Implications For Metasomatism Research Articles

Spinel and Ti-rich schorlomite from the Wajrakarur kimberlites, southern India: Implications for metasomatism, diamond potential and orangeite lineage

Kl-4 and P-5 mesoproterozoic kimberlite pipes along with several other well-known diamondiferous (ca. 1110 Ma) kimberlites in the Wajrakarur kimberlite field (WKF) intruded into the cratonic roots of Eastern Dharwar craton (EDC) in southern India. The groundmass minerals of the kimberlites exhibit inequigranular texture contain spinel, Ti-rich schorlomite garnet, two generations of olivine (macrocrysts and groundmass microphenocrysts), phlogopite, perovskite, clinopyroxene (diopside), ilmenite (low Mn) and rare apatite. We identified three distinct spinel associations in Kl-4 and P-5: (i) fine-grained (<50 μm) microcrysts in the groundmass; (ii) resorbed euhedral atoll spinel, consisting of titanomagnetite (magnesian-ulvospinel-magnetite to titanian-chrome-magnetite) which is isolated from the rim of magnetite by spongy lagoon phase of schorlomite, and (iii) larger chrome spinel macrocrysts/xenocrysts (>500 μm). The schorlomite garnet in both P-5 and Kl-4 represents solid solution of schorlomite-pyrope-almandine-grossular. Additionally, Kl-4 contains another Cr-rich schorlomite-pyrope-almandine-uvarovite solid solution. Macrocrystic spinel exhibits distinct composition of chromium (Cr2O3 up to 59.62 wt%), and poor in TiO2 (<1.19 wt%). The high chromium spinel macrocrysts from Kl-4 are confirmed to be fragments of mantle xenocrysts and their composition falls within the diamond stability field. Atoll-textured epitaxial mantled resorbed spinel associated with schorlomite suggests that they formed through the replacement of spinel possibly through interaction of late residual fluids/melts in the final stages of crystallization of the kimberlite magma. The significant enrichment of Fe and Ti in schorlomite suggests the involvement of metasomatized sub-continental lithospheric mantle. It is also inferred that spinel immiscibility played an important role in the metasomatic replacement. The Ti-rich minerals have orangeitic affinity, similar to those in the Kaapvaal craton of South Africa, and suggest the high Ti-, high Ca- and the low Al-bearing nature of the parent magma (Group II kimberlites). The groundmass tetraferriphlogopite is Al- and Ba-poor and spinel show compositions straddling between magnesian ulvöspinel (Group I kimberlite) and titanomagnetite (Group II kimberlite) comparable with orangeite and lamproites. The results presented in this study suggest that the P-5 and Kl-4 has orengeitic or lamproitic affinity. Our findings can be useful as an indicator mineral in diamond prospecting.

Scheelite and coexisting F-rich zoned garnet, vesuvianite, fluorite, and apatite in calc-silicate rocks from the Mogok metamorphic belt, Myanmar: Implications for metasomatism in marble and the role of halogens in W mobilization and mineralization

