association of quartz, cr-pyrope and cr-diopside in mantle xenolith in v.grib kimberlite pipe (northern east european platform): genetic models

The first results of mineralogical and geochemical studies of a unique xenolith of lithospheric mantle are presented illustrating the earlier non-described mineral association of quartz, cr-pyrope and cr-diopside. structural and textural features of the sample suggest a joint formation of these minerals. the calculated p-t-parameters of the formation of cr-diopside indicate the capture of xenolith from the depth interval ~ 95-105 km (31-35 kbar) corresponding to the stability field of coesite. this suggests that quartz in the studied xenolith can represent paramorphs after coesite. it was shown that quartz in this rock is not a product of postmagmatic processes. the transformation stage of the source lherzolite into garnet- and clinopyroxene-enriched rock/garnet pyroxenite as a result of exposure to a high-temperature silicate melt was reconstructed. subsequent stages of the influence of metasomatic agents were identified by the presence of a negative eu-anomaly in some garnet grains, which could result from the impact of subduction-related fluid and the enrichment of rock-forming minerals with light rare earth elements, sr, th, u, nb and ta as a consequence of fluid saturated with these incompatible elements. several models for the formation of sio2 phase (quartz/coesite) in association with high-chromium mantle minerals are considered including carbonatization of mantle peridotites/eclogites and melting of carbonate-containing eclogites at the stage of subduction and the impact of sio2-enriched melt/fluid of subduction genesis with peridotites of the lithospheric mantle.