TY - JOUR
T1 - Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240
T2 - Implications for their petrogenetic history on Mars
AU - Dunham, Emilie T.
AU - Balta, J. Brian
AU - Wadhwa, Meenakshi
AU - Sharp, Thomas
AU - McSween, Harry Y.
N1 - Funding Information:
Acknowledgments—The authors thank the associate editor (A. Ruzicka) and the reviewers (N. Castle and S. Symes) for helpful reviews that greatly improved the manuscript. We are also grateful to Richard Hervig and Kera Tucker for their assistance with SIMS analyses. The samples for this study were allocated to us by the Meteorite Working Group. We acknowledge the use of facilities within the Eyring Materials Center at Arizona State University supported in part by NNCI-ECCS-1542160. This work was supported by NASA grant NNX11AK75G to MW and NN09AG41G to TS.
Publisher Copyright:
© 2019 The Authors. Meteoritics & Planetary Science published by Wiley Periodicals, Inc. on behalf of The Meteoritical Society (MET).
PY - 2019/4
Y1 - 2019/4
N2 - Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.
AB - Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite finds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.
UR - http://www.scopus.com/inward/record.url?scp=85063674383&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=85063674383&partnerID=8YFLogxK
U2 - 10.1111/maps.13262
DO - 10.1111/maps.13262
M3 - Article
AN - SCOPUS:85063674383
SN - 1086-9379
VL - 54
SP - 811
EP - 835
JO - Meteoritics and Planetary Science
JF - Meteoritics and Planetary Science
IS - 4
ER -