HR: 0830h
AN: V31E-0985    [PDF]
TI: Chromite Macrocrysts from Kimberlites and Lamproites as Petrogenetic Indicators: Evidence from Mineral Inclusions
AU: * Logvinova, A M
EM: logv@uiggm.nsc.ru
AF: Institute of Mineralogy & petrography, Koptyug Ave., 3, Novosibirsk, 630090 Russian Federation
AU: Sobolev, N V
EM: sobolev@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, Planetary Geosciences Institute, 306 GS Bldg., Knoxville, TN 37996 United States
AB: Chromite megacrysts are the source of significant data relating to the petrogenesis of kimberlites. More than 10,000 chromite macrocrysts (0.25-1 mm) were randomly selected from heavy-mineral concentrates collected at several Yakutian and Arkhangelsk diamondiferous kimberlites, in addition to several in So. Africa, USA, Venezuela, and Australian lamproites. All chromites were systematically crushed; 350 unaltered mineral inclusions were extracted. Approximately 50 % of the inclusions are represented by Mg-olivines (Fo 86-94), following by Cr-pyropes (17 %), Cr-diopsides (16 %), enstatites (6 %), and phlogopites (2 %). Mg-calcite inclusions were discovered in 23 of the chromite grains from the XXIII Party Congress pipe (Yakutia), as well as associated with almost half of all the enstatites in chromites from the Newlands pipe (RSA). The inclusion-bearing chromites represent practically the entire range of compositions of heavy-mineral concentrates of chromites. Olivines and pyroxenes are also characterized by wide compositional ranges, albeit similar to olivines and pyroxenes from granular peridotite xenoliths and xenocrysts. Pyropic garnets possess a wide range in Cr2O3 (2.0-12.1 wt%) and CaO (1.2-15.6 wt%), and along with chromite megacrysts, they are representative of all known peridotitic parageneses: harzburgitic, lherzolitic, and wehrlitic (e.g., Sobolev et al., 1973, CMP 40). These belong to either the graphite or diamond stability fields, estimated using various geobarometers. Of special significance in this study is the discovery of abundant Mg-calcite inclusions, some of which are associated with pyrope in Yakutian kimberlites. This is interpreted to be a signature of involvement of a carbonatitic component in the genesis of the chromite-silicate association. There is increasing evidence worldwide for the involvement of carbonatitic fluid/melt with the formation of diamonds. The present study has discovered additional evidence for carbonatitic components having interacted with the mineralogy of the diamond host rocks in the mantle.
DE: 3620 Crystal chemistry
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 3665 Mineral occurrences and deposits
DE: 3902 Creep and deformation
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting