Abstract
We analyze a large database of abyssal peridotite clinopyroxene
compositions using principal component analysis (PCA) and k-means
clustering, to better understand clinopyroxene compositional systematics
in abyssal peridotites. We combine this analysis with open-system
melting models to investigate the potential sources of compositional
variation. PCA shows that 84% of the variation in clinopyroxene
compositions can be represented using only 2-dimensional information
(PC1 and PC2 values). We use k-means clustering to classify
clinopyroxene compositions into four clusters. Clusters 1–3,
representing 85% of the data, show progressive depletions in LREE/HREE,
and are associated with decreases in Na2O in
clinopyroxene, and general increases in Cr# of spinel. We interpret
peridotites with clinopyroxene compositions from clusters 1–3 to
represent residues of partial melt extraction. The degree of melt
extraction increases from cluster 1 to 3, and exerts a primary control
on compositional variations. The presence or absence of garnet-field
melting prior to spinel-field melting and the retained melt fraction
during partial melting exert secondary controls on clinopyroxene
compositions. Cluster 4 clinopyroxenes, representing 15% of data, show
less fractionated LREE/HREE with low-HREE abundances, elevated Sr, and
depleted signatures in their host peridotites. Clinopyroxene
compositions in cluster 4 are only reconstructed in our models where
melt-rock interaction follows partial melting, suggesting that
peridotites with cluster 4 clinopyroxenes have experienced both of these
processes. Clusters 1–4 are observed in most ridges, reflecting
compositional heterogeneity on each ridge. This variability reflects
variations in the degree of partial melting, amount of garnet-field
melting, retained melt fractions, and melt-rock interaction.