Compositional Data Analysis (CoDA) of Clinopyroxene From Abyssal Peridotites. (29th August 2022)

• Record Type: Journal Article
• Authors: Nishio, I.; Itano, K.; Waterton, P.; Tamura, A.; Szilas, K.; Morishita, T.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 23:Number 8(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-08-29
• Subjects: abyssal peridotite -- clinopyroxene -- mid‐ocean ridge -- mantle geochemistry -- compositional data analysis -- machine learning; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010472
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: We analyze a large database of abyssal peridotite clinopyroxene compositions using log‐ratio transformation, 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. 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 Na2 O 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 show less fractionated LREE/HREE with low‐HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt‐rock interaction, 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. Plain Language Summary: "Abyssal peridotites" are rocks that come from the Earth's mantle, beneath the oceanic crust. They are exposed on the seafloor near mid‐ocean ridges, the sites at which new oceanic crust forms by melting the mantle below. We analyzed a large database of chemical analyses of the mineral "clinopyroxene" from these abyssal peridotites using statistical methods, to try and understand what controls their chemical compositions. Our results simplify the complex chemical variations in these clinopyroxenes to just two main variables, and allow us to divide the compositions into 4 groups. The first three groups (85% of the clinopyroxene database) can be explained by forming in mantle rocks that melted by varying amounts, during the melting process that leads to the formation of oceanic crust. The last group (15% of the clinopyroxene database) cannot be explained by simple melting processes. Instead, these mantle rocks were first partially melted, and then reacted with melts that originated deeper in the mantle and passed through them. All clinopyroxene groups are present in most mid‐ocean ridges, showing that the composition of abyssal peridotites is extremely variable even on each ridge. Key Points: We classify database of abyssal peridotite clinopyroxene compositions using statistical methods and compare the data with melting models Eighty five percent of abyssal peridotites can be residues following melt extraction but 15% experienced both partial melting and melt‐rock interaction All clinopyroxene compositional clusters are observed in most ridges, reflecting compositional heterogeneities on each ridge