Scheelite, which is an important ore of tungsten and colored gemstone, is well developed in the calc-silicate rocks from the Mogok metamorphic belt (MMB), Myanmar. In this study, the textural, mineralogical, and compositional characteristics of scheelite and its associated minerals were systematically investigated to constrain the petrogenesis of scheelite-bearing calc-silicate rocks and the tungsten transfer and mineralization mechanism in a hydrothermal–metasomatic system. The petrological evidence, bulk and mineral geochemical signatures, and mass-transfer calculations indicate that the calc-silicate rocks formed by local metasomatism of marble via the introduction of an externally derived Si–Al–Fe–W–F-bearing, H2O-rich fluid phase. The distinct compositional zonations [F, Fe, Ca, and heavy rare earth elements (HREEs)] of garnet in the calc-silicate rocks record a two-stage metasomatic process and significant compositional variation in the associated fluid. The late-stage metasomatic fluid that led to the formation of the F-rich garnet rims, scheelite, and most of the calc-silicate minerals has noticeably higher fluorine activity (aF−), oxygen fugacity (fo2), and HREE content than the early-stage metasomatic fluid responsible for the garnet cores.The MMB scheelite exhibits typical “skarn-type” compositional characteristics with a high LaN/YbN ratio (100–180), a negative Eu anomaly (δEu=0.3–0.5), and a high Mo content (1100–1330ppm). These geochemical signatures are primarily controlled by the protolith, metasomatic fluid, redox conditions, and coexisting mineral phases. The enrichment of rare earth elements (REEs) and high field strength elements (HFSEs) in the MMB scheelite was dominated by two substitutionreactions: Ca2++W6+=REE3++HFSE5+ and 3Ca2+=2REE3++□Ca (where □Ca is a Ca-site vacancy). Considerable amounts of F and OH in the metasomatic fluid substituted for O in the garnet via the substitute reaction 4(F, OH)−=4O2−+Si4+, leading to a significant enrichment of F (up to 1.2wt.%) and OH (up to 0.32 for nOH) and a negative correlation between F and Si in the garnet.Detailed petrographic observations show that the occurrence of scheelite in the MMB calc-silicate rocks is always associated with the growth of F-rich minerals such as garnet rims (0.8–1.2wt.% F), vesuvianite (2.4–2.6wt.% F), fluorite (48–49wt.% F), apatite (3.9–4.1wt.% F), and titanite (2.6–3.4wt.% F). These textural characteristics, combined with the positive correlation of whole-rock F and W (as well as Sn, Mo) contents in the calc-silicate rocks, indicate that the elevated F contents increased the solubility of W in the infiltrating fluid, thereby allowing the W transfer in the hydrothermal–metasomatic system. The mineralization of scheelite was triggered by the crystallization of F-rich minerals during the formation of the calc-silicate rocks, which caused F depletion and consequent saturation of W in the metasomatic fluid. Our results suggest that, in the MMB metasomatic system, F rather than Cl is the key fluxing compound that facilitates the transfer of W and the mineralization of scheelite.

Actinolitic rinds on low-T mafic blueschist blocks in the Franciscan shale-matrix mélange near San Simeon: Implications for metasomatism and tectonic history

In the mélange terrane of the Franciscan accretionary prism, Mg-rich actinolite/tremolite “rinds” have long been known to be present on many high-T, garnet ± epidote-bearing, mafic blueschist and eclogite blocks, but they were thought to be absent on low-T lawsonite + chlorite blueschist blocks. By analogy with rinds on high-T blocks, these rinds on low-T blocks must have been produced by metasomatic reaction with ultramafic rock. The rinds on high-T blocks have led to the speculation that the high-T Franciscan blueschists were uplifted to the surface in serpentinite diapirs, while other processes exhumed the volumetrically more numerous low-T blocks. This paper reports the discovery of actinolitic rinds on the margins of nine low-T, lawsonite ± epidote Franciscan blueschists that are, or were, clearly encased in the shale-matrix mélange exposed in seacliffs and wavecut benches near San Simeon, California. Petrographic and geochemical analyses show these Mg-rich rinds are similar to those found on high-T garnet-bearing blocks.The bulk composition indicates the protolith of the blueschist blocks was mafic oceanic crust. The high Fe3+/∑Fe contents indicate it was probably the uppermost hydrated volcanic part of the crust. The block/ultramafic rock association was created by faulting during subduction initiation as fragments from the top of the subducting ocean crust were imbricated with mantle materials at the base of the ophiolitic leading edge of the North American plate. The chemistry of a block margin altered to actinolitic rind indicates Si, Na, Pb, Cu, and Sr were lost from the blocks while K, Rb, Cs, and Ba were added. Most pieces of actinolitic rind are Mg- and Ni-rich because they are metasomatized ultramafic material that was enriched in Si, K, Rb, Cs, and Ba. These metasomatic changes indicate the fluids that hydrated the mantle and altered the mafic blocks ascended from subducting and/or underplated sediments. Both high-T and low-T blueschists that have actinolitic rinds were uplifted to near the surface after they sank or were plucked from the base of the overriding plate by subduction-generated mélange that upwelled from depths of >15 km.

