Geochemistry of diamondiferous eclogites from Udachnaya Pipe, Siberia

Three types of garnet (Gnt) can be recognised in eclogites from the Udachnaya kimberlite pipe, with different major-, trace-element and O-isotope compositions and related to different stages of mantle metasomatism. Gnt1 and Gnt2 are usually found together with diamonds in metasomatic veins. Gnt2 forms Mg-rich rims on Gnt 1 (mg.67-0.80 vs 0.57-0.58 for Gnt1). The significant zoning in chemistry and its relationship to diamond with mantle-like carbon (13 C = -5.6 - -6.6) suggest that Gnt2 was produced by mantle fluids/melts shortly before eruption. Gnt2 has a wide range in oxygen isotope composition (18 O = 6.2-9.0), suggesting mixing of mantle fluids with the original isotopic system (Gnt1, 18 O =11.1-12.0). Gnt3 is found in corundum-bearing eclogites and has much higher Ca (CaO=15.61-16.56 wt%) and different isotopic characteristics (13 C = -3.6 - -5.3; 18 O = 3.2-5.7). This suggests a different metasomatic fluid from the one that produced Gnt2. Lu/Gd increases significantly from Gnt3 (0.06-0.12) to Gnt1 (4.0-4.2) to Gnt2 (5.5-9.0). The major- and TE compositions of garnets from 25 eclogites show a well-defined change from positive to negative HREE/LREE at a CaO content of 7.8-8.7%. This suggests either a change in fluid compositions or a change in Gnt/fluid partitioning related to garnet composition.