Formation of podiform chromitite deposits: implications from PGE abundances and Os isotopic compositions of chromites from the Troodos complex, Cyprus

Abstract

Podiform chromitite deposits occur in the mantle sequences of many ophiolites that were formed in supra-subduction zone settings (SSZ). We have measured PGE abundances and Os isotopic compositions of three major chromitite deposits (Kannoures, Hadji Pavlou, Kokkinorostos) and associated mantle peridotites from the Troodos Ophiolite Complex in order to investigate the petrogenesis of these rocks, and their genetic relationships and to examine the geochemical behaviour of the PGE. Spinels from the chromitite deposits have flat chondrite-normalized PGE patterns, but have distinct negative Pt anomalies. Thus, Pd, Os, Ru and Ir concentrations are very high compared to the Pt concentrations (Os: 13.7–104 ng/g, Ir 11.3–19.0 ng/g, Ru 34.3–83.6 ng/g, Pt 0.41–9.07 ng/g, Pd 11.1–76.8 ng/g). With the exception of Pd, this appears to be a general feature of chromitites from ophiolites worldwide. However, Pd concentrations determined in this study are high compared to other studies where whole rock samples were analysed. There is no simple explanation for this difference because mass balance constraints would not allow that this is solely due to Pd-depletion in the interstitial component. Rather, it implies that chromitites display large variations of relative PGE abundances, even on a local scale. Podiform chromitite deposits form as a result of the interaction of fluid-rich, percolating melts with surrounding mantle peridotites. Osmium, Ir, Ru and Cr concentrations decrease systematically from harzburgite to dunite surrounding the deposits. In addition, dunites and chromites have complementary PGE distribution patterns. Thus, the mantle peridotite is the source of these metals in chromitites. This also indicates that these elements behave incompatibly and are mobilized during continuous melt percolation. However, the low Pt concentrations in the chromitites suggest that Pt is not as effectively mobilized during melt percolation. Uniformly high Pt concentrations in harzburgite and dunite (ca. 11 ppb) also imply that most Pt remains in the mantle peridotite. This can be explained if residual Pt-rich phases, such as PtFe alloys, limit the mobility of Pt. PGE and Cr become concentrated when chromite and sulfide liquids precipitate as a result of the mixing of percolating melts in magma pools near the crust–mantle boundary. The 187Os/ 188Os ratios of the chromite separates (0.1265–0.1301) are less variable than those of the associated peridotites (0.1235–0.1546). The average isotopic composition of the chromites ( 187Os/ 188Os: 0.1284±0.0021) is superchondritic compared with the carbonaceous chondrite value ( 187Os/ 188Os: 0.1260±0.0013 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187) and similar to the average value measured for podiform chromitites worldwide (0.12809±0.00085 after Geochim. Cosmochim. Acta 66 (2002) 329; Geochim. Cosmochim. Acta 66 (2002) 4187). Radiogenic melts/fluids derived from the subducting slab trigger partial melting in the overlying mantle wedge and add significant amounts of radiogenic Os to the peridotites. Mass balance calculations suggest that a melt/rock ratio of approximately 17:1 (melt: 187Os/ 188Os: 0.163, Os: 0.01 ng/g, mantle peridotite: 187Os/ 188Os: 0.127, Os 4.2 ng/g) is necessary in order to increase the Os isotopic composition of the chromitite deposits to its observed average value. This value implies a surprisingly low average melt/rock ratio during the formation of chromitite deposits. The percolating melts likely have variable isotopic composition and PGE concentration. However, in the chromitite pods the Os from many melts is pooled and homogenized, which is the reason why the chromitite deposits show such a small variation in their Os isotopic composition. The results of this study suggest that the 187Os/ 188Os ratio of chromitites is not representative for the DMM, but only reflects an upper limit.