Near-solidus evolution of oceanic gabbros: Insights from amphibole geochemistry

The near-solidus evolution of plutonic rocks formed at slow-spreading ridges is investigated using the major and trace element compositions of amphiboles in a suite of gabbros from the Mid-Atlantic Ridge. These new data allow unambiguous geochemical discrimination between amphiboles of magmatic and hydrothermal origin. In turn, this allows the gabbro solidus to be constrained to 860 +/- 30 degreesC, using amphibole-plagioclase thermometry. This is consistent with temperatures from associated secondary clinopyroxene. Magmatic amphibole, which can be identified in almost all samples, formed during metasomatism of a low-porosity crystal mush by an evolved hydrous silicate melt. These amphiboles are characterised by high F, Nb, and F/Cl and low Cl contents. The amphibole-forming reaction involved melt, plagioclase, and clinopyroxene. Amphibole blebs with a geochemically magmatic signature are found enclosed in the cores of some primitive clinopyroxene crystals. There is no evidence for a seawater component in the magmatic amphibole, as would be expected if high-temperature seawater ingress leads to flux melting, as has recently been suggested. However, the ingress of seawater-derived fluids did occur at temperatures within error of the gabbro solidus forming amphibole in veins and replacing igneous phases. These amphiboles are characterised by high Cl, B, and Cl/F and low Nb, F, and Nb/La. The fluids involved in the formation of these amphiboles had compositions unlike seawater or hydrothermal vent fluids. Copyright (C) 2001 Elsevier Science Ltd.