Cr-pyrope xenocrysts with oxide mineral inclusions from the Chompolo lamprophyres (Aldan shield): Insights into mantle processes beneath the southeastern Siberian craton

Abstract

Pyrope xenocrysts (N = 52) with associated inclusions of Ti-and/or Cr-rich oxide minerals from the Aldanskaya dyke and Ogonek diatreme (Chompolo field, southeastern Siberian craton) have been investigated. The majority of xenocrysts are of lherzolitic paragenesis and have concave-upwards (normal) REE N patterns with increase in concentration from LREE to M-HREE (Group 1). Four Ca-rich (5.7-7.4 wt.% CaO) pyropes are extremely low in Ti, Na, Y and have sinusoidal REE N spectra, thus exhibiting distinct geochemical signatures (Group 2). A peculiar xenocryst s165 is the only sample to show harzburgitic derivation, whilst demonstrating normal to weakly sinusoidal REE N pattern and the highest Zr (93 ppm) and Sc (471 ppm). Chromite-magnesiochromite, rutile, Mg-ilmenite, and crichtonite-group minerals comprise a suite of oxide mineral inclusions in the pyrope xenocrysts. These minerals are characteristically enriched in Cr with 0.6-7.2 wt.% Cr 2 O 3 in rutile, 0.7-3.6 wt.% in Mg-ilmenite and 7.1-18.0 wt.% in the crichtonite-group minerals. The LILE-enriched complex titanates of the crichtonite group are high in Al 2 O 3 (0.9-2.2 wt.%), ZrO 2 (1.5-5.4 wt.%) and comprise a trend of compositions from the Ca-Sr-specific varieties to the Ba-dominant species (e.g. lindsleyite). In the studied pyrope xenocrysts the oxides coexist with silicates (clino-and orthopyroxene, olivine), hydrous silicates (talc, phlogopite, amphibole), carbonate (magnesite), sulfides (pentlandite, chalcopyrite, breakdown products of monosulfide and bornite solid solutions), apatite and graphite. P-T estimates imply the inclusion-bearing pyrope xenocrysts to have been derived from low-T peridotite assemblages that resided at ~600-800 °C and a pressure range of ~25-35 kbar in the graphite stability field. The pyrope genesis is linked to the metasomatic enrichment of peridotite protoliths by Ca-Zr-LILE-bearing percolating fluid-melt phases containing significant volatile components. These metasomatic agents are likely volatile-rich melts or supercritical C-O-H-S fluids that were released from a Paleo-subduction slab.

Botuobinskite and mirnyite, two new minerals of the crichtonite group included in Cr-pyrope xenocrysts from the Internatsionalnaya kimberlite

“…Both minerals have been documented in pyrope crystals from an ultramafic diatreme of the Garnet Ridge cluster, Arizona, USA (Wang et al , 1999). Botuobinskite has also been recognised in composite polymineralic inclusions in Cr-pyrope crystals from the Aldanskaya lamprophyre dyke, Chompolo field, southeastern Siberian craton (Rezvukhin et al , 2022). Apart from the findings in intimate association with pyrope, mirnyite occasionally occurs in association with the typical LIMA series (lindsleyite–mathiasite solid solution) Zr-enriched titanates in metasomatised peridotite xenoliths sampled by South African kimberlites (Haggerty et al , 1983; Haggerty, 1991; Gregoire et al , 2002; Konzett et al , 2013).…”

“…botuobinskite and mirnyite) are particularly notable for their association with Cr-pyrope of lherzolitic paragenesis in the lithospheric mantle assemblages (e.g. Wang et al , 1999 ; Rezvukhin et al , 2018, 2022). To the best of our knowledge, crichtonite-group titanates have never been documented in association with depleted harzburgitic or dunitic garnet.…”

“…In particular, the mantle-derived pyrope-hosted crichtonite-group titanates from the Internatsionalnaya kimberlite pipe are characterised by compositions rich in Na, Ca, Sr and REE, whereas those from South African kimberlites (LIMA series) are mostly Ba and K rich. The former also have larger Al contents than the latter, which probably reflects equilibrium with the host garnet (Rezvukhin et al , 2018, 2022). These differences indicate that the compositions of the metasomatic melts or fluids circulating in the lithospheric mantle of the Siberian and Kaapvaal cratons were not identical.…”