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AbstractThe cross sections formed by the contact asperities of two rough surfaces at an interference are islandshaped, rather than having the commonly assumed circular contour. These island-shaped contact surface contours show fractal behavior with a profile fractal dimension Ds. The surface fractal dimension for the asperity heights is defined as Dand the topothesy is defined as G. In the study of Mandelbrot, the relationship between Dand Dswas given as D= Ds+ 1 if these two fractal dimensions are obtained before contact deformation. In the present study, D, G,and Dsare considered to be varying with the mean separation (or the interference at the rough surface) between two contact surfaces. The D-Dsrelationships for the contacts at the elastic, elastoplastic, and fully plastic deformations are derived and the inceptions of the elastoplastic deformation regime and the fully plastic deformation regime are redefined using the equality of two expressions established in two different ways for the number of contact spots (N). The contact parameters, including the total contact force and the real contact area, were evaluated when the size distribution functions (n) for the three deformation regimes were available. The results indicate that both the Dand Dsparameters in these deformation regimes increased with increasing the mean separation (d*). The initial plasticity index before contact deformation (ψ)0is also a factor of importance to the predictions of the contact load (F*t) and contact area (At*) between the model of variable Dand G, non-Gaussian asperity heights and circular contact area and the present model of variable Dand G, non-Gaussian asperity heights and fractal contact area.

A ~5 cm thick garnet-rich layer, with relict spinel grains in its center, is intercalated within garnet pyroxenite at Rongcheng. Both the garnetite layer and the host garnet pyroxenite underwent the same P-T evolution, but exhibit quite different mineral assemblages, microtextures, and associated mineral reactions. Microtextures and mineral compositions confirm our previous suggestion that the Rongcheng garnet pyroxenite recrystallized from a spinel pyroxenite cumulate derived from a gabbroic magma. The present study concludes that: (1) The garnet-rich layer resulted from a net transfer reaction between primary spinel and clinopyroxene, Ca1.5(FeMg)0.5 Si2O6 + (FeMg)Al2O4 + TiO2 = Ca1.5(FeMg)1.5Al2(Si2Ti)O12, and mass-balance reactions between clinopyroxene end-members, 2[Ca0.75(FeMg)0.25][(FeMg)0.5Al0.5] (Si1.5Al0.5)O6 = Ca1.5(FeMg)1.5Al2Si3O12, under UHP conditions that produced garnet-I. (2) Garnet-I decomposed during exhumation of the UHP terrane, leading to the assemblage of Grt-II + Ilm + Cpx-II + Amp; ilmenites were partially released to the relict spinel along micro-fractures to form intergrowths of Spl-I + Ilm. A corona assemblage of Cpx-II + Spl-II + minor Amp developed along contacts between garnet and relict Spl-I at ~890°C and ~9 kbar. And (3), during a later stage of retrogression, aqueous fluid infiltrated heterogeneously through the garnet-rich layer, leading to the formation of hydrous minerals. Parageneses and mineral compositional maps suggest that Ti may have substituted for Si and entered the T site in the garnet structure, represented by a garnet component such as M3Al2[Si(3-x)Tix]O12, in addition to the commonly suggested M2+Ti4+ → 2Al3+ substitution (forming the garnet component M3-0.5x[Al2-xTix]Si3O12). Heterogeneous microtextures and mineral compositions indicate that the domain equilibrium was controlled by mass transfer at least at the mineral-grain scale due to various degrees of fluid infiltration.

Primary and exsolution fluid inclusions are recognized in omphacite of ultrahigh-pressure (UHP) eclogites from the main hole of the Chinese Continental Scientific Drilling Program, located in the southern Sulu orogenic belt. These oriented fluid inclusions occur as tubes, and coexist with exsolved quartz needles in the cores of host omphacite. Most complex primary fluid inclusions contain a gas bubble, a liquid phase, and one to several solids, such as quartz, halite, calcite, and opaque and unknown minerals, having compositions in the system of NaCl-CaCl2-CO2-H2O-SiO2 with possibly trace Fe and Mg; in contrast, simple fluid inclusions contain a gas, an aqueous liquid, and sometimes a calcite. We suggest that the complex fluid inclusions were trapped during the omphacite growth, whereas the simple aqueous inclusions and quartz needles exsolved from OH--rich supersilicic omphacite during early uplift of the UHP metamorphic rocks. Omphacite is one of the major fluid carriers during subduction of continental crust to mantle depths.

