Garnet and diopside-bearing diamondites (framesites)

Rocks consisting almost entirely of diamonds (diamondites) that contain min or amounts of silicates were analyzed for trace element abundances in the s ilicates by Laser Ablation ICP Mass Spectrometry for the first time. Diamon dites, previously described as polycrystalline diamond "aggregates" and "fr amesite", extend the range of monomineralic rocks known from the Earth's up per mantle. Our samples are intergrowths of diamonds with abundant open cav ities and some interstitial silicates. The most common silicate is pyrope w hich occurs in two different colors (and chemical compositions): orange and lilac similar to garnet inclusions in diamonds and garnets known from uppe r mantle eclogites and garnet peridotites, respectively. In our sample, the "peridotitic" garnet is accompanied by Cr-rich diopside whereas the "eclog itic" garnet is unaccompanied. Trace element abundances suggest that both t ypes of garnet formed from upper mantle fluids of similar origin which were rich in a carbonatitic component. The diamondites likely formed from the s ame fluids. Diamonds precipitated first and - in smaller amounts - contempo raneously with the silicates. Major upper mantle minerals like olivine, ort hopyroxene and omphacite are missing, possibly indicating that these minera ls behaved as refractory phases and were not mobilized by fluids. The chemi cal composition of "eclogite" and "peridotite" garnets differ in Cr and hig h field strength elements contents but not in the moderately compatible ele ments. They also have the same low Fe/Mg ratio which indicates a peridotiti c source for the fluids. The compositional difference in minor and trace el ements appears to be the result of different fluid processing rather than o f a different source, i.e., peridotite or eclogite.