Siberian flood basalt magmatism and Mongolia-Okhotsk slab dehydration

Experimental data combined with numerical calculations suggest that fast subducting slabs are cold enough to carry into the deep mantle a significant portion of the water in antigorite, which transforms with increasing depth to phase A and then to phase E and/or wadsleyite by solid-solid phase transition. Clathrate hydrates and ice VII are also stable at PT conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Some cold slabs are expected to deflect while crossing the 410 km and stagnate in transition zone being unable to penetrate through 660 km discontinuity. In this way slabs can move a long way beneath continents after long-lived subduction. With time, the stagnant slabs are heated to the temperature of the ambient transition zone and release free H~2~O-bearing fluid. Combining with transition zone water filter model this may cause voluminous melting of overlying upper mantle rocks. If such process operates in nature, magmas geochemically similar to island-arc magmas are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the south-eastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with fast subduction of the Mongolia-Okhotsk slab and originated by dehydration of the stagnant slab in the transition zone. We show that influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually reduced with increasing distance from the subduction zone

Hydrogen Contents of Mantle Minerals from Peridotite Xenoliths of Ichinomegata Volcano, NE Japan

Hydrogen contents of olivine, clinopyroxene, and orthopyroxene from peridotite and pyroxenite xenoliths from calc-alkaline andesite of Ichinomegata volcano have been measured by using FTIR technique. Positions of bands in the infrared spectra resemble those observed previously for the studied minerals. Olivine contains <17 ppm wt. H_2O, whereas clinopyroxene and orthopyroxene contain significant amount of H_2O (359-647 and 184-277 ppm wt., respectively). Partition coefficient of H_2O between clinopyroxene and orthopyroxene was found to vary between 1.8 and 3.2. We did not observe systematic compositional (major elements in bulk rocks and minerals) or textural dependences of hydrogen contents in minerals. The low hydrogen contents of olivine are consistent with hydrogen loss during transportation of xenoliths to the surface. The high hydrogen contents of pyroxenes are consistent with strong metasomatic enrichment of the mantle wedge beneath the Japan arc. However, hydration of nominally anhydrous pyroxenes may reflect earlier events than hydration of mantle wedge by metasomatic agents producing modal amphibole in peridotite and abundant uppermost mantle/lower crust amphibolites. Pyroxenes from Ichinomegata contain nearly maximum amount of H_2O among the others from mantle xenoliths in alkaline basalts and kimberlites.論文Articl

ロシア・東部バイカル地域におけるマントル進化

The genesis of major groups of xenoliths from the Late Cenozoic volcanic rocks of the eastern part of Baikal Rift System is considered on the basis of petrology and mineral chemistry. Reconstructed mantle sections beneath the Vitim field appear to be more complex than beneath the Udokan field due to significant modification by hydrous metasomatic melts. Peridotite xenoliths from the Miocene picrobasalts represent garnet and spinel depth facies. Pyroxenite xenoliths are interpreted as products of three types of the melt. The first forms Cr-diopside pyroxenites, it is an interstitial melt migrating through peridotite, and it is similar to melts described in peridotite massifs as undergoing percolative fractionation. Textural and compositional relationships indicate melt segregation and fractionation under the high-pressure conditions towards Al-rich pyroxenites and - at shallower levels - towards sp-websterites. The second and third melt types form the hydrous veins and megacrystic pyroxenites, respectively. Both were crystallized in larger channels relative to the first melt type, whereby the amphibole- and phlogopite-bearing assemblages may represent smaller channels on a scale of dozen centimeters. Three series of peridotite xenoliths from Pliocene basanites were determined: (1) high-T garnet and spinel lherzolites, (2) low-T spinel lherzolites and harzburgites, (3) low-T titaniferous spinel lherzolites. Protogranular peridotites of the Series 1 represent primitive to moderately depleted mantle from the depths 60-80km at T=1100-1250℃. Trace element patterns in clinopyroxenes are indicative of low degree partial melting of the primitive mantle. Peridotites of the Series 2 correspond to the depths 40-50km at T=800-900℃. Titaniferous peridotites enriched in pyroxenes and spinel were newly detected. They have a mosaic equigranular texture and are suggested to be a rare type of melt/mantle interaction. T-estimations within 750-850℃ projected to a geotherm allow to interprete their correspondence to the uppermost mantle section (40-50km depth). High Ti content (TiO_2=0.55% in the bulk rock) may characterize a metasomatizing melt generated from ilmenite- and/or phlogopite-bearing source. Clinopyroxene REE patterns (La/Yb)_n=0.01-0.08) reveal an evidence for MORB-like composition of coexisting melt. Xenoliths from the Pliocene basanites of Kuas Lake (Udokan field) show variation of unhydrous depletion and enrichment of lower lithosphere. Xenoliths correspond to spinel facies and may be divided into lherzolite, harzburgite and dunite, and websterite groups. Depleted nodules contain clinopyroxene enriched in LREE and depleted in HFSE. This enrichment is explained by reactive percolation of small melt fraction and accompanying melting of peridotites. Harzburgite-dunite veins seem to be located at the lower part of mantle column and have been formed by olivine-producing reaction with increasing of melt fraction. Lherzolite and websterite are located at the middle and upper part of column and have been formed by pyroxene-producing reaction with decreasing of melt fraction.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

