High-chlorine amphiboles from oceanic rocks; product of highly-saline hydrothermal fluids?

Abstract

High-precision analyses of chlorine and fluorine in amphiboles were performed by electron microprobe for five selected samples of metagabbro, amphibolite and metadiabase from the Mathematician Ridge failed spreading center, East Pacific Ocean. Fluorine is consistently very low, less than or equal to 0.2 wt.o/o in all amphiboles. Chlorine concentration is variable and forms a trimodal distribution that correlates with amphibole type in the following way: greenschist-grade actinolites generally contain less than or equal to 0.05 wt.o/o Cl; amphibolite-grade hornblendes contain 0.1 to 0.6 wt.o/o Cl; small irregular zones within actinolite bear up to 4.0 wt.o/o Cl. The high-chlorine amphiboles (hastingsite) probably formed as a result of locally elevated chlorine activity in hydrothermal fluids at upper greenschist-grade temperatures. The amphibolite-grade hornblendes with a few tenths of a percent chlorine may have formed in the presence of a saline hydrothermal fluid or a fluid of near seawater salinity. The amount and distribution in the ocean crust of saline hydrothermal fluids and their associated chlorine-bearing alteration products may be an important factor in understanding ocean hydrothermal systems, particularly in terms of the amount and nature of variabilitv in these svstems.