Kazuhito Ozawa

Olivine-spinel geospeedometry: Analysis of diffusion-controlled Mg-Fe2+ exchange

Subsolidus Mg-Fe2+ exchange between olivine and spinel is governed by Mg-Fe2+ interdiffusion. Incomplete exchange results in Mg-Fe2+ heterogeneity in both olivine and spinel, which provides information on the thermal histories of the host rocks. A composite sphere model has been developed to obtain quantitative cooling rates or heating duration from the Mg-Fe2+ heterogeneity. The model assumes that a spherical core of spinel and a surrounding semi-infinite spherical shell of olivine interact by diffusion-controlled exchange of Mg and Fe2+. The differential equations describing the model are solved numerically by finite difference approximations. The numerical solution reveals that cooling rates or heating duration can be estimated from the relationship between the grain size of spinel and temperature calculated from the chemical compositions of the core of a spinel grain and of olivine far away from it. The calculated temperature is employed in place of Mg(Mg + Fe2+) at the center of spinel to obtain the absolute temperature of thermal events. This olivine-spinel geospeedometer has been applied to peridotites. gabbro, and picrites from some ophiolite complexes in Japan to estimate their cooling rates. The estimated cooling rates for the peridotites range from 10−4 to 10−1 °C/yr, and those for the picrites from 103 to 104 °C/yr. The geospeedometer has been extended to estimate the heating duration of lherzolite xenoliths in basalt from the Ichinomegata crater, northeast Japan. The estimated heating duration of the xenoliths is less than one day.

Evaporation of forsterite in H2 gas

Kinetics of evaporation of forsterite in hydrogen gas was investigated by high temperature vacuum experiments in the pressure range plausible for the solar nebula. The evaporation rate at total pressure (Ptot) below 10−6 bar is nearly constant and is similar to that in vacuum, whereas the rate at 10−6 to 10−3 bar is dependent on Ptot. The evaporation rate, JexpFo, is fitted by JexpFo = 1.72Ptot1.199.87 × 10−7 (g · cm−2 · s−1) for Ptot below 10−4 bar. The condensation coefficient, α, which is a factor related to kinetics of surface reactions, is evaluated by using the Hertz-Knudsen equation for the kinetic theory of gas molecules. The ratio of the experimentally obtained evaporation rate to that calculated from chemical equilibrium in the system Mg2SiO4-H2 gives the α value of 0.06 in vacuum, which increases up to 0.2 with increasing Ptot from 10−3 to 10−4 bar. The apparent increase of forsterite evaporation rate with increasing H2 abundance is due mainly to increase of the equilibrium vapor pressure, which corresponds to increase in the driving force, and partly to increase in α (reduction of the kinetic barrier) for evaporation. The experimental results were applied to understand behavior of forsterite dusts with time in an abruptly heated model nebula mostly comprising forsterite and H2. The nebular system can be divided into complete and partial evaporation regimes, which is defined by a dust enrichment factor. For the complete evaporation regime (low dust enrichment), the minimum time for forsterite grains to totally evaporate is estimated as a function of total pressure, temperature, and initial grain size. The lifetime of forsterite grains (<10 μm in size) could be less than 1 h at 1700 °C. The experimental results were further applied to examine the possibility of isotopic fractionation for forsterite grains in the solar nebula. By evaluating the competition between evaporation from surface and elemental diffusion in forsterite, it is shown that forsterite grains could have isotopically fractionated to be heavier only for Mg, but not for Si and O.

Evaluation of olivine-spinel geothermometry as an indicator of thermal history for peridotites

Olivine and spinel in peridotites from the Miyamori ultramafic complex and the Ichinogemata crater of Northeast Japan show a systematic variation in the Mg/ (Mg+Fe) ratio which is correlated mainly with the grain size of spinel. This correlation can be explained by a diffusion model assuming a semi-infinite composite sphere under cooling or heating conditions. In order to obtain absolute temperatures of thermal events, the olivine-spinel geothermometer is applied to pairs of spinel core and olivine core (average composition). The calculated temperatures range over two hundred degrees and have a systematic relationship with the grain size of spinel. In the Miyamori complex, the calculated temperatures decrease monotonically with decrease in grain size of spinel, whereas in the Ichinomegata lherzolite nodule those of spinel smaller than 0.2 mm increase as the grain size decreases and those of spinel larger than 0.2 mm remain constant regardless of further increase in grain size. These observations, in the light of the diffusion model, suggest that the Miyamori complex may have cooled from higher than 800° C to lower than 600° C and that the lherzolite nodule from the Ichinomegata crater may have been in equilibrium at 900° C before it was heated above 1,100° C for less than a few days. These two examples indicate that olivine-spinel pairs of peridotites do not always indicate an appropriate equilibration temperature. We cannot interpret the supposed equilibration temperatures until the existence of isothermal stages in the thermal history of peridotites is established by carefully checking the chemical heterogeneity.

