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This content will become publicly available on April 27, 2017

Title: Ahrensite, γ-Fe2SiO4, a new shock-metamorphic mineral from the Tissint meteorite: Implications for the Tissint shock event on Mars

Ahrensite (IMA 2013-028), gamma-Fe2SiO4, is the natural Fe-analog of the silicate-spinel ringwoodite (gamma-Mg2SiO4). It occurs in the Tissint Martian meteorite, where it forms through the transformation of the fayalite-rich rims of olivine megacrysts or Ferich microphenocrysts in contact with shock melt pockets. The typical sequence of phase assemblages traversing across a Tissint melt pocket into olivine is: quenched melt or fayalite-pigeonite intergrowth → bridgmanite + wustite → ahrensite and/or ringwoodite double right arrow highly-deformed olivine + nanocrystalline ringwoodite → deformed olivine. We report the first comprehensive set of crystallographic, spectroscopic, and quantitative chemical analysis of type ahrensite, and show that concentrations of ferric iron and inversion in the type material of this newly approved mineral are negligible. We also report the occurrence of nanocrystalline ringwoodite in strained olivine and establish correlations between grain size and distance from melt pockets. The ahrensite and ringwoodite crystals show no preferred orientation, consistent with random nucleation and incoherent growth within a highly strained matrix of olivine. As a result, grain sizes of ahrensite immediately adjacent to melt pockets are consistent with growth during a shock of moderate duration (1-10 ms).
Authors:
 [1] ;  [2] ;  [1] ;  [1] ;  [1] ;  [3] ;  [3] ;  [4]
  1. California Inst. of Technology (CalTech), Pasadena, CA (United States)
  2. Univ. of Nevada, Las Vegas, NV (United States)
  3. Carnegie Institution of Washington, Argonne, IL (United States)
  4. Univ. of Tennessee, Knoxville, TN (United States)
Publication Date:
OSTI Identifier:
1332393
Grant/Contract Number:
NA0001982
Type:
Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 184; Journal Issue: C; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Research Org:
Univ. of Nevada, Las Vegas, NV (United States)
Sponsoring Org:
USDOE National Nuclear Security Administration (NNSA)
Contributing Orgs:
NSF EAR-0318518 DMR-0080065 U.S. DOE DE-AC02-06CH11357 National Nuclear Security Administration under the Stewardship Science Academic Alliances program through DOE Cooperative Agreement DE-NA0001982 NASA Cosmochemistry grants NNX11AG58G NNX12AH63G NSF grant EAR-1322082 JPL
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES Ahrensite; gamma-Fe2SiO4; new high-pressure mineral; shock metamorphism; nanomineral; ringwoodite group; Tissint meteorite; martian shergottite; synchrotron diffraction