HR: 0800h
AN: V41E-1515    [Abstracts]
TI: Eclogitic Diamond Inclusions from the Komsomolskaya Pipe, Yakutia
AU: * Logvinova, A
EM: logv@uiggm.nsc.ru
AF: Institute of Mineralogy & Petrography, Koptyug Ave., 3, Novosibirsk, 630090 Russian Federation
AU: Taylor, L A
EM: lataylor@utk.edu
AF: University of Tennessee, Dept of Earth & Planetary Sci. 306 EPS Bldg., Knoxville, TN 37996 United States
AU: Sovolev, N V
EM: sobolev@uiggm.nsc.ru
AF: Institute of Mineralogy & Petrography, Koptyug Ave., 3, Novosibirsk, 630090 Russian Federation
AU: Floss, C
EM: floss@wustl.edu
AF: Washington University, Laboratory for Space Sciences Physics Department, CB 1105, St. Louis, MO 63030 United States
AB: Mineral inclusions from diamonds (DIs) provide important constraints on the conditions prevailing at the time of the diamond's formation. Diamonds from all major operating mines of Yakutia, including the Mir, Udachnaya, Sytykanskaya, and Aykhal, contain an extremely high ratio of U/P (ultramafic/peridotitic) type inclusions versus E (eclogitic) type - i.e., '99/1' for the -3+1 mm-size fraction - based upon prior study (Yefimova and Sobolev, 1977, Dokl. Akad. Nauk SSSR, 237). However, the recently developed Komsomolskaya mine has attracted special attention because of the first discovery of majoritic (Si = 3.13 pfu) E-type garnet inclusions among Yakutian diamonds, as well as for the unusually high proportion of E-type garnet DIs, reaching 15% (Sobolev et al., 2004, Lithos, 77). All the mines mentioned above have a narrow emplacement-age range within 344-365 Ma (Agashev et al., 2004, Dokl. Earth Sci., 399). We report here major- and trace-element variations in eclogitic DIs from 30 Komsomolskaya diamonds. Most of these DIs were studied in-situ, on a single polished surface of each diamond. About half of these samples contain multiple inclusions, with up to 4 grains of garnet and/or omphacite in a single diamond. Only one DI of touching grains of omphacite + coesite was detected, but another sample contains isolated coesite and omphacite DIs. Kyanite associated with high Ca-garnet was also found. Examination of the diamond polished surfaces with CL reveals relative times-of-encapsulation for the multiple DIs. All multiple DIs in a single diamond have similar major-element contents. However, between diamonds, the garnet inclusions have a wide range of 3.5 to 16.2 wt.% CaO, a span that covers practically the entire range of E-type DIs worldwide. Omphacites from 15 samples, however, possess a restricted compositional range in Na2O from 3.75 to 6.18 wt% only. Significant variations in trace elements occur in both garnets and pyroxenes in these DIs. Garnets range in Y, 24-65; Zr, 15-68; and Sr 0.4-4.3 ppm. Abundances in pyroxenes also exhibit broad ranges: Y, 1.6-4.0; Zr, 6-17; and Sr, 93-390 ppm. Here, as with the major elements, the ranges for the E-type DIs from this one pipe span almost the entire range for E-type garnets and pyroxenes from worldwide sources (Stachel et al., 2004, Lithos, 77). These wide ranges in the chemistry of DIs from a single pipe bring into question the possible validity of diamond-age determinations based upon Rb-Sr & Sm-Nd of garnets and clinopyroxenes gained collectively from hundreds of diamonds in a one pipe.
DE: 1040 Radiogenic isotope geochemistry
DE: 1042 Mineral and crystal chemistry (3620)
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005