The origin of amphibole in a sequence of gabbronoritic rocks recovered at Ocean Drilling Program Site 894 was evaluated on the basis of textural and geochemical criteria. Major element compositions indicate that all the amphibole is metamorphic whereas textural relations suggest that amphibole formed either by magmatic or metamorphic processes. The rare earth element (REE) content of amphibole, clinopyroxene, apatite, and zircon were determined by ion microprobe to assess these interpretations. Amphibole shows broad range in REE concentrations (up to 1,000 times chondritic values) with (La/Sm)_N ratios from 0.1 to 1.4. There is no systematic relationship between REE content and textural type. The origin of amphibole with REE contents less than or equal to clinopyroxene cannot be distinguished by compositional criteria. By contrast, the REE content of all other amphibole can only be explained by magmatic processes. Calculated melts in equilibrium with igneous amphibole indicate that amphibole crystallized from melts with progressively higher REE contents; in some samples melts evolved without significant fractionation of the REEs whereas in other samples melts became light-REE-enriched. An increase in the total abundance of the REE melts is consistent with the crystallization of plagioclase, clinopyroxene, and orthopyroxene from progressively more evolved melt. The light-REE-enriched melts may have been influenced by the crystallization of accessory phases or by interaction with exsolved magmatic fluids. These results indicate that there was a continuum between magmatic and hydrothermal processes and that groundmass amphibole records a complex history of crystallization from a melt and its exsolved magmatic fluids, followed by interaction with seawater-derived hydrothermal fluids.

Date of initial receipt: 25 August 1994
Date of acceptance: 2 March 1995

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