Abstract
Mineralogical and chemical data are presented for a suite of Na–Cr-rich clinopyroxenes associated with chromite, winchite (sodium-calcium amphibole), titanite and calcite in Mg-Cr-rich silicocarbonatites from the Samalpatti carbonatite complex, Tamil Nadu, South India. The Mg-Cr-rich silicocarbonatites occur as 10–30 cm large enclaves in pyroxenites. The chemical composition of the pyroxenes differs among individual enclaves, with variable proportions of diopside, kosmochlor and jadeite-aegirine end-members. These compositions fill a previously unoccupied space in the kosmochlor-diopside-jadeite+aegirine ternary plot, indicating a distinct origin of kosmochlor-rich pyroxene compared with previous findings from diverse settings. The Na–Cr-rich clinopyroxene has low ΣREE = 9.2 ppm, with slight enrichment in LREE (LaN = 7), coupled with low HREE (YbN = 0.6), and flat HREE, paralleled by a significant fractionation of Nb/Ta (2408) and Th/U (26.5). Sodic metasomatism (fenitization) associated with either carbonatite emplacement at shallow levels or during carbonatite ascent through the upper mantle most likely was the major process operating in the area. We suggest two scenarios of the formation of Na–Cr-rich pyroxene: (1) from mantle-derived chromian mineral phases (spinel and/or garnet) through fenitization, with subsequent corrosion by growing winchite due to volatile influx; (2) via metasomatic reaction of Cr-rich garnet in mantle peridotite due to reaction with Na-rich carbonatite melt. Collectively, the appearance of kosmochlor may play an important role in deconvolving metasomatic processes, and fenitization in particular. If combined with petrologic experiments, it could improve our understanding of the origin and subsequent history of chemical signatures of carbonate-rich materials in the mantle.
Highlights
Chromium enters the crystal structure of pyroxenes in variable amounts
Sodium-Cr2 O3 (Cr)-rich pyroxene in samples IC03B and IC04A is mainly associated with chromite and titanite, and is enclosed in calcite and/or randomly oriented needles of sodium-calcium amphibole winchite [{CaNa}{Mg4 Al}(Si8 O22 )(OH)2 ] (Figure 3); minor plagioclase, K-feldspar, and accessory apatite are present
The Na–Cr-rich pyroxene grains have an irregular shape with residual crystals corroded by winchite or calcite (Figure 3a,b), and frequently contain inclusions of titanite (Figure 3c,d)
Summary
Chromium enters the crystal structure of pyroxenes in variable amounts. Only small admixtures of Cr can be found in common calcic clinopyroxenes with a structural formula M2M1(Si,Al) O6 , where M2 site is occupied mainly by Na and Ca, and M1 site by Mg, Fe2+ , Al, V, Ti, Fe3+ andCr3+ , respectively. Cr plays a more important role in the structure of sodic clinopyroxenes, where kosmochlor/ureyite (NaCrSi2 O6 ) is a chromian end-member of this group. The other two end members are jadeite (NaAlSi2 O6 ) + aegirine (NaFeSi2 O6 ) [1] and diopside (MgCaSi2 O6 ), and kosmochlor forms a solid solution with each of these. Minerals 2018, 8, 355 that pyroxenes from kimberlites are very rich in chromium and have up to 42 mol % of kosmochlor component. They stated that pyroxenes from kimberlites do not fall close to the most common diopside–kosmochlor binary join and that, as a consequence, these pyroxenes contain other components (aegirine, jadeite) in tens of mol %. In their study, a complete isomorphic diopside–kosmochlor join was identified for the first time in the nature (Sludyanka compley, Baikal region, Russia)
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