Mica-Amphibole-Rutile-Ilmenite-Diopside Research Articles

Modal metasomatism in the Kaapvaal craton lithosphere: constraints on timing and genesis from U-Pb zircon dating of metasomatized peridotites and MARID-type xenoliths

Modal metasomatism in the Kaapvaal craton lithosphere is well documented in upper mantle xenoliths sampled by both group I (mainly late Cretaceous) and group II (mainly early Cretaceous to late Jurassic) kimberlites in the Kimberley area. The metasomatic style is characterized by introduction of K, H and large ion lithophile/high field strength (LIL/HFS) elements into the lithospheric mantle leading to the crystallization of hydrous potassic phases such as phlogopite and/or K-amphibole. Textures indicate that the hydrous phases either replace pre-existing assemblages in peridotites, forming the metasomatized peridotite suite (phlogopite–K-richterite–peridotites: PKPs) or crystallize from K-rich melts, forming the mica–amphibole–rutile–ilmenite–diopside (MARID) suite of xenoliths. These K-rich assemblages become potential low melting source components for alkaline incompatible trace element enriched magmas. The timing of metasomatism and its temporal and possible genetic relation to kimberlite magmatism is poorly constrained because of the rarity of phases in the metasomatic assemblages suitable for precise dating. Here we present precise sensitive high resolution ion microprobe (SHRIMP) U–Pb formation ages of 88 ± 2 (1σ=1 standard deviation) and 82 ± 3 Ma data for zircons from a K-richterite–phlogopite-bearing metasomatized peridotite (PKP) and a MARID xenolith respectively, sampled by a group I kimberlite. Both average PKP and MARID zircon ages are indistinguishable from emplacement ages of group I kimberlites in the Kimberley area dated at 83 ± 4 (2σ) and 84 ± 0.9 Ma. One exceptionally old age spot of 102 ± 5 Ma from a PKP zircon provides evidence for modal metasomatism predating group I kimberlite emplacement by several millions of years with minor resetting of the U–Pb isotopic system of most analyzed PKP zircons to a group I emplacement age. Detailed textural and mineral chemical analysis, including high energy X-ray mapping and analysis of fluid inclusion daughter crystals, indicates a complex reaction history for both PKPs and MARIDs. U–Pb zircon ages from this study combined with literature data and experimentally derived models for MARID formation are used to suggest that MARID-formation is concurrent and genetically related to both group I and II kimberlite magmatism in the Kimberley area. MARID and PKP zircon ages are also consistent with the idea first proposed by Dawson and Smith (Geochim Cosmochim Acta 41: 309–323, 1977) that metasomatized peridotites may form from interaction of hydrous fluids expelled by solidifying MARID-type melts with peridotitic wall rocks.

An oxygen barometer for rutile–ilmenite assemblages: oxidation state of metasomatic agents in the mantle

Oxygen fugacity has been calculated for rutile–ilmenite assemblages from the reaction 2Fe 2O 3 (in ilmenite) + 4TiO 2 (rutile) = 4FeTiO 3 (in ilmenite) + O 2. The equation log fO 2=22.59−25925/ T−3.09log T+0.0016535 P+48.836 P/ T−4log a Ilm FeTiO 3 +2log a Ilm Fe 2O 3 +4log a Rut TiO 2 , where T is in kelvin and P is in kbar, was derived from available thermodynamic data. The hypothetical end-member rutile–ilmenite reaction is located between the magnetite–hematite and Ni–NiO (NNO) buffers. The rutile–ilmenite oxygen barometer has been applied to ilmenite-bearing assemblages in mantle xenoliths from kimberlites, including the metasomatic MARID (mica–amphibole–rutile–ilmenite–diopside) suite and a MORID (mica–orthopyroxene–rutile–ilmenite–diopside) vein, along with rutile–ilmenite assemblages in eclogites and in Granny Smith diopside megacrysts. The oxygen fugacities of MARID and MORID lie around the NNO buffer and are comparable to those in metasomatized spinel lherzolites. Most MARID and MORID assemblages yield a more oxidizing fO 2 than the EMOD (enstatite–magnesite–olivine–diamond) buffer, such that MARID and MORID fluid or melt would tend to destroy diamond or graphite by oxidation.

