Mineralogy and geochemistry of the inclusion-bearing Cr-pyropes from the Chompolo lamprophyres, Aldan shield, Siberian сraton

Inclusion assemblages within Cr-pyrope xenocrysts from the Aldanskaya and Ogonek lamprophyres (Chompolo field, Aldan shield of Siberian craton, Yakutia) are characterized by the wide list of minerals. Partially the inclusion assemblages with graphite within Cr-pyropes in Chompolo lamprophyres were previously described (Nikolenko et al., 2017).

Here we present the results of a trace-elements study of 54 pyrope grains with Cr-spinel inclusions. The majority of studied pyropes are lherzolitic with small amount of wherlitic and harzburgitic ones, according to the classification schemes (Sobolev et al 1973; Grutter et al., 2004). The concentration of Cr₂O₃ ranges from 1.58 to 7.56 wt% at Mg # = 69.6-84.4 and Ca # = [100Ca / (Ca + Mg + Fe + Mn)] = 8.6-26.3. The TiO₂ content does not exceed 0.36 wt%. The MnO contents in the pyropes studied is in the range of 0.35–0.69 wt%, which indicates rather low temperature conditions (Grutter at al., 2004).

Studied mineral inclusions can be divided in two groups by their morphology and position within the host pyrope grain. Majority of the studied Cr-spinels within pyropes are represented by the single-mineral inclusions (CrSp-I), which have clear octahedral morphology but some of them can be described by more complex morphology that looks as irregular or rounded. Single Cr-spinel inclusions are commonly large and range in size from 100 to 500 µm. Another inclusions type represents joint associations of Cr-spinels (CrSp-II) with silicates, carbonates, sulphides, graphite, volatile-bearing minerals and series of Ti-oxides. Size of Cr-spinels II in this samples is usually 10-50 µm and rarely reaches 100 µm.

The distribution of the rare earth elements (REE) for pyropes containing CrSp-I inclusions in chondrite-normalized REE-diagram has a sinusoidal pattern and is characterized by the chondrite-normalized ratio Sm_N/Er_N > 1 at low Ti/Eu values, which is a sign of carbonatite metasomatism (Shchukina et al., 2017). Pyropes containing complex polyphase inclusions with CrSp-II carry signs of silicate (melt) metasomatism, expressed in elevated contents of Y (up to 20.5 ppm) and Zr (9.5–44.6 ppm) and an increased Ti impurity. Pyropes with CrSp-II inclusions have typical for lherzolites distribution of REE with  Sm_N/Er_N ratio in the range of 0.5-1.

Cr-spinel inclusions within pyropes were also studied in detail and revealed some differences in the chemical composition between two groups.

Temperatures estimated for the pyropes containing mineral inclusions using Ni-in-garnet thermometer ranges from 640-910 °C. Temperatures were also estimated for Cr-spinel inclusions by use the Zn-in-spinel thermometer (Ryan et al., 1996). The temperature distribution for CrSp-I and CrSp-II groups shows different values with maximum frequency at 650-700 and 750-800 °C respectively.

The geochemical features, the composition of inclusions and the results of thermometry of the two described pyrope populations with Cr-spinel inclusions indicate different metasomatic processes associated with their formation.

Complex studies of mineral inclusions in Cr-pyropes and major element analyses of Cr-pyropes and Cr-spinels were supported by the Russian Science Foundation, grant No 18-77-10062. Trace-elements studies of Cr-pyropes were supported by the Russian Science Foundation, grant No 18-17-00249.