Cogenetic silica-rich and carbonate-rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths: Implications for metasomatism in the oceanic upper mantle

In an attempt to characterize metasomatic agents for the oceanic upper mantle, we have undertaken a study of melt and fluid inclusions trapped in metasomatized peridotite nodules (anhydrous spinel lherzolites and harzburgites) from the Kerguelen Islands (southern Indian Ocean). These xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene. These are silicate melt inclusions, carbonate-rich inclusions and CO 2 fluid inclusions. These inclusions are secondary in nature and form trails along fracture planes in the sheared peridotites. Heating experiments conducted on silicate melt inclusions give an estimation of the entrapment temperatures ( ≈ 1250°C) and indicate that there is no genetic relationship between the inclusions and their host minerals. The chemical composition of the silicate melt inclusions is characterized by normative quartz and feldspar components, with SiO 2 ≈ 60wt%, Al 2O 3 ≈ 20wt%, Na 2O and K 2O each ≈ 4–5wt%, FeO and MgO < 3 wt%, Cl > 1000 ppm, H 2O ⩾ 1.2%, and oversaturation of the melt with CO 2. The trace element signature is characterized by LREE enrichment, negative HFSE (Ti and Zr) anomalies and a Ti Zr value of 17. The trapped melt has crystallized the following minerals: K-rich amphibole (kaersutite), diopside, rutile, ilmenite and carbonate (magnesite). Carbonate-rich inclusions, interpreted as trapped carbonate melt, have crystallized calcite. The carbonate-rich inclusions are often physically connected with the silicate melt inclusions, indicating the former existence of a homogeneous melt which later unmixed into two separate melts. Cogenetic relationships between CO 2 inclusions and both carbonate melt inclusions and silicate melt inclusions yield a minimum trapping pressure for all types of inclusions of 12.5 kbar at 1250°C, corresponding to upper mantle depths. Based on their daughter mineral types, their chemical composition and high volatile element contents, the silicate-carbonate melt inclusions trapped in the ultrabasic xenoliths of the Kerguelen Islands are interpreted as small amounts of a metasomatic melt phase. These melt inclusions cannot result from melting of the anhydrous peridotite assemblages in which they have been trapped. They must represent an exotic, migrating metasomatic melt phase in the oceanic lithosphere below the Kerguelen Islands.

The mineral chemistry of new titanates from the jagersfontein kimberlite, South Africa: Implications for metasomatism in the upper mantle

New members of the crichtonite mineral series are described in which K, Ba, Ca and REE are in significant concentrations (5 wt% oxides) filling the A formula position in AM 21 O 38. These phases are chromium (16 wt% Cr 2O 3) titanates (58 wt% TiO 2) enriched in ZrO 2 (5 wt%) and constitute a mineral repository for refractory and large ion lithophile elements in the upper mantle. The mineral senes coexists with Mg-Cr-ilmenite, Nb-Cr-rutile, and Ca-Cr (NbZr) armalcolite that have equally unusual chemistries. Kimberlitic crichtonites are depleted in the intermediate lanthanides but highly enriched in LREE and HREE with chondrite normalized abundances of 10 3 to 10 5. Crichtonite, armalcolite, and Nb-Cr-rutile occupy a compositional range in TiO 2 contents bridging the gap between ilmenite and rutile, two minerals having a widespread distribution in kimberlites and mantle-derived nodule suites. In common with other associations, and based on similarities in mineral chemistry, it is concluded that these minerals formed at P = 20–30 kb, 900–1100°C by reaction of peridotite with metasomatizing fluids. Kimberlitic crichtonite may be expressed as spinel + Cr-ferropseudobrookite, and armalcolite is equivalent to Cr-geikielite + rutile in the system (FeMg)-TiO 2-Cr 2O 3. This system contains a number of Cr-Ti compounds not found as minerals but it is proposed that the ubiquitous occurrence of ilmenite intergrowths in kimberlitic rutile results from decomposition of high pressure αPbO 2-type crystallographic shear structures. The new minerals have exotic chemistries and the high K-affinities broaden the scope for the origin of alkalic rocks, the generation of highly potassic magmas in the upper mantle, and suggest that alkali metasomatism may be pervasive.