Geochemistry and thermobarometry of mantle xenocrysts and xenoliths from the Mir pipe

Geochemistry and thermobarometry of mantle xenocrysts and xenoliths from Mir  kimberlite pipe were studied using new EPMA, SEM and LA ICP MS analyses

The PTX plot for the Mir pipe (Malo-Botuobinsky field) (Ashchepkov et al., 2010; 2014; 2019; 2022; 2023) shows the large interval from 8 to 1.1 GPa. The garnets show rathe narrow PT and P-Fe# but very wide P-Ca plots starting from the middle pyroxenitic layer to LAB. The Cr-Cpx and Cr-Sp are coinciding in Fe# in general. But the eclogites show very wide range of compositions trend. Diamond inclusions (DIA) (Sobolev, et al., 1976; 1997; Bulanova et al., 2002; Logvinova et al., 2004) the DIA pyropes have an opposite trend. In the P (GPa)-CaO plot largest variations in CaO are in the lower part of the mantle section. The most magnesian dunite varieties form an interval from 6.5 to 5 GPa, and then above them, the harzburgitic garnets again appear in the middle part of SCLM. There is high proportion of peridotite Cr-bearing varieties of ortho-and clinopyroxenes in the middle SCLM, which suggests that pyroxenites originated from peridotite partial melts. Omphacites together with garnets form an ascending P-Fe# plot. The geothermal conditions traced by DIA also form two branches. Even Cr-garnets partly trace the convective branch, although this is not evident in the middle part. The Cr-garnets are found at higher temperature conditions at deeper part of the SCLM. However, most of them plot between the 35–40 mWm−2 geotherms. The Cr-pyroxenites and Cr-diopsides form the colder branches to 35 mWm−2 geotherms or even lower. In the P-fO2 diagram, the less oxidized conditions correspond to the eclogitic clinopyroxenes in middle SCLM. At high pressures, the Cr-rich garnets give the lowest fO2 conditions.

The REE patterns of the pyropes show wide range of compositions from S-shaped dunitic to semi – rounded lherzolitic and flattened HREE harzburgitic and LREE enriched pyroxenitic. In multicomponent diagram they show peaks in Th-U and Pb and troughs in Sr and highly synchronously varying HFSE.

The Cpx form Gar lherzolites are showing several groups commonly inclined La/Ybn ~100 (normalization to primitive mantle (McDonough and Sun, 1995) with the hump at Ce- to Nd. In MCD they show very wide variations even in LILE from peak in Ba Rb to deep troughs and the same for U, Th. Pb, Sr. The HFSE are mostly moderately depleted (Zr <Hf) or deep minima Nb-Ta. Some low -Gar pyroxenites display less inclined patterns. There are more flattened patterns and REE -low for Sp lherzolites.

 The ilmenites in REE show different inclinations from positive (Lan ~100 and La/Ybn 70-100) to negative (Lan ~1-2 and La/Ybn ~0.1). They all show very high Ta-Nd peaks to 1000/PM and a bit less Zr-Hf (150-1000/PM).

Work was done on state assignments of IGM SB RAS FWZN-2026-0007. Russian Science Foundation Grant (24-27-00411).