Mantle metasomatism beneath the Siberian platform

Author

Vladimir Sobolev

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Geology

Major Professor

Lawrence A. Taylor

Committee Members

Harry Y. McSween, Claudia I. Mora, Alexander W. VanHook

Abstract

The composition and evolution of the upper mantle is recorded in deep-seated xenoliths, brought to the surface of the Earth by kimberlites. These xenoliths have experienced a wide variety of processes at varied temperatures and pressures during their post-origin evolution. Metasomatism is one of these processes, and it is responsible for most of the chemical and mineralogical changes to the xenoliths both in the mantle and on their way to the surface. Diamondiferous eclogite xenoliths provide a unique suite of samples from which to obtain the information about such processes. Based upon mineralogy and petrography of 29 eclogite xenoliths from the Udachnaya kimberlite, five different petrographic groups are defined, based on texture, mineral color, and degree of alteration. Chemical compositions of eclogitic minerals span a complete range from high to low jadeite content in clinopyroxenes and from pyrope-almandine to grossular in garnets; however, these minerals display both inter- and intragrain homogeneity. The oxygen isotope (δ¹⁸O) values of individual minerals vary from 4.8 to 7.0 %_o. The carbon isotope (δ¹³3) values of diamonds vary from -1 to -7 %_o, and are not indicative of a biogenic, crustally-derived component. The values for ⁸⁷Sr/^{86,/sup>Sr are extremely low (0.70150-0.70311), which suggest a long-lived, depleted (low Rb/Sr) component consistent with upper mantle, as well as very old reconstructed whole-rock model ages. However, these features, taken as whole, are not unequivocal strong evidence for the mantle origin of the Udachnaya eclogites. In fact, only two samples. which are classified as Group A (as per Taylor and Neal, 1989), are believed to represent a primary mantle material. From this large suite of eclogites, ten samples were chosen for quantitative analyses in order to possibly discern the nature of the metasomatic fluids which affected Udachnaya eclogites. The quantitative approach involved comparisons of reconstructed whole-rock ("pristine") compositions versus measured whole-rock analyses. These comparisons yielded systematic depletions in SiO₂, Na₂O, and FeO and enrichments in TiO₂, K₂O, MgO, and LREE. Numerous secondary phases are interpreted as the direct result of modal metasomatism (i.e., typical metasomatic minerals such as phlogopite, amphibole, djerfisherite, sodalite, etc.). It is believed that the metasomatizing fluids which modified the Udachnaya eclogites probably represent older mantle metasomatism, and its introduction, in timing and composition, was different from that of the host kimberlite. A different type of metasomatism altered garnet peridotite A-143, from the Mir kimberlite. This peridotite contains unusual "reversed" chemical zonation in garnet (low- Cr and high-Cr rim) and kosmochloric clinopyroxene, containing the highest Na₂O (6.08 wt.%) and Cr₂O₃ (7.08 wt.%) contents reported to date for peridotites. Metasomatism, that affected peridotite A-143, could have had a "kimberlitic" component, with (La/Nd)_n = 5-10, and Nd≈100xCl chondrites, or a Na-enriched "eclogitic" component. This peridotite xenolith consists of a mineral assemblage with a unusual chemistry and textural features that offer another example of the extremely complex process, called metasomatism.}

Recommended Citation

Sobolev, Vladimir, "Mantle metasomatism beneath the Siberian platform. " PhD diss., University of Tennessee, 1997.
https://trace.tennessee.edu/utk_graddiss/9611