Petrogenesis of eclogite xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, Russia

Author

David Andrew Remley

Date of Award

5-1999

Degree Type

Thesis

Degree Name

Master of Science

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Claudia Mora, Harry McSween

Abstract

Eclogite xenoliths from the Obnazheimaya kimberlite, Siberia, provide an opportunity to study the composition of the mantle beneath the Siberian platform, near the edge of the craton. This study includes detailed petrography, and major-, minor-, andtrace-element geochemistry of a suite of nine eclogite xenoliths from Obnazheimaya.Geothermometric and geobarometric calculations indicate the xenoliths formed from 29 to40 kilobars pressure and 815 to 1130°C. These calculations also reveal that the geothermal gradient near the edge of the Siberian platform differs markedly from the center of the craton, with the edge possessing a higher geothermal gradient.Core-to-rim compositional variations in clinopyroxene include decreasing AI₂O₃,Na₂O, and FeO, and increasing MgO and CaO. Major-element zoning in garnet is only observed in Group B eclogite samples. Core-to-rim variations in these garnets include increasing MgO and decreasing CaO and FeO (i.e., increasing Mg#). These variations are such that core compositions lie in the Group B composition field, whereas the rim values extend toward the Group A field, suggesting that it may be possible to modify a Group Beclogite into a Group A eclogite.The minerals composing the rocks of this suite are also zoned in REE. The Observed zonations are the result of complex metasomatic alteration possibly involving crustal fluids (positive Eu anomalies) and a Mg-, Cr-, and LREE-rich mantle-fluid.Based on mineral chemistry, the eclogite suite can be divided into three groups; Group A eclogites (three samples). Group B eclogites (two samples), and a previously unrecognized group with Group A major-element and Group B trace-element signatures(four samples).The mineral chemistry of the Group A and Group B eclogites can be compared with peridotite and websterite xenoliths from the same locality. REE analyses reveal that Group A eclogites from Obnazhennaya are similar to gamet-peridotite xenoliths from the same locality. Based on the similarities in major- and trace-element compositions, it is believed that the Group A eclogites represent low-Cr modal variants of the peridotite suite. The Group B eclogites have chemical affinities with the mid-section of an ophiolite complex and exhibit positive Eu anomalies and δ¹⁸ values outside the accepted mantlerange. The oxygen isotopic compositions and Eu anomalies indicate that the Group Bsamples are of crustal origin, implying that subduction could have occurred beneath the northern edge of the Siberian platform as recently as the middle-to-late Proterozoic. The Third group has Mg- and Cr-contents diagnostic of Group A eclogites, which are thought to be of mantle origin. However, these eclogites also exhibit positive Eu anomalies andcrustal δ¹⁸ signatures. Such rocks may represent relicts of recycled oceanic crust that have been metasomatized by a Mg-, Cr-, and LREE-rich fluid.

Recommended Citation

Remley, David Andrew, "Petrogenesis of eclogite xenoliths from the Obnazhennaya kimberlite pipe, Yakutia, Russia. " Master's Thesis, University of Tennessee, 1999.
https://trace.tennessee.edu/utk_gradthes/10008