Global variations in abyssal peridotite compositions
Date
2016-01-09
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Elsevier
Abstract
Abyssal peridotites are ultramafic rocks collected frommid-ocean ridges that are the residues of adiabatic decompression
melting. Their compositions provide information on the degree of melting and melt–rock interaction
involved in the formation of oceanic lithosphere, as well as providing constraints on pre-existing mantle heterogeneities.
This review presents a compilation of abyssal peridotite geochemical data (modes, mineral major
elements, and clinopyroxene trace elements) for N1200 samples from 53 localities on 6 major ridge systems.
On the basis of composition and petrography, peridotites are classified into one of five lithological groups: (1) residual
peridotite, (2) dunite, (3) gabbro-veined and/or plagioclase-bearing peridotite, (4) pyroxenite-veined
peridotite, and (5) other types of melt-added peridotite. Almost a third of abyssal peridotites are veined, indicating
that the oceanic lithospheric mantle is more fertile, on average, than estimates based on residual peridotites
alone imply. All veins appear to have formed recently during melt transport beneath the ridge, though some pyroxenites
may be derived from melting of recycled oceanic crust.
A limited number of samples are available at intermediate and fast spreading rates, with samples from the East
Pacific Rise indicating high degrees of melting. At slow and ultra-slow spreading rates, residual abyssal peridotites
define a large (0–15% modal clinopyroxene and spinel Cr#=0.1–0.6) compositional range. These variations
do not match the prediction for how degree of melting should vary as a function of spreading rate. Instead, the
compositional ranges of residual peridotites are derived from a combination of melting, melt–rock interaction
and pre-existing compositional variability, where melt–rock interaction is used here as a general term to refer
to the wide range of processes that can occur during melt transport in the mantle. Globally, ~10% of abyssal peridotites
are refractory (0% clinopyroxene, spinel Cr# N 0.5, bulk Al2O3 b 1wt.%) and someridge sections are dominated
by harzburgiteswhile lacking a significant basaltic crust. Abyssal ultramafic samples thus indicate that the
mantle ismulti-component, probably consisting of at least three components (lherzolite, harzburgite, and pyroxenite).
Overall, the large compositional rangeamong residual andmelt-added peridotites implies that the oceanic
lithospheric mantle is heterogeneous, which will lead to the generation of further heterogeneities upon subduction
back into the mantle.
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Warren, Jessica M. "Global variations in abyssal peridotite compositions." Lithos 248 (2016): 193-219.