PT conditions and trace element variations of picroilmenites and pyropes from placers and kimberlites in the Arkhangelsk region, NW Russia

摘要

Compositions of picroilmenite and pyrope concentrates from Carboniferous sandstones in the Arkhangelsk kimberlite province were analyzed by EPMA and LAM ICP MS in Analytic Center of V.S. Sobolev's Institute of Geology and Mineralogy, SD RAS, Novosibirsk. The results from single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) for garnet, spinel, ilmenite and clinopyroxene suggest heating of the base of the lithospheric mantle to 1400 ℃ (45 mw/m~2) at 7.0-7.5 GPa and to 900 ℃ (35 mw/m~2) at 3.5-5.5 GPa in an interval corresponding to a lens enriched in chromite and clinopyroxene. The pipes from the eastern fields reveal smoother mantle geotherms and lower temperature PT paths. Mantle columns beneath the kimberlites from northern (Verkhotinskoe field) and western pipes (Kepinskoe field) show heating from the lithosphere base to 5.0 GPa and stepped PT paths shown by chromites probably due to interaction with magmas which caused local Ti-enrichment near 3.0 and 5.5 GPa. The PT paths in the mantle columns beneath the alnoeite pipes reveal higher temperature and relatively shallow PT conditions with two major clusters around 3.0 and 5.0 GPa. Trace element patterns for garnets vary from S-type typical of harzburgites to those with a hump in MREE (middle REE) typical for pyroxenites. Lherzo-litic garnets with sinusoidal decrease of LREE show distinctive HFSE enrichment. Trace element ratios (Sm/Er)_n and (La/Yb)_n of garnets correlate positively with pressures estimates by single grain thermobarometry (Ashchepkov et al., 2010, 2011, 2012) but only poorly with Cr_2O_3 content. Enrichment in HFSE of all garnets is related to metasomatism that accompanied the picroilmenite-forming event. llmenites reveal two compositional trends. One corresponds to fractionation within conduits at the lower mantle (6.0-7.0 GPa) without contamination. A second trend at <6.0 GPa, formed due to assimilation fractional crystallization (AFC), is characterized by Fe and Cr increase with decreasing pressure. Similar trace element patterns of the various in HREE in ilmenites, possibly partly due to garnet assimilation from wall rock peridotites. The PT conditions and geochemistry for the minerals from the Carboniferous sediments are similar to those from the Lomonosovskoe deposit and Arkhangelskaya pipe (Lehtonen et al., 2009).