Potassium Amphibole Stability in the Upper Mantle: an Experimental Study in a Peralkaline KNCMASH System to 8.5 GPa

Experiments were performed from 1·0 to 8·5 GPa in a peralkaline INTRODUCTION system K2O‐Na2O‐CaO‐MgO‐Al2O3‐SiO2‐H2O (KNCMASH) K-rich amphiboles are exotic members of the amphibole to investigate the stability and composition of richteritic amphiboles family in terms of their frequency of occurrence at in the MARID (mica‐amphibole‐rutile‐ilmenite‐diopside) as- the Earth’s surface. With few exceptions they are semblage amphibole + phlogopite + clinopyroxene. The results represented by K-rich varieties of richterite Nawere compared with phase relations and the composition of natural CaNaMg5Si8O22(OH)2, katophorite NaCaNaMg4‐ MARIDs to assess possible mechanisms of formation for MARID- AlSi7AlO22(OH)2 and, more rarely, arfvedsonite type rocks. K-richterite is stable in a wide range of bulk K/Na NaNa2Mg4Fe 3+ Si8O22(OH)2. The vast majority of known ratios in the MARID assemblage to 8·5 GPa and 1300∞C. In occurrences is confined to extremely alkaline, Kthis assemblage the amphibole can accommodate significant amounts dominated mainly magmatic environments which are of K on the M(4)-site and shows a systematic increase in the K/ poor in Al and partly Si undersaturated. Experimental Na ratio with increasing pressure. At P >7·0 GPa, K-richterite results on K-richterite stability show that this amphibole can coexist with garnet. Phase relations of K-richterite in a natural can act as a principal reservoir for alkalis and water MARID composition are consistent with those in the simplified as well as for a number of incompatible trace elements system and confirm the potential stability of K-richterite and K- in the Earth’s mantle down to transition zone depths, richterite + garnet within the diamond stability field. The as- and may be involved in deep-seated magma generation semblage K-richterite + phlogopite + clinopyroxene is incompletely processes: K-richterite (or Ti- and/or F-rich varieties)

Fluid processes in diamond to spinel facies shallow mantle

Topography on the lithosphere-asthenosphere boundary and resultant variations in the architecture of the lithosphere are inextricably linked to heat and mass transfer processes involving the convecting upper mantle. This is primarily accomplished by the movement of silicate and nonsilicate (i.e. carbonate) melts, derivatives of which are trapped in the shallow mantle beneath tectonically active and inactive parts of continents. In diamond facies mantle (180–220 km), melt character is inferred from solid inclusions (e.g. apatite, carbonate, mica) in megacrystic diamonds and the mineralogy and chemistry of diamondiferous peridotites and eclogites. Deep lithospheric processes have involved silicate (K-rich and hydrous) and carbonate melts. In garnet facies mantle (75–200 km) information on melt transfer processes occurs in kimberlite-borne xenoliths, e.g. phlogopite-richterite garnet peridotite, MARID (mica-amphibole-rutile-ilmenite-diopside) pyroxenites and IRPS (ilmenite-rutile-phlogopite-sulphide) pyroxenites. Polybaric fractionation and crystallization of silicate melts in propagating fractures in the lower lithosphere can explain the character of several xenolith suites. Transfer of potassic silicate melts (equiv. kimberlite/lamproite) are thought to be closely linked to the genesis of MARIDs and hydrous garnet peridotites, and, the transfer of alkaline silicate melts (equiv. basalt) may explain the character of IRPS pyroxenites. In spinel facies mantle (<75 km) kaersutite-pargasite-carbonate spinel peridotites and amphibole-mica-apatite pyroxenites constrain the nature and origin of shallow mantle fluid processes. While transfer of mobile silicate (equiv. basalt) melts accounts for the chemistry of many spinel peridotites and pyroxenites, highly mobile carbonate melts are believed to have played a pivotal role in the formation of apatite pyroxenites/wehrlites (converted peridotites) and carbonate-bearing peridotites (reacted wallrock).

