Li isotopic variations in single pyroxenes from the Northwest Africa 480 shergottite (NWA 480); a record of degassing of Martian magmas?
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|Authors:||Beck, P.; Barrat, J. A.; Chaussidon, M.; Gillet, P.; Bohn, M.|
|Author Affiliations:||Primary: |
Ecole Normale Supérieure de Lyon, Laboratoire des Sciences de la Terre, Lyons, France
CNRS Géodynamique et Planétologie, France
Ifremer-Centre de Brest, France
|Volume Title:||Geochimica et Cosmochimica Acta|
|Source:||Geochimica et Cosmochimica Acta, 68(13), p.2925-2933. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037|
|Note:||In English. 35 refs.; illus., incl. 2 tables|
|Summary:||Lithium abundances and isotopic compositions were measured by ion microprobe in individual grains of pyroxene, and in a few maskelynites and Ca-phosphates grains, from the Martian meteorite Northwest Africa 480 (NWA 480).In pyroxenes Li abundances are nearly constant from core to rim with concentrations ranging between 3 and 4 µg/g. In contrast, a significant isotopic zoning is observed with δ7Li values increasing within single crystals from ∼ -17 per mil in the core to ∼ +10 per mil in the rim, most of the variability being observed in the core. Plagioclase (now maskelynite) and phosphate crystals, which co-crystallized with the pyroxene rims, display similar δ7Li values. Because of the incompatible behavior of Li, the present constancy of Li concentrations within zoned pyroxenes rules out any simple crystallization model in a closed system for Li. The large Li isotopic variations observed within pyroxenes support this conclusion. There is no evidence in support of secondary alteration of NWA 480 to explain the Li isotopic variations, which thus most likely reflect magmatic processes on Mars. Degassing might explain the Li systematics observed in NWA 480 pyroxenes. Because Li has a strong affinity with water-rich fluids, a significant loss of Li from NWA 480 parental melt can happen upon melt emplacement and cooling. Such a Li loss could compensate the effect of crystal fractionation and thus help to maintain constant the Li content of the melt. Li isotopic fractionation is anticipated to accompany this process, 7Li being depleted relative to 6Li in the volatile phase. The magnitude of the isotopic change of the fractionating melts is difficult to predict because it depends on the value of the Li isotopic fractionation and on the amount of Li loss, but at first glance it seems consistent with the increase of δ7Li values observed in NWA 480 pyroxenes with increasing fractionation. The present data suggest that degassing prevailed not only during the crystallization of shergottites like Zagami and Shergotty, but also during the crystallization of the other types of basaltic shergottites. Abstract Copyright (2004) Elsevier, B.V.|
|Sections:||Meteorites and tektites|
|Subjects:||Achondrites; Alkali metals; Chain silicates; Concentration; Degassing; Experimental studies; Geochemistry; Ion probe data; Isotope fractionation; Isotopes; Kinetics; Li-7; Lithium; Magmas; Mars; Martian meteorites; Mass spectra; Metals; Meteorites; Northwest Africa Meteorites; Parent bodies; Planets; Pyroxene group; Shergottite; Silicates; Single-crystal method; SNC Meteorites; Spectra; Stable isotopes; Stony meteorites; Terrestrial planets; NWA 480|
|Copyright Information:||GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands|
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