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SUMMARY:Ivanova et al. (2025) Geochemistry 85\, 126319
DTSTART;VALUE=DATE-TIME:20260907T070000Z
DTEND;VALUE=DATE-TIME:20260907T080000Z
UID:271193420922
DESCRIPTION:文献：Ivanova et al. (2025) Geochemistry 85\, 126319題目
 ：Metamorphism and aqueous alteration of the unusual CY carbonaceous chon
 drite Northwest Africa 4757要旨：The first CY chondrite from the Northw
 est Africa region was studied (NWA 4757). It is a small (5 g) fine-grained
  monomict microbreccia consisting of abundant matrix (∼95 vol%) and rare
  pseudomorphic chondrules (up to 200 μm). The meteorite has main characte
 ristics of CY chondrites. The oxygen isotopic composition of NWA 4757 is 1
 6O-poor (δ18O = 23.83\, δ17O = 12.84\, Δ17O = 0.45 to δ18O = 26.96\, 
 δ17O = 14.50\, Δ17O = 0.48 ± 0.03) which is the heaviest among other CY
  chondrites. NWA 4757 contains abundant sulfides (∼20 vol%) and its bulk
  chemical composition is enriched in sulfur compared to CM chondrites like
  most other CY chondrites except for Dhofar 225 and Dhofar 735.NWA 4747 wa
 s affected by very intensive aqueous alteration corresponding to petrologi
 c type 1.0 before it was affected by thermal metamorphism. Only a few grai
 ns of unaltered olivine (Fa10) survived in NWA 4757. In contrast to other 
 CYs\, troilite from NWA 4757 are Mn-rich. The matrix is divided into light
  and dark types in texture and composition. The light matrix is Ca-rich\, 
 and Fe-poor compared to the dark matrix. The dehydrated phyllosilicates ha
 d serpentine composition with a low saponite content like CY2s. The presen
 ce of high-Ni metal and sulfides\, chromite and ilmenite and absence of ma
 gnetite indicate that under equilibrium conditions\, the redox state of th
 e alteration system in the NWA 4757 parent body did not exceed the fugacit
 y of the iron-wustite buffer.After aqueous alteration NWA 4757 experienced
  intensive thermal metamorphism resulting in a low H2O (1.9 wt%) compared 
 to usual CI and CM chondrites. The Fourier Transform Infrared (FT-IR) spec
 troscopy of the matrices of NWA 4757 and Mighei (CM2) showed that the NWA 
 4757 matrix is dominated by Fe-rich fine-grained olivine. According to a c
 lassification system for thermally metamorphosed hydrated carbonaceous cho
 ndrites based on X-ray Powder Diffraction (XRD) data\, NWA 4757 is charact
 erized by heating stage IV (&gt\;750 °C) like CY2s. Since calcite (and ev
 en dolomite) survived metamorphism\, it should indicate that the peak temp
 erature was not higher than 800 °C. Thus\, NWA 4757 is the first CY chond
 rite of thermal stage IV (CY2) which had properties of extremely altered C
 M1 chondrite before metamorphism and suggestively should be CY2-m1.話者
 ：大塚 勝仁 (PERC)言語：日本語
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