Transformation of olivine to diopside via interaction with carbonatite melts.

Numerous empirical and experimental studies have documented the consumption of orthopyroxene and production of clinopyroxene in mantle peridotite during carbonatite metasomatism, whereas little attention has been paid to whether and how olivine is compositionally modified through the same process of carbonatite metasomatism. Here, we present the discovery of well-preserved reaction features in olivine from typical calciocarbonatites from Sri Lanka. The olivine grains experienced serpentinization along the cracks and in their rims and are generally surrounded outwardly by serpentine, tremolite and diopside zones. The inner serpentine zone is dominated by serpentine with tiny magnetite, whereas the mantle tremolite zone is characterized by euhedral tremolite and intergrowth of dolomite with small amounts of serpentine relics and diopside. The outer diopside zone shows spongy texture and commonly encloses olivine, serpentine, tremolite and dolomite, which are also observed in interstitial diopside grains in the carbonatites. Compositional profile analyses reveal that these olivine grains display decreasing FeO and increasing MgO contents and thus elevating Fo values from their cores to the reaction rims, while the serpentine inherits compositions from the olivine. Elemental mapping images demonstrate outward diffusion of Si and Mg in the serpentine zones and contribution of Ca from carbonatite melts during tremolite formation. Further interaction with carbonatite melts produced Si- and Mg-poor and Ca-rich diopside relative to the reactant tremolite. These observations suggest that the interaction with carbonatite melts first transformed olivine to diopside through serpentinization, followed by tremolitization. The diopside formed through olivine-carbonatite melt interaction is anomalously depleted in Na and Al and enriched in Ca, which are distinct from those generated in interactions between orthopyroxene and both carbonatite and silicate melts. This implies that for the first time some wehrlites and clinopyroxenites in the mantle might originate from intense interaction between peridotites (including dunite) and carbonatite melts. This mechanism can be invoked to explain the formation of other global occurrences of wehrlite and clinopyroxenite suites. [ABSTRACT FROM AUTHOR]