[S-CG47] Hard-Rock Drilling Science: Continental and Deep-Sea Drilling, and Ophiolite

[SCG47-11] Trace element variations in clinopyroxene from abyssal peridotites:
Insights from multivariate statistical analyses

★Invited Papers

*Ikuya Nishio^1,2, Keita Itano³, Pedro Waterton², Akihiro Tamura¹, Kristoffer Szilas², Tomoaki Morishita¹ (1.Kanazawa University, 2.University of Copenhagen, 3.Akita University)

Keywords:Clinopyroxene, Mantle, Peridotite, Statistical analysis

Abyssal peridotites are rocks that come from the Earth's upper mantle, beneath the oceanic crust. Their compositions provide information on processes involved in the formation of oceanic lithosphere and trace element abundances in clinopyroxenes from abyssal peridotites are the important source of the information (Warren, 2016). We analyze a large dataset of abyssal peridotite clinopyroxene compositions using statistical methods, principal component analysis (PCA) and k-means clustering, to better understand clinopyroxene trace element systematics in abyssal peridotites (Nishio et al., 2022). We combine this statistical analysis with partial melting models to investigate the potential sources of trace element 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. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from clusters 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 show less fractionated LREE/HREE with low-HREE abundances and elevated Sr. The compositions 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. However, positive Sr anomalies shown in cluster 4 clinopyroxenes cannot be reproduced in our melt-rock reaction models using expected melt/rock compositions at the mid-ocean ridge. This characteristic is observed in forearc peridotites, where they indicate fluid involvement. We therefore speculate that cluster 4 peridotites could be recycled arc mantle materials that interacted with hydrous melts, and/or experienced hydrous melting at the mid-ocean ridge.

References
Warren, J. M. (2016). Global variations in abyssal peridotite compositions. Lithos, 248, 193– 219. https://doi.org/10.1016/j.lithos.2015.12.023
Nishio, I., Itano, K., Waterton, P., Tamura, A., Szilas, K., & Morishita, T. (2022). Compositional Data Analysis (CoDA) of Clinopyroxene From Abyssal Peridotites. Geochemistry, Geophysics, Geosystems, 23(8), e2022GC010472. https://doi.org/10.1029/2022GC010472