Qingzhen and Yamato-691: A tentative alphabet for the EH chondrites

Petrological investigations of unequilibrated EH chondrites revealed the presence of three subgroups. They are identified based on the presence of different concentrations of MnS in niningerite. These differences were produced by partitioning of Mn between niningerite and enstatite as a result of different f_S2 and f_O2 during their formation. In order of increasing MnS-contents and hence increasing f_S2 and decreasing f_O2 these groups are : (A) Yamato (Y)-691 and Abee, (B) Indarch, and (C) Yamato-74370,South Oman, Qingzhen, Kota Kota, Kaidun III, and St. Marks. In the third subgroup the meteorites follow an equilibration and evolution sequence; Y-74370 the most primitive and St. Marks the most equilibrated. Y-691 is the most primitive in its subgroup. Differences in the chemical compositions of minerals in Y-691 and Qingzhen reveal a dichotomy in the compositions of niningerite, djerfisherite, kamacite, and perryite. Niningerites in Y-691 contain the least MnS (3.6-6.7 mole%) and counterparts in Qingzhen the most (12-14 mole%). K/Na ratios in djerfisherite are lower in Qingzhen than in Y-691. The Si concentration in kamacite in Qingzhen is higher than in Y-691. Ni in perryite in Qingzhen is higher than in Y-691. Na and K are highly fractionated between two sulfide lithologies. Na resides mainly in chondrules in caswellsilverite, in a Cl-bearing glass in the chondrules, and in Cr-rich sulfides in the matrix. In contrast, K is confined to djerfisherite, which occurs only in sulfide-rich objects in the matrix, and is highly depleted in chondrules. Two new layer structure minerals were discovered in Y-691 : (a) Na-Cu-Cr-sulfide with the general formula (NaCu) CrS_2,and (b) a Na-Cu-Zn-Cr-sulfide. An evolution scheme was constructed for the EH chondrites in the solar nebula and in their parent bodies. Niningerite and oldhamite condensed first and probably acted as nucleation sites for condensing sulfides, metals and silicates. Both minerals are abundant in chondrules, indicating that chondrule formation preceded all other sulfide- and metal-rich objects. For the first time, planetary metamorphic events were recognized. The Qingzhen Reaction, a breakdown of djerfisherite to troilite, covellite, idaite, bornite, and other unidentified phases, was discovered in Qingzhen and Y-691. Thermal episodes took place in the parent bodies at 1.4 Ba (Qingzhen), and 800 Ma (Y-691). Reverse zoning in niningerite indicates that Fe diffused from troilite to niningerite during the thermal event. In Y-691 sphalerite also formed during the metamorphic episode due to mobilization of Zn (and other volatiles). EH chondrites condensed in a chemically inhomogeneous region of the solar nebula where considerable variations in sulfur and oxygen fugacities existed

Petrography of Cape York and Grant: Irons with Simple Pd-Ag Systematics

The presence of excess ^(107)Ag from the decay of extinct ^(107)Pd(t_(1/2) = 6.5 my) has been determined in IVB, IV A and two anomalous irons (Kaiser and Wasserburg, 1983). Excesses of ^(107)Ag in groups IIIAB and IIB meteorites, Cape York and Grant, and Derrick Peak, respectively, show the widespread presence of ^(107)Pd in the early solar system (Chen and Wasserburg, 1983)

Relaxation in the glass-former acetyl salicylic acid studied by deuteron magnetic resonance and dielectric spectroscopy

Supercooled liquid and glassy acetyl salicylic acid was studied using dielectric spectroscopy and deuteron relaxometry in a wide temperature range. The supercooled liquid is characterized by major deviations from thermally activated behavior. In the glass the secondary relaxation exhibits the typical features of a Johari-Goldstein process. Via measurements of spin-lattice relaxation times the selectively deuterated methyl group was used as a sensitive probe of its local environments. There is a large difference in the mean activation energy in the glass with respect to that in crystalline acetyl salicylic acid. This can be understood by taking into account the broad energy barrier distribution in the glass.Comment: 8 pages, 3 figures, Submitted to Phys. Rev.