Paleozoic lawsonite-bearing low-grade metabasalts with rare flattened pillow structures occur in the Chugoku Mountains, Southwest Japan. The meta-mafic rocks are divided into meta-pillow basalt core (PBC), meta-pillow basalt rim (PBR), and metabasaltic breccia (BB). The PBC have MORB-like major and trace element concentrations, contain 5 wt% water, and a Na-amphibole-free mineral assemblage: Ca-Na pyroxene (maximum 29% jadeite component) + lawsonite + chlorite + quartz. In contrast, the PBR and BB contain a lawsonite-blueschist-facies mineral assemblage: Na-amphibole ± lawsonite + pumpellyite ± chlorite + albite + quartz. Parageneses and compositions of minerals yield equilibrium P-T conditions of T< 280°C and P = 0.6-0.7 GPa for all three rock types. Phase relations in the 2Al-Ca-2Fe3+-(Fe+Mg) tetrahedron suggest that the Na-amphibole-free mineral assemblage in PBC occurs only under relatively oxidized conditions. This mineral assemblage has been described from low-grade metabasalts in several high-P-low-T terranes, and might be a low-T equivalent of the lawsonite eclogite mineral assemblage. We propose that interaction of fluids with subducting old oceanic crust first produces the Ca-Na pyroxene + lawsonite + chlorite assemblage by hydration reactions at shallow depths in a cold subduction zone. With subduction to greater depths, such intensely hydrated metabasalts with the Na-amphibole-free mineral assemblage transforms to lawsonite eclogite below 300°C.

The 5158 m deep main hole of the Chinese Continental Scientific Drilling Project (CCSD-MH) at Maobei, southwestern Sulu Belt, penetrated five major lithologic units: eclogite, amphibolite ± retrograde eclogite, ultramafic rock, paragneiss, and orthogneiss. All analyzed samples are overprinted to various extents by amphibolite-facies retrograde metamorphism. Ultrahigh-pressure (UHP) index minerals are preserved as inclusions in zircons separated from 137 core samples. These index assemblages include: Coe + Phe + Grt from Grt-Ep-Bt paragneiss; Coe + Jd + Grt + Ap, Coe + Jd + Phe + Ap and Coe + Grt + Jd + Phe from Grt-Ep-Hbl paragneiss; Coe + Phe, Coe + Ky + Ap, and Coe + Ky + Ttn from orthogneiss; and Coe + Grt + Omp, Coe + Grt + Phe, Coe + Omp + Rt, and Coe + Grt + Omp + Phe from Bt-Ep amphibolite and other retrograde eclogites. The common occurrences of these UHP assemblages throughout the section indicate that the drill hole did not penetrate through the UHP tectonic slice. All of the UHP mineral assemblages, whether from retrograde ecologites or from the surrounding gneisses, record similar metamorphic conditions of 773-843°C and 3.2-4.0 GPa. Zircons separated from surface samples and cores from other shallow holes in the area also contain similar coesite-bearing UHP mineral inclusions. This widespread occurrence of UHP assemblages in the southwestern Sulu area suggests that voluminous continental materials were subducted and metamorphosed at mantle depths 100 km, and then were rapidly exhumed to crustal levels.

