Computer Science
Scientific paper
Mar 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993gecoa..57.1297b&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 57, Issue 6, pp.1297-1306
Computer Science
9
Scientific paper
Accessory chromium spinel occurring in serpentinites is usually altered. The extent of this alteration and its relationship to alteration processes such as serpentinization and metamorphism is not defined, complicating the use of altered spinel as a petrogenetic indicator. Here, the relationship between different alteration processes and spinel alteration is evaluated in terms of accessory chromium spinel occurring in serpentinites of progressive metamorphism in the eastern Central Alps. Two coexisting Cr-spinels were identified, i.e., an altered, Fe-rich Cr-spinel (I) with Cr > Al, and an Fe-poor, Cr-spinel (II) with Al > Cr, which exsolved a Cr-Fe 3+ phase and which resisted alteration. Alteration of Cr-spinel (I) is evident from decreasing Al, Mg, and Cr values, with a trend leading to ferrite-chromite (trend a). An enrichment in Cr (trend b) related to CO 2 in the serpentinizing fluid, indicated by the presence of carbonate, talc or tremolite, is superimposed to (a). Alteration is a decomposition of the spinel structure caused by the dissolution of Mg, Al, and Cr during serpentinization. Dissolution of Cr is favored in a reduced environment whereas reprecipitation occurs under locally oxidizing conditions, indicated by the presence of CO 2 . SiO 2 detected in spinel is a relative measure of the degree of alteration. Metamorphism up to amphibolite facies recrystallized and homogenized preexisting spinel phases to various degrees. Whether or not and to what extent a spinel alters depends on its Fe content. Fe 2+ , stabilized in the octahedral site (high CFSE), is responsible for an inverse and less stable spinel structure. Fe-rich spinels (Cr-spinel I) are, therefore, readily subjected to alteration, whereas Fe-poor spinels (Cr-spinel II) are resistant to alteration during the serpentinization processes.
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