The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: implications for the structure and composition of asteroid 2008 TC 3

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC3). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC3 was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC3. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ18O = 13.53‰, δ17O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH− in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC3 consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm−3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm−3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC3 in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands

Links Between Hydrothermal Environments, Pyrophosphate, Na+, and Early Evolution

The discovery that photosynthetic bacterial membrane-bound inorganic pyrophosphatase (PPase) catalyzed light-induced phosphorylation of orthophosphate (Pi) to pyrophosphate (PPi) and the capability of PPi to drive energy requiring dark reactions supported PPi as a possible early alternative to ATP. Like the proton-pumping ATPase, the corresponding membrane-bound PPase also is a H+-pump, and like the Na+-pumping ATPase, it can be a Na+-pump, both in archaeal and bacterial membranes. We suggest that PPi and Na+ transport preceded ATP and H+ transport in association with geochemistry of the Earth at the time of the origin and early evolution of life. Life may have started in connection with early plate tectonic processes coupled to alkaline hydrothermal activity. A hydrothermal environment in which Na+ is abundant exists in sediment-starved subduction zones, like the Mariana forearc in the W Pacific Ocean. It is considered to mimic the Archean Earth. The forearc pore fluids have a pH up to 12.6, a Na+-concentration of 0.7 mol/kg seawater. PPi could have been formed during early subduction of oceanic lithosphere by dehydration of protonated orthophosphates. A key to PPi formation in these geological environments is a low local activity of water

Meteorit. Planet. Sci.

Opaque minerals in the Qingzhen (EH3) and MacAlpine Hills (MAC) 88136 (EL3) enstatite chondrites were studied and compared with other EH and EL chondrites. All opaque minerals usually occur in multi-sulfide-metal clasts and nodules in the matrix between chondrules (El Goresy et al., 1988). The higher abundance of opaque minerals, the occurrence of niningerite and various alkali-sulfides (e.g., caswellsilverite, phases A and B, djerfisherite) are diagnostic criteria for EH chondrites, while alabandite is characteristic for EL chondrites. In addition, EH chondrites are characterized by enrichments of Si in both kamacite and perryite, and alkali elements in sphalerite and chalcopyrite. The Mn contents of daubreelite and sphalerite are lower in EH than in EL chondrites. These are consistent with lower oxygen fugacity and higher H2S fugacity of EH than EL chondrites. In contrast, the discovery of sphalerite and Zn- rich daubreelite in MAC 88136 indicates that their absence in EL6 chondrites is probably related to thermal metamorphism in the parent body. Schreibersite microspherules are commonly enclosed in most sulfides in Qingzhen, but are absent in MAC 88136. They were once molten, and probably predated all sulfide host phases. The petrographic setting and chemical compositions of the sulfide hosts of the schreibersite microspherules in EH3 chondrites are consistent with formation by condensation. The earliest sulfide condensates oldhamite and niningerite occupy the interiors of the clasts and nodules, whereas the rims consist of troilite and djerfisherite. In addition, in Qingzhen, some other troilite, djerfisherite and sphalerite assemblages coexist with perryite. They were produced by sulfurization of metallic Fe-Ni in the nebula. In MAC 88136, sulfurization of Si-bearing Fe-Ni metal is less pronounced, and it produced troilite, schreibersite and less abundant perrylte. Two kinds of normal zoning and a reverse zoning trends of niningerite, and both normal and reverse zoning of sphalerite were found in clasts and nodules in Qingzhen. The coexistence of normal and reverse zoning profiles in niningerite grains in the same meteorite strongly suggests that they formed before accretion in the parent body, because an asteroidal metamorphic or an impact event in the parent body would have erased these contrasting profiles and destroyed the textural settings. In contrast, alabandite in MAC 88136 shows only normal zoning, with the FeS content decreasing to 9.3 mol% toward troilite, indicating very slow cooling at low temperature