Abstract
Here, the petrological features of numerous primitive achondrites and highly equilibrated chondrites are evaluated to review and expand upon our knowledge of the chondrite–achondrite transition, and primitive achondrites in general. A thermodynamic model for the initial silicate melting temperature and progressive melting for nearly the entire known range of oxidation states is provided, which can be expressed as Tm = 0.035Fa2−3.51Fa + 1109 (in °C, where Fa is the proportion of fayalite in olivine). This model is then used to frame a discussion of textural and mineralogical evolution of stony meteorites with increasing temperature. We suggest that the metamorphic petrology of these meteorites should be based on diffusive equilibration among the silicate minerals, and as such, the chondrite–achondrite transition should be defined by the initial point of silicate melting, not by metal–troilite melting. Evidence of silicate melting is preserved by a distinctive texture of interconnected interstitial plagioclase ± pyroxene networks among rounded olivine and/or pyroxene (depending on ƒO2), which pseudomorph the former silicate melt network. Indirectly, the presence of exsolution lamellae in augite in slowly cooled achondrites also implies that silicate melting occurred because of the high temperatures required, and because silicate melt enhances diffusion. A metamorphic facies series is defined: the Plagioclase Facies is equivalent to petrologic types 5 and 6, the Sub-calcic Augite Facies is bounded at lower temperatures by the initiation of silicate melting and at higher temperatures by the appearance of pigeonite, which marks the transition to the Pigeonite Facies.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 857-885 |
| Number of pages | 29 |
| Journal | Meteoritics and Planetary Science |
| Volume | 55 |
| Issue number | 4 |
| DOIs | |
| State | Published - Apr 1 2020 |
| Externally published | Yes |
Bibliographical note
Funding Information:Colin MacRae and Nick Wilson are thanked for assistance with the electron microprobe work. Junnel Alegado is thanked for preparing many thin sections. NASA JSC is thanked for provision of some of the samples used in this study; US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program, which has been funded by NSF and NASA, and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson Space Center. This project was funded in part by National Geographic Research and Exploration Grant 9680‐15, which funded fieldwork that resulted in recovery of some of the meteorites used in this study. We acknowledge ARC LIEF grant LE130100087, which provided funds to purchase the electron microprobe used in this study. We acknowledge use of the Monash Centre for Electron Microscopy, funded centrally by Monash University. We also thank our crowd‐sourced funding supporters who donated to our Space Rocks campaign with Pozible; the meteorites recovered through that project were invaluable in this study. T.E.J. acknowledges support from The Institute of Geosciences, Curtin University, and Open Fund GPMR210704 from the State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The authors declare no conflicting interests.
Funding Information:
Colin MacRae and Nick Wilson are thanked for assistance with the electron microprobe work. Junnel Alegado is thanked for preparing many thin sections. NASA JSC is thanked for provision of some of the samples used in this study; US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program, which has been funded by NSF and NASA, and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson Space Center. This project was funded in part by National Geographic Research and Exploration Grant 9680-15, which funded fieldwork that resulted in recovery of some of the meteorites used in this study. We acknowledge ARC LIEF grant LE130100087, which provided funds to purchase the electron microprobe used in this study. We acknowledge use of the Monash Centre for Electron Microscopy, funded centrally by Monash University. We also thank our crowd-sourced funding supporters who donated to our Space Rocks campaign with Pozible; the meteorites recovered through that project were invaluable in this study. T.E.J. acknowledges support from The Institute of Geosciences, Curtin University, and Open Fund GPMR210704 from the State Key Lab for Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The authors declare no conflicting interests.
Publisher Copyright:
© The Meteoritical Society, 2020.