was the last major event during Earth’s accretion, and its effects on the composition of the Earth and the newly forming Moon would be measureable today. Recent work on lunar samples has revealed that the Moon’s water was not lost as a result of this giant impact. Instead, the Moon appears to contain multiple hydrogen reservoirs with diverse deuterium-to-hydrogen (D/H) ratios. For the first time, we incorporate hydrogen isotopic measurements of lunar samples to help constrain the composition of Theia. We show that the Moon incorporated very low-D/H (δD ≈ -750‰) materials that only could have derived from solar nebula H₂ ingassed into the magma ocean of a large (∼0.4 M_E) planetary embryo that was largely devoid of chondritic water. We infer Theia was a very large body comparable in size to the proto-Earth, and was composed almost entirely of enstatite chondrite-like material. These conclusions limit the type of impact to a “merger” model of similarly-sized bodies, or possibly a “hit-and-run” model, and they rule out models that mix isotopes too effectively.     

Occurrence and origin of keilite, (Fe>0.5,Mg<0.5)S, in enstatite chondrite impact-melt rocks and impact-melt breccias

Keilite (Fe_>0.5,Mg_<0.5)S, the iron-dominant cubic analog of niningerite, (Mg_>0.5,Fe_<0.5)S, occurs in enstatite chondrites [Shimizu, M., Yoshida, H., Mandarino, J.A., 2002. The new mineral species keilite, (Fe,Mg)S, the iron-dominant analog of niningerite. Can. Mineral. 40, 1687–1692]. I find that keilite occurs only in enstatite chondrite impact-melt rocks and impact-melt breccias. Based on the phase relations in the system MgS–MnS–CaS–FeS [Skinner, B.J., Luce, F.D., 1971. Solid solutions of the type (Ca,Mg,Mn,Fe)S and their use as geothermometers for the enstatite chondrites. Am. Mineral. 56, 1269–1296], I conclude that keilite formed from niningerite or alabandite (Mn_>0.5,Fe_<0.5)S by reaction with troilite (FeS) at elevated temperatures of well above 500 °C (the lowest equilibration temperature of keilite), but it is likely that the maximum temperatures during melting experienced by keilite-bearing impact-melt rocks and impact-melt breccias were considerably higher, perhaps >1500 °C, as indicted by the occurrence of euhedral enstatite that formed from a melt [McCoy, T.J., Dickinson, T.L., Lofgren, G.E., 1999. Partial melting of the Indarch (EH4) meteorite: a textural, chemical, and phase relations view of melting and melt migration. Meteorit. Planet. Sci. 34, 735–746]. Based on the classifications of the keilite-bearing meteorites as impact-melt rocks and impact-melt breccias and my own textural observations, I conclude that this elevated temperature was reached as a result of impact and not internal heating and melting, followed by fast cooling, thus, quenching in keilite. Enstatite chondrite impact-melt rocks and impact-melt breccias that do not contain keilite may have been more deeply buried after impact and, hence, cooled slowly and were annealed so that FeS exsolved from keilite, concomitant with the formation of niningerite, alabandite or various (Mn,Mg,Fe) mixed sulfides.