Formation of Amphibole-Bearing Peridotite and Amphibole-Bearing
Pyroxenite through Hydrous Melt-Peridotite Reaction and In Situ
Crystallization: An Experimental Study
Abstract
Amphibole is a common hydrous mineral in mantle rocks. To better
understand the processes leading to the formation of amphibole-bearing
peridotites and pyroxenites in mantle rocks, we have undertaken an
experimental study reacting lherzolite with hydrous basaltic melts in
Au-Pd capsules using the reaction couple method. Two melts were
examined, a basaltic andesite and a basalt, each containing 4 wt% of
water. The experiments were run at 1200°C and 1 GPa for 3 or 12 h, and
then cooled to 880°C and 0.8 GPa over 49 h. The reaction at 1200°C and 1
GPa produced a melt-bearing orthopyroxenite-dunite sequence. The cooling
stimulates crystallization of orthopyroxene, clinopyroxene, amphibole,
and plagioclase, leading to the formation of an amphibole-bearing
gabbronorite–orthopyroxenite–peridotite sequence. Compositional
variations of minerals in the experiments are controlled by temperature,
pressure, and reacting melt composition. Texture, mineralogy, and
mineral compositional variation trends obtained from the experiments are
similar to those from mantle xenoliths and peridotite massif from the
field including amphibole-bearing peridotites and amphibole-bearing
pyroxenite and amphibolite that are spatially associated with
peridotites, underscoring the importance of hydrous melt-peridotite
reaction in the formation of these amphibole-bearing rocks in the upper
mantle. Amphiboles in some field samples have distinct textual and
mineralogical features and their compositional variation trends are
different from that defined by the melt-peridotite reaction experiments.
These amphiboles are either crystallized from the host magma that
entrained the xenoliths or product of hydrothermal alterations at
shallow depths.