Trace element abundances in garnets and clinopyroxenes from diamondites - a signature of carbonatitic fluids

摘要

Silicates intergrown with diamonds from 10 diamondites (polycrystalline diamonds, framesites) have been analysed for trace element contents by laser ablation ICP-MS. The diamondites are fine- (<100 mu m) to coarse-grained (> 1 mm) rocks with abundant pores and cavities. The walls of the open cavities are covered by euhedral diamond crystals. Silicates (commonly garnets) are mostly interstitial or occupy the space in cavities and often contain inclusions of euhedral diamonds. Four diamondites contain lilac "peridotitic" garnets with low CaO contents (3.6-5.7 wt percent), high Mg-numbers (0.83-0.84) and high Cr_2O_3 contents (3.9-6.4 wt percent). Occasionally, they are accompanied by Cr-diopside. "Peridotitic" garnets have heavy rare earth element-enriched and light rare earth element-depleted chondrite-normalised patterns, occasionally with a small hump at Eu and Sm. The remaining six diamondites contain orange coloured "eclogitic" garnets with low Cr_2O_3 contents (<1 wt percent). "Eclogitic" garnets can be divided into two subgroups: E-I garnets have high Mg-numbers (0.84-0.85, as high as those of the "peridotitic" garnets) and higher Cr_2O_3 and TiO_2 and lower heavy rare earth element contents than the E-II garnets. The chondrite-normalised trace element patterns of the two subgroups of "eclogitic" garnets are similar to each other, all are depleted in light rare earth elements with respect to the heavy rare earth elements and show significant positive anomalies of Zr and Hf. "Eclogitic" garnets are more depleted in highly incompatible elements (light rare earth elements, Nb and Ta) than the "peridotitic" garnets. Diamondites and their silicates very likely crystallised from a fluid phase. The trace element contents of the hypothetical fluids in equilibrium with the "peridotitic" garnets are similar to the trace element contents of kimberlitic and carbonatitic liquids. Thus, crystallisation of these diamondites from a highly alkaline liquid in the presence of carbonates can be suggested. Hypothetical melts in equilibrium with "eclogitic" garnets are highly magnesian but depleted in light rare earth elements and other highly incompatible elements relative to the typical kimberlitic, lamproitic or carbonatitic liquids. This is an unexpected result because eclogites are richer in trace elements than peridotites and fluids in equilibrium with these rocks should reflect this. The different trace element contents of fluids which precipitated, beside diamonds, "peridotitic" and "eclogitic" garnets, respectively, therefore, must be the result of differences in the properties of these fluids rather then of different source rocks, as was already suspected by Kurat and Dobosi (2000).