Ultramafic tectonite of the Miyamori ophiolitic complex in the Kitakami Mountains, Northeast Japan: hydrous upper mantle in an island arc

The ultramafic tectonite of the Miyamori ophiolitic complex is divided into two types, one bearing aluminous spinel (Cr/(Cr+Al)< 0.4) and the other, chromian spinel(Cr/(Cr + Al)<0.4) (denoted ASPP and CSPP respectively). ASPP consists mainly of harzburgite and lherzolite and occurs as isolated kilometric patches in CSPP, which can be subdivided into massive and layered types. Massive CSPP consists mainly of magnesian harzburgite and dunite, whereas layered CSPP commonly is stratified and consists of less magnesian harzburgite, dunite, wehrlite, lherzolite, websterite, and clinopyroxenite. The 2 km thick layered CSPP occurs within the massive CSPP, and their lithologies are transitional. The structural and lithologic features of CSPP and the chemical variations of its olivine and spinel suggest that the layered CSPP crystallized from segregated partial melt, leaving the massive CSPP as a strongly depleted residue. Hornblende is invariably present in both the ASPP and CSPP, whereas phlogopite ispresent only in CSPP. The hornblende in CSPP is distinctly richer in K2O (0.4–1.0 wt%) than that in ASPP(<0.1 wt%), but residual peridotite of CSPP is more depleted in major elements than that of ASPP. The low TiO2/K2O ratio of hornblende and the presence of TiO2 poor phlogopite suggest that partial melting, melt segregation, and crystallization to form CSPP took place in the upper mantle beneath an island arc. By contrast, ASPP could be the source material of CSPP which formed as slightly depleted residue beneath a back-arc basin.

P-T history of a mantle diapir: the Horoman peridotite complex, Hokkaido, northern Japan

The Horoman peridotite complex, Hokkaido, Japan is divided into Lower and Upper zones on the basis of contrasting geological features. The complex recorded a consecutive decompression history in chemical zoning of pyroxenes and plagioclase in plagioclase lherzolite, which is interpreted to have been derived from garnet lherzolite by subsolidus decompression reactions. In the Lower Zone, and earlier decompression history is clearly preserved in large pyroxene porphyroclasts, which show marked M-shaped Al zoning characterized by low Al concentration at the core (Al=0.12/6 oxygens), gradual increase toward the marginal region, and rapid decrease toward the rim. The Ca content in the core is nearly constant (Ca=0.03/6 oxygens) with slight increase toward the margin followed by abrupt decrease toward the rim. The Al and Ca contents in the core of orthopyroxene in plagioclase lherzolite from the Upper Zone (Al=0.22, Ca=0.055/6 oxygens) are much higher than those for the Lower Zone, and the Al content typically decreases monotonously from the core to the rim with several exceptions that show poorly developed M-shaped zoning profiles. The earliest P-T conditions, inferable from the core compositions of pyroxenes are 900–950°C and ∼20 kbar for the Lower Zone and 1100–1150°C and ∼20 kbar for the Upper Zone. The increase of Al from the core to the margin is inferred to have resulted from nearly adiabatic decompression from these conditions into spinel peridotite facies. The complex experienced further decompression from the spinel stability field into the plagioclase stability field, which is inferred from plagioclase zoning in fine-grained aggregates composed mostly of plagioclase, chromite spinel, and olivine with minor pyroxenes. The Na-Ca ratio of each plagioclase grain decreases from the core to the rim, suggesting continuous decompression reaction producing olivine and plagioclase from pyroxenes and spinel. The sharp increase in Ca content toward the rim indicates that fairly rapid cooling associated with decompression is necessary to form and preserve the marked zoning. The sharp decrease in Al and Ca contents toward the rim of orthopyroxene was also formed during this final ascent of the complex. The systematic changes of the mineralogic and petrographic features that are gradational between the Lower and Upper zones suggest that the Horoman complex retains a temperature variation from the upper mantle. The Upper Zone is interpreted to have followed a higher temperature decompression path than the Lower Zone and probably represents a relatively hotter portion of a mantle diapir ascending from a depth greater than 60 km in the upper mantle.