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

中生代から新生代にかけてのプレート沈み込みに起因するマグマの特徴

Mesozoic through Cenozoic magmatic evolution of East Asia was governed by oceanic plate subduction responsible for generation of a high-velocity anomaly, known as "the stagnated slab", at the mantle transition zone and the low-velocity Transbaikal mantle domain at depth of 200-350km. Based on spatial-temporal distribution of magmatic activity, the anomalous mantle region is suggested to be a time-integrated expression of subduction processes. High- and low-velocity material could be stored firstly during closing of the Mongolia-Okhotsk Ocean finalized at ca. 140Ma. After terrane accretion and structural reorganization at 113-107Ma, subduction of the Kula-Izanagi plate defined the northern margin of the anomalous mantle region. Low-velocity anomalies extended from a continental margin landward over 1000km beneath Aldan shield of the Siberian craton. The structural reorganization between 65 and 50Ma took place contemporaneously with accretion of the Okhotsk Sea plate to Eurasia. Block rotation and extension at the continental margin were accompanied by formation of the oblique Sikhote-Alin slab flexure of the Pacific plate. Afterwards, the slab flexure was widening to the south due to landward growing of the directly subducted Honshu-Khingan slab fragment. The latter resulted in development of the southern margin of the anomalous mantle region. The structural reorganization between 21 and 15Ma was coeval to accretion of the Philippine Sea plate to Eurasia with formation of the Japan-Korea oblique slab flexure, trench rolling-back effect, block rotations, and extension at the continental margin. The present-day subduction activity of the Pacific slab is focused at the oblique Japan-Korea and direct Hokkaido-Amur flexures.論文Articl

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

Phase relations and melting of carbonated peridotite between 10 and 20GPa: a proxy for alkali- and CO2-rich silicate melts in the deep mantle

We determined the melting phase relations, melt compositions, and melting reactions of carbonated peridotite on two carbonate-bearing peridotite compositions (ACP: alkali-rich peridotite+5.0wt% CO2 and PERC: fertile peridotite+2.5 wt% CO2) at 10-20GPa and 1,500-2,100°C and constrain isopleths of the CO2 contents in the silicate melts in the deep mantle. At 10-20GPa, near-solidus (ACP: 1,400-1,630°C) carbonatitic melts with40wt% CO2 gradually change to carbonated silicate melts with>25wt% SiO2 and15GPa. Similar to hydrous peridotite, majorite garnet is a liquidus phase in carbonated peridotites (ACP and PERC) at 10-20GPa. The liquidus is likely to be at~2,050°C or higher at pressures of the present study, which gives a melting interval of more than 670°C in carbonated peridotite systems. Alkali-rich carbonated silicate melts may thus be produced through partial melting of carbonated peridotite to 20GPa at near mantle adiabat or even at plume temperature. These alkali- and CO2-rich silicate melts can percolate upward and may react with volatile-rich materials accumulate at the top of transition zone near 410-km depth. If these refertilized domains migrate upward and convect out of the zone of metal saturation, CO2 and H2O flux melting can take place and kimberlite parental magmas can be generated. These mechanisms might be important for mantle dynamics and are potentially effective metasomatic processes in the deep mantle