Petrogenesis of Glasses and Microlites in Mantle Xenoliths from Baikal-Mongolia Region : a Review

Petrogenesis of various glasses, microlite assemblages and fluid and melt inclusions in minerals of mantle xenoliths from alkaline volcanics of Baikal-Mongolian region is discussed. Diversity and compositional variations of glasses vary widely and their origin can be addressed to most of previously reported hypotheses. Study of the fluid inclusions indicates that depth of trapping is usually well correlated with the depth of Moho estimated from seismological and petrological data. The density of CO_2 fluid inclusions indicates minimum pressures of their origin of 10-15kbar. Glasses from melt inclusions in minerals of Dzhilinda xenoliths (Vitim flied) can represent direct melts formed by in situ partial melting of peridotite at about 10kbar, the pressure estimated from fluid inclusions, whereas glasses from melt inclusions in minerals of garnet Iherzolite of Bereya Quarry (Vitim flied) can represent melts formed at pressures above 15kbar. The SiO_2 contents of these glasses (51-57 wt.%) are consistent with those of experimental partial melts from low-degree partial melting of peridotite. Origin of SiO_2-rich (up to 74 wt.%) glasses from harzburgite xenoliths is consistent with experimental study by Shaw et al. (1998) indicating that reaction of orthopyroxene with silica-undersaturated melt at ambient pressure results in olivine, clinopyroxene, spinel and intermediate of silicic melt. We describe unusual clinopyroxene-olivine-sanidine symplectites in hightemperature spinel Iherzolites from Burkal locality. Calculated bulk compositions of the symplectites indicate that they can represent breakdown after K-bearing pyroxene solid solution from deep mantle.論

Mineralogical Study of Interstitial Phase Assemblages in Titaniferous Peridotite Xenoliths from Pliocene Basanites of Vitim Volcanic Field (Transbaikalia, Russia)

Origin of microlite assemblages in Ti-rich peridotite xenoliths from basanite of Dzilinda River (Vitim volcanic field) is discussed. Major minerals in the melt pockets are olivine, clinopyroxene, Cr-spinel, rutile, plagioclase, and sanidine. Rarely ilmenite, armalcolite, loveringite, apatite, and goethite are also observed. Ni-bearing sulfide inclusions form chains in the primary minerals and coexist with amphibole and goethite. Studied melt pockets are most likely represent products of a reaction of primary minerals with small melt fraction during transport to the surface of shortly before entrainment. Occurrence of abundant Ti-oxide minerals may reflect primary mineralogy of Ti-rich peridotite. The coarse rutile grains can be primary in the xenoliths and could be formed long before transport of xenoliths to the surface. Sulfides in Dzhilinda xenoliths form chains together with pseudosecondary fluid and recrystallized glass inclusions and are not related to interstitial assemblages. Pentlandite and monosulfide solid solution represent product of crystallization of immiscible sulfide melt in the mantle. Viterite can be formed by alteration of pentlandite. Rounded goethite inclusions may represent pseudomorphs on sulfide. This is supported by presence of amphibole in the inclusions (which provide water for goethite) and high CuO content of goethite.論文Articl

ロシア・南バイカルKhenteyドーム、Burkal川流域に産するマントル捕獲岩中の単斜輝石の微量元素研究

Trace element chemistry of clinopyroxene in the mantle xenoliths from melanephelinites of the Burkal volcanic group has been studied. The Burkal group is composed of several local outcrops of 5-8 Ma melanephelinites within the Khentey domal uplift near the Russia/Mongolia boundary. Cr-diopside group xenoliths include garnet and spinel lherzolite, spinel harzburgite and dunite, and garnet and spinel pyroxenites. Hydrous minerals were not detected, however shallow mantle feldspatic metasomatism is present. Clinopyroxene from garnet lherzolites has high TiO_2, Al_2O_3, and Na_2O relative to clinopyroxene from spinel lherzolites. Olivine has composition of Fo_. Spinel has Mg#=60-80 and contains 10-46 wt.% Cr_2O_3. Clinopyroxene from garnet lherzolites has REE patterns typical for fertile peridotites. Trace element patterns of clinopyroxene from depleted spinel peridotites show progressive depletion in HREE and HFSE and enrichment in LREE toward more depleted varieties of harzburgites and dunites. REE patterns of clinopyroxene in harzburgites are strongly U-shaped and have (La/Sm)n=5-36 and (Sm/Yb)n=0.4-2.1. Clinopyroxene in harzburgites has also extremely low Zr content (0.4-3.4ppm) and high Ti/Zr ratio ranged in 190-240. The patterns of clinopyroxene in depleted peridotites are indicative of significant partial melting (up to 15-20%) of the primary substrate followed by cryptic metasomatic enrichment by silicate or carbonatitic melt. Estimation of T-P parameters for garnet lherzolites reveals equilibration at 17-23 kbar (60-90km depths) and 1050-1150℃. T-estimations for harzburgites and dunites indicate, that they may form veins at 50-70km depth, whereas shallow mantle (low-T) depleted peridotites were not detected. The uppermost mantle may be composed of fertile spinel lherzolites.論文Articl

