292 D43. Tectonics I (Posters): Advances in Tectonics from Mineralogy and Geochronology

Tuesday, 24 October 2017
9:00 AM - 6:30 PM
Halls 4EF

ORIGIN OF DIAMONDS IN SKENDERBEU MASSIF, MIRDITA OPHIOLITE, ALBANIA: IMPLICATIONS FROM ISOTOPES AND INCLUSION COMPOSITIONS

Paper number: 292-9
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Here we report a new type of diamond occurrence from the Skenderbeu massif in Mirdita ophiolite. The host diamond hosting ophiolite originated from shallow mantle depth. Skenderbeu massif is considered to be a part of Jurassic Tethys oceanic domain in West Albania, a tectonic setting distinct from that of other currently discovered diamonds. More than twenty microdiamonds sourced from the Skenderbeu massif, which contain micro- and nanoinclusions. These inclusions consist of a nickel-based manganese-cobalt alloy, coexisting with nanocrystals (20 x 20 nm) of calcium silicate in orthorhombic perovskite structure and fluid. Qualitative analysis of the fluid shows that it is composed of C-Si-Ca-Cl-O-Mn, Mg and K. The fluid coexisting with that alloy proves that the diamonds in Skenderbeu massif are indigenous rather than synthetic diamond. Diamond crystallized from a redox-sensitive Ni-Mn-Co alloy melt in the deep mantle at least in the diamond stability field. All of the diamonds (n= 12) are composed of isotopically light carbon, and have a unimodal distribution heavily skewed towards δ13CPDB ~ −25‰. The light carbon isotopic compositions are interpreted as evidence of diamond growth from organic carbon added to the subducted oceanic crust. The diamonds (n= 8) contain 37.0 – 286.4 ppm nitrogen (N) and show an exceptionally large range in δ15NAir (-12.9‰ to +25.5‰). These characteristics mentioned above suggests that the Skenderbeu diamonds nucleated and grew from a C-saturated nickel–manganese–cobalt-rich melt in a highly reduced environment. Nitrogen isotopic composition data is as evidence for nitrogen isotope heterogeneity in Skenderbeu diamond-forming fluids/melts on localized scales. We suggest here that the Ni-Mn-Co alloy originated from the melting of the Mn-nodules in the subducted oceanic crust. High-density fluids (HDFs) released from the subducted oceanic crust contain Cl+O+Si+K+Ca+CO32-+H2O. The HDFs react with the alloy melt and the alloy melt reduces the HDFs thus releasing carbon which is dissolved in the metal alloy. Diamond precipitated from the C supersaturated melt when PT changed. It is suggested that subsequent rapid upward transport in channeled networks may explain the formation and preservation of Skenderbeu diamonds.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-299891

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