Diamonds: Minor elements in silicate inclusions: Pressure‐temperature implications

Abstract

New electron and ion microprobe analyses of 36 olivines, 9 pyroxenes, and 10 garnets included in diamond were obtained. Olivine inclusions from kimberlites with Precambrian ages have higher mean Cr2O3 (850 versus 360 ppmw), Al2O3 (200 versus 90 ppmw), CaO (490 versus 210 ppmw), Na2O (110 versus 30 ppmw), and Li (686 versus 466 counts per second) than olivine inclusions found in Mesozoic kimberlites, perhaps indicating a temporal change of either temperature or chemistry or both in the mantle source regions of diamonds. In contrast to lherzolitic olivines, aluminum and sodium in olivines from diamonds are insufficient to couple with trivalent chromium; perhaps divalent chromium is inferred. Some olivines show zoning to Ca‐rich edges. Temperature and pressure estimates for inclusions in four diamonds fall in the combined pressure‐temperature range of coarse and porphyroclastic garnet lherzolites, and temperature estimates for inclusions in seven diamonds lacking a pressure estimate cover most of the temperature range for lherzolites. Taken at face value, the pressure and temperature estimates for inclusions in diamonds indicate that some diamonds might have crystallized in the presence of a liquid containing H2O, CO2, or both. Other estimates fall in the subsolidus field even for H2O‐saturated peridotite. Although the pressure‐temperature estimates overlap for lherzolites and ultramafic‐type inclusions in diamonds, differences in trace element content of olivines require that the crystallization environments were not identical. Perhaps the Cr content of the olivine inclusions indicates that the host diamonds grew in a reducing environment not found in typical lherzolites.

Trace element compositions of submicroscopic inclusions in coated diamond: A tool for understanding diamond petrogenesis

“…Thompson and Barnes (1984) investigated the distribution of Ni and Fe between sulfides and olivine, and calculated a Ni/Fe partition coefficient of Ϸ9.8 (KD sulfide/olivine ϭ sulfide X Ni /X Fe /Olivine X Ni /X Fe , where X denotes mole fractions). The calculated Ni/Fe ratio of olivine in equilibrium with a sulfide melt with the compatible element composition of the core fluid (Ni/Fe ϭ 0.2 to 0.6) is 0.02 to 0.06, this is within the range observed for olivine inclusions in natural diamond (0.02-0.06; Hervig et al, 1980). Fleet and MacRae (1988) have determined this partition coefficient experimentally and give a higher value of Ϸ30, which they suggest is appropriate to the upper mantle.…”

The Orthopyroxene-Garnet Geobarometer for Peridotites

“…For thermobarometric calculations from (acces sible) published data on inclusions in diamonds, per taining to Phanerozoic kimberlites, we chose those in which orthopyroxene and garnet (+ olivine) form intergrowths, which guarantees thermal and diffusional equilibrium between them: sample AB-75 from Yakutia [21] and samples 23 [53] and F 5-75 [35] from southern Africa. The compositions of the orthopyroxenes and garnets from these samples (the raw data for the present calculations of crystallochemical coefficients) are given in the table.…”

“…Equilibrium pressures and temperatures of peridotite inclusions in diamonds. Gr + Opx + Ol paragenesis from Phanerozoic kimberlites (AB-75 [21], F 5-75 [35], 23 [53]). 1) Pressure calculated from proposed geobarometer (double dash shows maximum permissible error of geobarometer); temperatures calculated from geothermometer of [47]; 2) error in esti mating pressure, depending on equilibrium temperature (pressure calculated from proposed geobarometer for arbitrary temperature values); 3, 4) analogous calculations using geobarometer in [33]; 5, 6) ditto, using geobarometer in [45]; 7) pres sure calculated from geobarometer in [24] for arbitrary temperature values; 8) conductive geotherm for surface heat flow of 40 mW/m 2 [51]; 9) pressure calculated from proposed geobarometer, temperature from [58].…”

Advanced Ceramics III

“…(3) Silicate phases such as garnet, enstatite and other phases are found as inclusions in diamond in kimberlites and lamproites and are the basis for establishing the P-T conditions for the growth of diamond at depth in the earth (Boyd et aI., 1985;Cohen and Rosanfeld, 1979;Hervig et at., 1980;Meyer & Tsai, 1976). …”