12 research outputs found
Metasomatic origin of garnet xenocrysts from the V. Grib kimberlite pipe, Arkhangelsk region, NW Russia
AbstractThis paper presents new major and trace element data from 150 garnet xenocrysts from the V. Grib kimberlite pipe located in the central part of the Arkhangelsk diamondiferous province (ADP). Based on the concentrations of Cr2O3, CaO, TiO2 and rare earth elements (REE) the garnets were divided into seven groups: (1) lherzolitic ādepletedā garnets (āLz 1ā), (2) lherzolitic garnets with normal REE patterns (āLz 2ā), (3) lherzolitic garnets with weakly sinusoidal REE patterns (āLz 3ā), (4) lherzolitic garnets with strongly sinusoidal REE patterns (āLz 4ā), (5) harzburgitic garnets with sinusoidal REE patterns (āHzā), (6) wehrlitic garnets with weakly sinusoidal REE patterns (āWā), (7) garnets of megacryst paragenesis with normal REE patterns (āMegā). Detailed mineralogical and geochemical garnet studies and modeling results suggest several stages of mantle metasomatism influenced by carbonatite and silicate melts. Carbonatitic metasomatism at the first stage resulted in refertilization of the lithospheric mantle, which is evidenced by a nearly vertical CaO-Cr2O3 trend from harzburgitic (āHzā) to lherzolitic (āLz 4ā) garnet composition. Harzburgitic garnets (āHzā) have probably been formed by interactions between carbonatite melts and exsolved garnets in high-degree melt extraction residues. At the second stage of metasomatism, garnets with weakly sinusoidal REE patterns (āLz 3ā, āWā) were affected by a silicate melt possessing a REE composition similar to that of ADP alkaline mica-poor picrites. At the last stage, the garnets interacted with basaltic melts, which resulted in the decrease CaO-Cr2O3 trend of āLz 2ā garnet composition. Cr-poor garnets of megacryst paragenesis (āMegā) could crystallize directly from the silicate melt which has a REE composition close to that of ADP alkaline mica-poor picrites. P-T estimates of the garnet xenocrysts indicate that the interval of ā¼60ā110Ā km of the lithospheric mantle beneath the V. Grib pipe was predominantly affected by the silicate melts, whereas the lithospheric mantle deeper than 150Ā km was influenced by the carbonatite melts