Meteoric hydrothermal origin of calcites in “Green Tuff” formations,Miocene age,Japan

Oxygen and carbon isotope compositions were determined for calcites from the Green Tuff formations of Miocene age in Japan. Values of ¹⁸O from 24 calcites in altered rocks from 5 districts range from –2 to +16_SMOW, in most cases from 0 to +8_SMOW. The low ¹⁸O values rule out the possibility of their low-temperature origin or any significant contribution of magmatic fluid in the calcite precipitation. These values, coupled with their mineral assemblages, suggest that the calcites formed from meteoric hydrothermal solutions which caused propylitic alteration after the submarine strata became emergent.Values of ¹³C from the calcites show a wide variation from –17 to 0_PDB. Calcites from different districts have different ranges of ¹³C values, indicating that there was no homogeneous reservoir of carbon at the time the calcite formed, and that the carbon had local sources. Carbon isotopic compositions of calcite within ore deposits in the Green Tuff formations range from –19 to 0_PDB, similar to those of calcite in the altered rocks in the same district, suggesting that the carbon in ore calcites was likely supplied from the surrounding rocks through activity of meteoric hydrothermal solutions.     

Late quaternary uplift in eastern Indonesia

Radiocarbon dates of fifteen samples representing raised shorelines on various islands of the tectonically mobile region of eastern Indonesia suggest rates of tectonic uplift ranging up to 12.5 mm/year. Low rates of 0.35–1.2 mm/year are from Biak Island and are averages for the last 31,000–36,000 years. The low rates may be explained to indicate subsidence alternating with uplift, both of which occurring under influence of the large Irian fault zone that passes to the immediate south of the island. Very young elevated strandlines (250 years BP) indicate rates of uplift between 8 and 12.5 mm/year which seem to demonstrate the episodic character of vertical diastrophic movements. Intermediate rates of uplift are in the order of 5 mm/year and may represent averages for eastern Indonesia.     

Possible sub-arc origin of podiform chromitites

Abstract The sub-arc mantle condition possibly favors the formation of podiform chromitites. The Cr/(Cr + Al) atomic ratio (= Cr#) of their chromian spinel frequently is higher than 0.7, which is comparable with the range for arc-related primitive magmas. This almost excludes the possibility of their sub-oceanic origin, because both oceanic peridotites and MORB have chromian spinel with the Cr# < 0.6. Precipitation of chromitite and associated dunite enhances a relative depletion of high-field strength elements (HFSE) to large-ion lithophile elements (LILE), one of chemical characteristics of arc magmas, for the involved magma. This cannot alter completely, however, the MORB to the arc-type magma, especially for Ti and Zr. The presence of chromitite xenoliths, similar both in texture and in chemistry to podiform chromitites of some ophiolitic complexes, in some Cenozoic alkali basalts from the southwest Japan arc indicates directly that the upper mantle beneath the Japan arcs has chromitites.     

Insights into Petrological Characteristics of the Lithosphere of Mantle Wedge beneath Arcs through Peridotite Xenoliths: a Review

The petrological characteristics of peridotite xenoliths exhumedfrom the lithospheric mantle below the Western Pacific arcs(Kamchatka, NE Japan, SW Japan, Luzon–Taiwan, New Irelandand Vanuatu) are reviewed to obtain an overview of the supra-subductionzone mantle in mature subduction systems. These data are thencompared with those for peridotite xenoliths from recent orolder arcs described in the literature (e.g. New Britain, WesternCanada to USA, Central Mexico, Patagonia, Lesser Antilles andPannonian Basin) to establish a petrological model of the lithosphericmantle beneath the arc. In currently active volcanic arcs, thedegree of partial melting recorded in the peridotites appearsto decrease away from the fore-arc towards the back-arc region.Highly depleted harzburgites, more depleted than abyssal harzburgites,occur only in the frontal arc to fore-arc region. The degreeof depletion increases again to a degree similar to that ofthe most depleted abyssal harzburgites within the back-arc extensionalregion, whether or not a back-arc basin is developed. Metasomatismis most prominent beneath the volcanic front, where the magmaproduction rate is highest; silica enrichment, involving themetasomatic formation of secondary orthopyroxene at the expenseof olivine, is important in this region because of the additionof slab-derived siliceous fluids. Some apparently primary orthopyroxenes,such as those in harzburgites from the Lesser Antilles arc,could possibly be of this secondary paragenesis but have beenrecrystallized such that the replacement texture is lost. TheTi content of hydrous minerals is relatively low in the sub-arclithospheric mantle peridotites. The K/Na ratio of the metasomatichydrous minerals decreases rearward from the fore-arc mantleas well as downward within the lithospheric mantle. The lithosphericmantle wedge peridotites, especially metasomatized ones frombelow the volcanic front, are highly oxidized. Shearing of themantle wedge is expected beneath the volcanic front, and isrepresented by fine-grained peridotite xenoliths. KEY WORDS: mantle wedge; lithospheric mantle; peridotite xenoliths; melting; metasomatism     