A suggested origin of MARID xenoliths in kimberlites by high pressure crystallization of an ultrapotassic rock such as lamproite

Chemical, mineralogical and isotopic studies have been made on nodules of the MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) xenolith suite in southern African kimberlites. All are ultramafic and ultrapotassic (MgO= 20–25%, K2O=4–9%), with bulk compositions reflecting the wide variation in relative proportions of the five minerals amongst the nodules. They are comparable in major element compositions to magnesian lamproites, in particular the ultrabasic olivine-lamproites of Western Australia. In a number of high pressure experimental studies on ultra-potassic rocks, the phases produced between 25–30 kbar from compositions comparable to those of MARID rocks (in the presence of additional water), were predominantly phlogopite and diopside (±K-richterite, ±ilmenite, ±rutile). Furthermore the compositions of experimental minerals produced in the synthetic-biotite-mafurite-H2O system by Edgar et al. (1976) are similar to those in MARID rocks.

Volatile contents of phlogopite micas from South African kimberlite

Phlogopite micas from nodules in South African kimberlites were analyzed for major elements with the electron microprobe and for volatile contents by high temperature mass spectrometry. The micas are from primary- (deformed) and secondary- (undeformed) textured grains in perodotite xenoliths, glimmerites, MARID (mica-amphibole-rutile-ilmenite-diopside) suite nodules and a mica megacryst. The major element and volatile contents of micas exhibiting these modes of occurrence overlap to a greater extent than indicated in previous studies. Concentrations of volatile species occupying structurally defined crystallographic sites (H2O, F, Cl) are greater for many of the micas than predicted on the basis of the mica formula, particularly for the glimmerite and MARID suite samples. A correlation exists between micas with tetrahedral and octahedral cation deficiencies and those with excess H2O, F and Cl. Substitution of H+ for tetrahedral and possibly octahedral cations may be responsible for the excess H2O in these micas. Except for one sample, the major element and volatile data for the peridotite, glimmerite and MARID suite micas indicate that they crystallized at oxygen fugacities below the quartz-fayalite-magnetite buffer. F and K2O are in the correct proportion in the micas to provide the source for these elements in alkali basalts, but not in mid-ocean ridge basalts. Kaersutite amphibole is a more likely source of potassium and fluorine in mid-ocean ridge basalts.

Trace element and isotope studies on veined, metasomatic and “MARID” xenoliths from Bultfontein, South Africa.

Concentrations of K, Rb, Sr, U, Pb and the REE as well as Sr, Nd and Pb isotope ratios are reported for separated mineral fractions from mantle-derived xenoliths showing textural and/or mineralogical evidence of metasomatism (e.g. veining, K-richterite or phlogopite) and from MARID xenoliths (Mica Amphibole Rutile Ilmenite Diopside, believed to be igneous). All samples are from the dumps of the Bultfontein kimberlite mine in Kimberley. Mineral phases analysed include mainly K-richterite, phlogopite, diopside and olivine. The study concerned the nature of the metasomatic processes and the character of the metasomatic fluid or liquid and that generating the MARID samples. REE and LILE concentrations vary greatly even within single mineral species (except diopside), but strong relative light REE enrichment is prevalent throughout, even in pre-metasomatic olivines. A single type of liquid or fluid may, however, have been responsible for the metasomatism in all samples and the generation of the MARID material, if a subsolidus reaction producing K-richterite from phlogopite is assumed. The liquid appears to have had kimberlitic affinities, and the metasomatism and generation of MARID material is probably younger than 200 Ma. The Sr and Nd isotopes form a pattern consistent with mixing of material from a slightly depleted mantle source (the metasomatising agent) with mantle material showing ancient enrichment in incompatible elements (typical of subcontinental lithosphere). K-richterite appears to be a sink for Pb and its absence or presence in old K-enriched mantle may possibly control correlations or anticorrelations between Sr and Pb isotope systems such as observed in volcanic rocks.