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

Coogan, Laurence A., Wilson, Robert N., Gillis, Kathryn M. and MacLeod, Christopher John 2001. Near-solidus evolution of oceanic gabbros: insights from amphibole geochemistry. Geochimica et Cosmochimica Acta 65 (23) , pp. 4339-4357. 10.1016/S0016-7037(01)00714-1

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Abstract

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°C, 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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Publisher: Elsevier
ISSN: 0016-7037
Last Modified: 04 Jun 2017 02:51
URI: http://orca-mwe.cf.ac.uk/id/eprint/12742

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