Antarctic meteorite research 

About: Antarctic meteorite research is an academic journal. The journal publishes majorly in the area(s): Chondrite & Meteorite. It has an ISSN identifier of 1343-4284. Over the lifetime, 173 publications have been published receiving 2714 citations.

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

Abstract: 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 f82 and f o2 during their formation. In order of increasing MnS-contents and hence increasingfs2 and decreasingfo2 these groups are: (A) Yamato (Y)-691 and Abee, (B) Indarch, and (C) Yamato-74370, South Oman, Qingzhen, Kata Kata, Kaidun Ill, 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 equili­ brated. 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 com­ positions 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) CrS2 , 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 pro­ bably acted as nucleation sites for condensing sulfides, metals and silicates. Both minerals are abundant in chondrules, indicating that chondrule formation preceded all other sulfideand 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.

Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism

Abstract: A petrographic and mineralogical study of Yamato-86720 (Y-86720) shows that it may be a CM carbonaceous chondrite that has experienced a con­ siderably different alteration history from most CM chondrites. Y-86720 has an unusually high abundance of troilite ( 9 vol%); most troilite occurs in submicron to micron grains dispersed throughout the meteorite. Ca-Mg carbonates and minor Fe-Ni metal, mostly taenite, are present, but magnetite and PCP are absent. This meteorite contains chondrules and aggregates that were completely replaced by optically translucent materials, presumably phyllosilicates. Thus Y-86720 was probably extensively affected by aqueous alteration; the degree of alteration may be the highest of the CM chondrites. Matrix consists largely of fine grains of Mg-Fe olivine, a nearly amorphous Si­ Mg-Fe-rich material, and an Fe-rich material; the latter may be ferrihydrite. Phyllosilicates are rare. The textures suggest that the olivine and the Si-Mg-Pe­ rich material were produced by alteration of phyllosilicates by heating. The replacement products of chondrules and aggregates show lower (Mg+Fe)/(Si+ Al) ratios than serpentine and relatively high Na contents (up to 2.0 wt% as Na20), suggesting the presence of not only serpentine but another type of phyllosilicate. However, the replacement products show consistently high analytical totals; thus they may have been dehydrated and partially altered to anhydrous phases. These results suggest that Y-86720 has experienced mild thermal metamorphism after the aqueous alteration; it appears to have been heated above 500C in a reduced condi­ tion. Most fine-grained troilite probably segregated from matrix during the metamorphic process. Many of the troilites are replaced by a ferrihydrite-like material; thus, Y-86720 may have been affected by additional mild aqueous altera­ tion after the thermal metamorphism.