The Earliest Generation of Diamond: The First Find of a Diamond Inclusion in Kimberlitic Olivine

Today, it is known that the majority of diamonds are crystallized mostly from a metasomatic agent close in the main characteristics to carbonatite melts acting upon mantle rocks, and therefore, diamonds are located in the interstitial space of these rocks. So far, diamond has never been found included in other kimberlitic or xenolithic minerals. We have found a diamond inclusion inside the kimberlitic olivine grain, which is the first find of its kind. The diamond crystal is to have been captured by the growing olivine at quite high temperatures (more than 1400 °C) early in the history of the cratonic lithospheric mantle formation. The event had taken place long before the depleted peridotite cooled down to the temperature of the Middle Archean cratonic geotherm corresponding to the diamond stability field at depths where carbonatite melts can react with depleted peridotite, making it a diamond-bearing rock. On the one hand, this find provides evidence that diamonds can crystallize from the high-temperature silicate melt with some carbonate component. On the other hand, the diamond was found coexisting with a sulfide inclusion in the same olivine, i.e., crystallization from a sulfide melt may be another way of diamond formation

Error sources in single-clinopyroxene thermobarometry and a mantle geotherm for the Novinka kimberlite, Yakutia

A new suite of 173 clinopyroxene grains from heavy-mineral concentrates of the diamondiferous Novinka kimberlite (Upper Muna field, Yakutia) has been analyzed for major and minor elements with an electron microprobe to perform a thermobarometric study and model the thermal structure of the Archean Upper Muna lithospheric mantle. Scrupulous evaluation of propagation of analytical uncertainties on pressure estimates revealed that (1) the single-clinopyroxene geobarometer can be very sensitive to analytical uncertainties for particular clinopyroxene compositions, and that (2) most clinopyroxenes from Novinka have compositions that are sensitive to analytical uncertainties, notwithstanding their apparent compositional suitability for single-clinopyroxene thermobarometry based on previously proposed application limits. A test on various mantle clinopyroxenes containing different proportions of the sensitive elements Cr, Na, and Al allowed us to identify clinopyroxene compositions that produce unacceptably high propagated errors and to define appropriate analytical conditions (i.e., higher beam currents and longer counting times for specific elements) that allow precise P-T estimates to be obtained for sensitive compositions. Based on the results of our analytical test, and taking into account the intrinsic limitations of the single-clinopyroxene thermobarometer, we have designed a new protocol for optimum thermobarometry, which uses partly revised compositional filters. The new protocol permits precise computation of the conductive paleogeotherm at Novinka with the single-clinopyroxene thermobarometer of Nimis and Taylor (2000). Thermal modeling of the resulting P-T estimates indicates a ~34 mW/m2 surface heat flow, a thermal lithosphere thickness of ~225 km, and an over 100 km thick \u201cdiamond window\u201d beneath Novinka in the middle Paleozoic (344\u2013361 Ma). We estimate that appropriate analytical conditions may extend the applicability of single-clinopyroxene thermobarometry to over 90% of clinopyroxene-bearing garnet peridotites and pyroxenites and to ~70% of chromian-diopside inclusions in diamonds. In all cases, application to clinopyroxenes with Cr/(Cr+Al)mol 4.5 GPa

ヴィティム火山地域の後期中新世ピクロ玄武岩の鉱物学予報・始原マグマの進化について

Primary mineralogy of unique picrobasalt from Vitim volcanic field has been described. The picrobasalts contain abundant mantle xenoliths. Their amount is up to 50 modal % of the picrobasalts. K-Ar estimation of the aphanitic groundmass of the picrobasalt indicates the age of 16.3 Ma. This is earliest magmas in the Vitim volcanic field. The picrobasalt contains abundant olivine phenocrysts. Modal olivine percentages range from 25 to 42% (of picrobasalt minus xenocrysts total). The groundmass composed of clinopyroxene (29% in average), plagioclase (21%), Ti-magnetite (4%), biotite (3%), Cr-spinel (1%), apatite (1.5%), glass (8%) and others (<1.5%). Mg number of olivine phenocrysts ranges from 75 to 82. Calculated Mg number of olivine coexisting with picrobasalt is 85-89. This indicates partial disequilibrium, which is probably connected with contamination of the magma by Mg-rich xenolith materials. Interstitial glass from picrobasalt groundmass is SiO_2- and alkali-rich (55-61 wt.H% SiO_2; 3.0-5.1 wt.% Na_2O and 4.0-4.5 wt.%K_2O). Crystallization of picrobasalt magma was started from olivine and spinel in the middle crust magma chambers (5-8kbar at 1150-1200℃). Phases of groundmass crystallized under the surface conditions. Crystallization started from clinopyroxene (+minor Ti-magnetite and apatite), following by clinopyroxene+plagioclase, plagioclase only and glass.論文Articl

Armalcolite and rutile-bearing mantle peridotites, Vitim volcanic field, South Russia: petrography and mineralogy

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