Mantle earthquakes frozen in mylonitized ultramafic pseudotachylytes of spinel-lherzolite facies

F. Seifert and Bayerisches Geoinstitut (University of Beyreuth); Japan Society for the Promotion of Science grant 17340159; Progetti di RilevanteInteresse Nazionale grant 2005044945 and a Progetti di Eccellenza Fondazione Cassa di Risparmio di Padova e Rovigo (CARIPARO) grant.

Relationships between tectonite and cumulate in ophiolites: the Miyamori ultramafic complex, Kitakami Mountains, northeast Japan

Abstract The Miyamori ultramafic complex forms the basal ultramafic portion of an ophiolite. The complex consists of a tectonic member which is composed dominantly of harzburgite and dunite, and a cumulate member which is composed of interlayered wehrlite, dunite and clinopyroxenite. The tectonite member is overlain by the cumulate member and characterized by tabular granular or porphyroclastic textures, a strong lineation and magnesian olivine (Mg/Mg + Fe = 0.88–0.93). In contrast, the cumulate member exhibits igneous textures and shows no evidence of a penetrative deformation. The olivine is less magnesian than that of the tectonite member (Mg/Mg + Fe = 0.82–0.89). At the boundary of the two members, harzburgite xenoliths have been found in wehrlite of the cumulate member. The minerals at the core of a few large harzburgite xenoliths preserve the compositional characteristics of typical harzburgites in the tectonic member. The occurrence of the harzburgite xenolith in vehrlite and the structural and textural features of the two members indicate that the tectonite member had already been deformed before a magma intruded into the tectonite member and formed a magma chamber in which cumulates were deposited together with harzburgite fragments on the floor of the tectonite. The xenoliths show a fine grained mosaic texture, which may be attributed by the heat of the intruded magma. This hiatus implies that the magma which made the cumulate member did not originate directly from the underlying harzburgite.

Contribution of oceanic gabbros to sea-floor spreading magnetic anomalies.

The contribution of oceanic gabbros, representative rocks for layer 3 of the oceanic crust, to sea-floor spreading magnetic anomalies has been controversial because of the large variation in magnetic properties. Ocean Drilling Program (ODP) Leg 118 contains a continuous 500.7-meter section of oceanic gabbro that allows the relations between magnetization and petrologic characteristics, such as the degree of metamorphism and the magmatic evolution, to be clarified. The data suggest that oceanic gabbros, together with the effects of metamorphism and of magmatic evolution, account for a significant part of the marine magnetic anomalies.

Tectonic implications of the hydrated garnet peridotites near Mt Kinabalu, Sabah, East Malaysia

Abstract Hydrated garnetiferous peridotites form part of an ultramafic complex in the Mt Kinabalu area of Sabah, East Malaysia. Garnet peridotites are associated with abundant spinel lherzolites and rare plagioclase-bearing spinel lherzolites. The complex is in fault contact with the surrounding Tertiary strata. High-temperature peridotite mineral assemblages are overprinted by lower temperature hydrous assemblages characterized by abundant hornblende which is found filling interstices or thin veinlets. The small amounts of K, Na, Cl and F in the hornblende suggest that the metasomatic fluid may have been depleted in these elements. Carbon dioxide-rich, incompatible element-enriched, fluids are not thought to be the cause of the metasomatism since metasomatic phlogopite and carbonates, commonly reported from garnet peridotites and metasomatized alpine-type ultramafic rocks, are absent. The Mt Kinabalu garnet peridotites represent part of the sub-crustal mantle beneath Kalimantan which was metasomatized during ascent due to tectonism. The metasomatic fluid, poor in incompatible elements and H 2 O-rich, may have been introduced into a sub-continental mantle wedge after dehydration of a subducted oceanic slab.

Long-term eruption rates and dimensions of magma reservoirs beneath quaternary polygenetic volcanoes in Japan

Abstract Long-term eruption rates of 18 Quaternary volcanoes in Japanese islands are estimated to range from 5 to slightly greater than 0.05 km 3 /10 3 y. The long-term rate correlates well with the variation of phenocryst assemblage, which reflects the crystallization temperature. The long-term eruption rate and temperature of magma from the volcanoes in northeastern Japan are higher than those in southwestern Japan. In northeastern Japan, the long-term rate is higher at volcanoes located near the colcanic front than those on the back-arc side. Applying the estimated long-term eruption rates to the model calculation of continuous magma supply, the appropriate radius of the magma chamber is estimated to be less than 3.5 km.