Megacrysts and xenoliths in kimberlite, Elliott County, Kentucky: A mantle sample from beneath the Permian Appalachian Plateau
Two kimberlite pipes in Elliott County contain rare ultramafic xenoliths and abundant megacrysts of olivine (Fo85 93), garnet (0.21 9.07% Cr2O3), picroilmenite, phlogopite, Cr-poor clinopyroxene (0.56 0.88% Cr2O3), and Cr-poor orthopyroxene (<0.03 0.34% Cr2O3) in a matrix of olivine (Fo88 92), picroilmenite, Cr-spinel, magnetite, perovskite, pyrrhotite, calcite, and hydrous silicates. Rare clinopyroxene-ilmenite intergrowths also occur. Garnets show correlation of mg (0.79 0.86) and CaO (4.54 7.10%) with Cr2O3 content; the more Mg-rich garnets have more uvarovite in solution. Clinopyroxene megacrysts show a general decrease in Cr2O3 and increase in TiO2 (0.38 0.56%) with decreasing mg (0.87 0.91). Clinopyroxene megacrysts are more Cr-rich than clinopyroxene in clinopyroxene-ilmenite intergrowths (0.06 0.38% Cr2O3) and less Cr-rich than peridotite clinopyroxenes (1.39 1.46% Cr2O3). Orthopyroxene megacrysts and orthopyroxene inclusions in olivine megacrysts form two populations: high-Ca, high-Al (1.09 1.16% CaO and 1.16 1.18% Al2O3) and low-Ca, low-Al (0.35 0.46% CaO and 0.67 0.74% Al2O3). Three orthopyroxenes belonging to a low-Ca subgroup of the high-Ca, high-Al group were also identified (0.86 0.98% CaO and 0.95 1.01% Al2O3). The high-Ca, high-Al group (Group I) has lower mg (0.88 0.90) than low-Ca, low-Al group (Group II) with mg=0.92 0.93; low mg orthopyroxenes (Group Ia) have lower Cr2O3 and higher TiO2 than high mg orthopyroxenes (Group II). The orthopyroxene megacrysts have lower Cr2O3 than peridotite orthopyroxenes (0.46 0.57% Cr2O3). Diopside solvus temperatures indicate equilibration of clinopyroxene megacrysts at 1,165° 1,390° C and 1,295° 1,335° C for clinopyroxene in clinopyroxene-ilmenite intergrowths. P-T estimates for orthopyroxene megacrysts are bimodal: high-Ca, high-Al (Group I) orthopyroxenes equilibrated at 1,165° 1,255° C and 51 53 kb (± 5kb) and the low-Ca, low-Al (Group II) orthopyroxenes equilibrated at 970° 1,020°C and 46 56 kb (± 5kb). Garnet peridotites equilibrated at 1,240° 1,360° C and 47 49 kb. Spinel peridotites have discordant temperatures of 720° 835° C (using spinel-olivine Fe/Mg) and 865° 1,125° C (Al in orthopyroxene). Megacrysts probably precipitated from a fractionating liquid at >150 km depth. They are not disaggregated peridotite because: (1) of large crystal size (up to 1.5 cm), (2) compositions are distinctly different from peridotite phases, and (3) they display fractionation trends. The high mg, low T orthopyroxenes and the clustering of olivine rims near Fo89 90 reflect liquid changes to higher MgO contents due to (1) assimilation of wall-rock and/or (2) an increase in Fe3+/Fe2+ and subsequently MgO/FeO as a result of an increase in f o.