Petrogenesis of group A eclogites and websterites: evidence from the Obnazhennaya kimberlite, Yakutia

Taylor, Lawrence A.; Snyder, Gregory A.; Keller, Randall; Remley, David A.; Anand, Mahesh; Wiesli, Rene; Valley, John and Sobolev, Nikolai V. (2003). Petrogenesis of group A eclogites and websterites: evidence from the Obnazhennaya kimberlite, Yakutia. Contributions to Mineralogy and Petrology, 145(4) pp. 424–443.

DOI: https://doi.org/10.1007/s00410-003-0465-y

Abstract

Mantle xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, possess a large range of mineralogical and chemical compositions, from both group A and B eclogites. Major-element contents of the group A eclogites exhibit transitional features between the group B eclogites and peridotite. The Mg# of clinopyroxenes is 0.86–0.94, with 0.60–0.84 for garnets. Differences in concentration of LREEs exist between the Obnazhennaya group A and the well-studied group B eclogites from the Udachnaya kimberlite pipe. In general, garnets in the group A eclogites contain lower LREEs than those from the group B eclogites; however, the trend for clinopyroxene is reversed. High d 18O (5.46–7.81) values, and the positive Eu anomalies in the garnets and clinopyroxenes (Eu/Eu* 1.2–1.4) demonstrate the involvement of an oceanic crustal component in the formation of the group A eclogites. The group A eclogites formed between 21.0 and 37.6 kbar, and 711 and 923 °C, in a time interval of 1,071–1,237 Ma. An innovative model is proposed to explain the formation of the group A eclogites and websterites. It involves the reaction of a depleted mantle peridotite with TTG and carbonatite melts closely related to the subduction of oceanic crust.

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