Chromitites in ophiolites: How, where, when, why? Part I. A review and new ideas on the origin and significance of platinum-group minerals

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• 作者：Jos茅 Mar铆a Gonz谩lez-Jim茅nez ; William L. Griffin ; Fern ; o Gervilla ; Joaqu铆n A. Proenza ; Suzanne Y. O'Reilly ; Norman J. Pearson
• 关键词：Platinum-group minerals ; Os isotopes ; Melt mixing ; Assimilation ; Postmagmatic veining ; Subsolidus recrsytallisation
• 刊名：Lithos
• 出版年：15 February, 2014
• 年：2014
• 卷：189
• 期：Complete
• 页码：127-139
• 全文大小：1144 K

文摘

Platinum-group minerals are the dominant control on the budget of platinum-group elements in ophiolitic chromitites. They vary widely in distribution, morphology, chemistry and Os-isotope compositions. Their distribution in different types of chromitites, and different microstructural sites within them may reflect the role of chromite as a physical 鈥渃ollector鈥?of the platinum-group elements during crystallisation of both phases from melts. The platinum-group minerals show significant Os-isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains. This micro-scale isotopic heterogeneity, and their random spatial distribution within the chromites, argue strongly against an origin of the platinum-group minerals via exsolution from the chromite lattice during post-magmatic cooling.

A re-interpretation of their microstructures and Os isotopes analysed in situ had led us to review the traditional models, and to propose several origins for the platinum-group minerals in chromitites. (1) The suites of platinum-group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite. (2) Some platinum-group minerals in chromitites may have been scavenged from wall-rock peridotite during migration of the parental melts of the chromitite. (3) Other platinum-group minerals may have precipitated from metasomatic fluid/melts that infiltrated existing chromitites. The recrystallization of pre-existing platinum-group minerals during polyphase metamorphism or 鈥渞ecycling鈥?of the chromitite into deeper mantle levels can explain the presence of micrometric platinum-group minerals with distinct Os-isotope compositions.