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At Mt. Vulture volcano (Basilicata, Italy) calcite globules (5–150 μm) are hosted by silicate glass pools or veins cross-cutting amphibole-bearing, or more common spinel-bearing mantle xenoliths and xenocrysts. The carbonate globules are rounded or elongated and are composed of a mosaic of 2–20 μm crystals, with varying optical orientation. These features are consistent with formation from a quenched calciocarbonatite melt. Where in contact with carbonate amphibole has reacted to form fassaitic pyroxene. Some of these globules contain liquid/gaseous CO2 bubbles and sulphide inclusions, and are pierced by quench microphenocrysts of silicate phases. The carbonate composition varies from calcite to Mg-calcite (3.8–5.0 wt.% MgO) both within the carbonate globules and from globule to globule. Trace element contents of the carbonate, determined by LAICPMS, are similar to those of carbonatites worldwide including ΣREE up to 123 ppm. The Sr–Nd isotope ratios of the xenolith carbonate are similar to the extrusive carbonatite and silicate rocks of Mt. Vulture testifying to derivation from the same mantle source. Formation of immiscibile silicate–carbonatite liquids within mantle xenoliths occurred via disequilibrium immiscibility during their exhumation. 相似文献
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This paper presents the textural, mineralogical and chemical study of veinlets cross-cutting peridotite xenoliths from the lithospheric mantle and brought to the surface by alkaline basalts (Persani Mountains, Romania). The veinlets utilized pre-existing zones of weakness in the host rocks or display a random distribution, lining grain boundaries or cross-cutting any mineral, and always forming an interconnected network. They are filled with carbonate patches included in a silicate matrix. Both products are holocrystalline. Carbonate products have alkali-poor calciocarbonatitic to sövitic compositions, while the silicate matrix composition ranges from monzodioritic to monzonitic and alkali feldspar syenitic, depending on the host-sample, i.e., within a rather alkaline silica-saturated series. The mineral phases present in the silicate matrix (F-apatite, armalcolite, chromite, diopside–enstatite series, plagioclase–sanidine series) are usually present in the carbonate zones, where forsterite is also found. Some minerals cross-cut the interface between both types of zones. Only the matrix is different, feldspathic (oligoclase to sanidine) in the former and pure calcite in the latter. Thus, mineralogical and textural relationships between both products are consistent with an origin with equilibrium liquid immiscibility. Mantle minerals cross-cut by veinlets are sometimes resorbed at grain boundaries, and at the contact of the most alkaline silicate and carbonate melts, subhedral diopside/augite formed at the expense of mantle enstatite or olivine. In terms of mineral chemistry, the compositional variations recorded by vein minerals vary along a continuous trend. They generally superpose to those observed from lherzolites to harzburgites, and exhibit the same range of composition as that observed between rims and cores of mantle minerals cross-cut by veinlets. In detail, the Ca-rich pyroxenes of veinlets are Al-poor and Mg-rich; cpx in the carbonate zones are slightly more Ca-rich than those in the silicate matrix; spinels are relatively Al- and Mg-poor but rather Cr- and Fe-rich. Existence of only one titanium oxide (armalcolite) and various pairs of pyroxenes suggest crystallization temperatures in the range 1100–1200°C and pressures between 10–15 kb. Feldspar compositions in silicate materials, which vary continuously from labradorite to sanidine, are consistent with hypersolvus and dry crystallization conditions. All of these results provide evidence that immiscibility occurred at mantle depth as the liquid was forcibly injected during hydraulic fracturing of the mantle. The compositions of conjugate melts suggest a very large miscibility gap, as expected at high pressure in a dry environment from the experiments of Kjarsgaard and Hamilton [Kjarsgaard, B.A., Hamilton, D.L., 1988. Liquid immiscibility and the origin of alkali-poor carbonatites. Mineral. Mag. 52, 43–55; Kjarsgaard, B.A., Hamilton, D.L., 1989. The genesis of carbonatites by immiscibility. In: Bell, K. (Ed.), Carbonatites: Genesis and Evolution. Unwyn Hyman, London, pp. 388–404.]. The parental melt was carbonate, silica-undersaturated and rich in F, Cl and CO2. Both immiscible melts were water-undersaturated. The cooling rate until total crystallization in veinlets was very slow, limited and necessarily occurred at mantle depth. Wall rock reactions leading to the formation of Ca-rich pyroxene at the expense of mantle enstatite or olivine occurred only at the contact with somewhat alkali-rich carbonatitic or silicate melts. Calcite, always anhedral, is the last mineral to crystallize. It is a differentiation product formed by magmatic crystallization or wall rock reaction. In some cases, given the rarity of any other minerals, it may be the product of the crystallization of a pure sövite immiscible melt. 相似文献
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