Nickel enrichment in mantle olivine beneath a volcanic front

We found abnormally Ni-rich olivine (Fo = 93) with up to 5.3 wt% of NiO, ten times higher than the ordinary mantle value (0.4 wt%), in a highly metasomatized mantle peridotite xenolith from Avacha volcano, the Kamchatka arc, Russia. The Ni enrichment displays outward diffuse circular domains (<1 mm across) in fine-grained (mostly <100 μm) olivine-rich parts. Associated metasomatic orthopyroxene also shows high NiO (<1.1 wt%). Such high Ni concentrations in olivine cannot be attained in ordinary residual or cumulus peridotites, but are achievable via diffusion from Ni-rich sulfide melt. Sulfur-bearing silicic melt, the main metasomatic agent for the Avacha peridotites, separated sulfide melt, which was fractionated to be Ni-rich at relatively low temperatures. This is a new way of mobility and redistribution of Ni in the mantle, which is active in the mantle wedge, especially beneath a volcanic front.     

Seasonal variation of bromocarbons at Hateruma Island,Japan: implications for global sources

High-frequency measurements of dibromomethane (CH₂Br₂) and bromoform (CHBr₃) at Hateruma Island, in the subtropical East China Sea, were performed using automated preconcentration gas chromatography/mass spectrometry. Their baseline concentrations, found in air masses from the Pacific Ocean, were 0.65 and 0.26 ppt, respectively, in summer and 1.08 and 0.87 ppt, respectively, in winter. Air masses transported from Southeast Asia were rich in bromocarbons, suggesting strong emissions in this area. The passage of cold fronts from the Asian continent was associated with sharp increases in observed concentrations of bromocarbons derived from coastal regions of the continent. Comparison of the relationships between [CH₂Br₂]/[CHBr₃] and [CHBr₃] in the Hateruma Island data with those in monthly mean data from 14 globally distributed U.S. National Oceanic and Atmospheric Administration ground stations suggested that these gases are produced primarily from a common process on a global scale.     

Shock-wave compression of silica gel as a model material for comets

A shock-wave compression experiment using synthesized silica gel was investigated as a model for a comet impact event on the Earth’s surface. The sample shocked at 20.7 GPa showed considerable structural changes, a release of water molecules, and the dehydration of silanol (Si–OH) that led to the formation of a new Si–O–Si network structure containing larger rings (e.g., six-membered ring of SiO₄ tetrahedra). The high aftershock temperature at 20.7 GPa, which could be close to 800 °C, influenced the sample structure. However, some silanols, which were presumed to be the mutually hydrogen-bonded silanol group, remained at pressures >20.7 GPa. This type of silanol along with a small number of water molecules may remain even after shock compression at 30.9 GPa, although the intermediate structure of the sample recovered was similar to that of silica glass.     

Vesicle layering in solidified intrusive magma bodies: a newly recognized type of igneous structure

 We report a novel type of layering structure in igneous rocks. The layering structure in the Ogi picrite sill in Sado Island, Japan, is spatially periodic, and appears to be caused by the variation in vesicle volume fraction. The gas phase forming the vesicles apparently exsolved from the interstitial melt at the final stage of solidification of the magma body. We call this type of layering caused by periodic vesiculation in the solidifying magma body "vesicle layering." The presence of vesicle layering in other basic igneous bodies (pillow lava at Ogi and dolerite sill at Atsumi, Japan) implies that it may be a fairly common igneous feature. The width of individual layers slightly, but regularly, increases with distance from the upper contact. The layering plane is perpendicular to the long axes of columnar joints, regardless of gravitational direction, suggesting that the formation of vesicles is mainly controlled by the temperature distribution in the cooling magma body. We propose a model of formation of vesicle layering which is basically the same as that for Liesegang rings. The interplay between the diffusion of heat and magmatic volatiles in melt, and the sudden vesiculation upon supersaturation, both play important roles. Received: 15 February 1996 / Accepted: 24 June 1996