EarthRef.org Reference Database (ERR) -- Gillis & Meyer 2001

We report new compositional and textural observations on amphiboles hosted in oceanic gabbros recovered from the slow spreading Mid-Atlantic (MARK area) and Southwest Indian (Hole 735B) ridges and the fast spreading East Pacific Rise (Hess Deep) in an effort to find a set of reliable criteria that unequivocally distinguishes magmatic from hydrothermal amphibole. We find that there is no simple set of criteria that may be applied to distinguish between these parageneses. Rather, it is necessary to assess both a magmatic and hydrothermal origin for all amphibole grains in order to establish appropriate criteria for a particular sample suite. The rare earth element (REE) content of ~75% of our amphibole grains may be accounted for by crystallization from residual melts; some of these melts were enriched in the light rare earth elements by the crystallization of apatite and/or amphibole. Of significance, only a small subset of these grains crystallized at magmatic temperatures (>825?C). We conclude that the grains that equilibrated at lower temperatures were initially magmatic but that their major elements were reset during subsolidus, hydrothermal alteration, and deformation. This decoupling of the major and rare earth elements was most prevalent in the Hess Deep and MARK suites, both of which were tectonically exhumed at amphibolite to greenschist facies conditions. The remaining REE data identify hydrothermal amphibole that formed by mineral-scale reactions involving clinopyroxene, fluids, and ? plagioclase at subsolidus conditions. Evaluation of fluids in equilibrium with amphibole shows that deep hydrothermal fluids are unlike those venting at the seafloor in terms of REE contents and/or patterns