Mineral inclusions in zoned zircons reveal that both ultrahigh-pressure (UHP) and non-UHP orthogneisses occur in cores from drill hole CCSD-PP1 at Donghai, southwestern Sulu terrane. Zircons from the UHP orthogneiss have inherited magmatic cores containing low-P mineral inclusions, narrow mantles with UHP mineral inclusions, and rims that contain low-P minerals or are inclusion free. In contrast, zircons from non-UHP orthogneiss have only magmatic cores and metamorphic rims, both of which are characterized by low-P mineral inclusions, or are inclusion free. SHRIMP U-Pb analyses of zircons from the UHP orthogneiss identify three discrete and meaningful age groups: Proterozoic magmatic protolith ages 753 Ma in the cores, a UHP metamorphic event at 227 ± 8 Ma in the coesite-bearing mantles, and an amphibolite-facies overprint at 213 ± 7 Ma in the overgrowth rims. Zircons from non-UHP orthogneiss record only two discrete age groups, inherited magmatic cores with Proterozoic protolith ages 748 Ma, and metamorphic rims with an age of about 211 ± 6 Ma. The metamorphic rims have the same Triassic age as the late amphibolite-facies overprint of the UHP orthogneiss within analytical uncertainty.These data, together with previous studies, suggest that Neoproterozoic supracrustal protoliths of the Yangtze craton experienced Late Triassic (227 ± 8 Ma) subduction to depths of at least 120 km, and were then rapidly exhumed to mid-crustal depths where both UHP rocks and a few non-UHP Neoproterozic granitic intrusives were subjected to coeval amphibolite-facies retrograde metamorphism at 211 ± 6 Ma. The exhumation rate deduced from the zircon age data and previously obtained metamorphic P-T data is estimated to be 6.3-7.1 km/m.y. This rapid exhumation suggests that the Sulu UHP metamorphic rocks returned to crustal depths as part of a buoyant sliver, possibly produced by slab breakoff.

In order to understand the dehydration process related to phengite decomposition during subduction of crust into the deep mantle, we systematically investigated mineral inclusions in zircons from Kokchetav diamond-grade eclogites. In the eclogites, phengite is absent from the matrix but only occurs as inclusions in clinopyroxene. The clinopyroxene is zoned; the core augite contains K2O up to 0.24 wt%, whereas the rim omphacite contains secondary K-feldspar inclusions. Phengite may have been consumed during prograde reactions, and K was fixed in clinopyroxene or in a fluid/melt phase. Inclusions of clinopyroxene and garnet were identified in zircon cores, whereas garnet, rutile, quartz, and composite inclusions are present in the mantles and quartz occurs in the rims. Distribution of mineral inclusions in zoned zircons indicates that zircon cores grew at the peak UHP stage, whereas the rims grew in the quartz stability field during decompression. The composite inclusions have assemblages of albite + glass + epidote, rutile + ilmenite, quartz + rutile, and rutile + albite. The former three-phase composite inclusions have rounded outlines and triple-junction grain boundaries, suggesting crystallization from fluid or melt. During the growth of zircon from core to mantle, phengite was consumed, and new garnet, rutile and fluid/melt were formed.

An Eocene submarine boninite-series volcanic center is exposed on the island of Chichi-jima, Bonin Islands, Japan. Five rock types—boninite, bronzite andesite, dacite, quartz dacite, and rhyolite—were distinguished within the boninite volcanic sequence on the basis of petrography and geochemistry. Boninites contain high Mg, Ni, and Cr contents indicative of primitive melts, but have high Si contents relative to other mantle-derived magmas. All boninite-series rocks contain very low incompatible-element concentrations, and concentrations of high-field-strength elements in primitive boninites are less than half of those found in depleted mid-ocean ridge basalts. Abundances of large-ion lithophile elements are relatively high in boninite-series rocks, similar to the enrichments observed in many island-arc lavas. Trends for both major- and trace-element data suggest that the more evolved members of the boninite magma series were derived primarily through high-level fractional crystallization of boninite. Textural features, such as resorption and glomeroporphyrocrysts, and reverse chemical zonations suggest that magma mixing contributed to the development of the quartz dacites.

The Pre-pilot Hole No. 1 of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) in the southern Sulu terrane recovered a continuous core of eclogite, garnet peridotite, orthogneiss, paragneiss, and minor schist and quartzite. Geochemical characteristics indicate that the garnet peridotite was derived from depleted mantle; all other rock types are metamorphosed supracrustal rocks with continental affinities. The eclogite consists of garnet, omphacite, phengite, quartz (coesite), amphibole, rutile, and zircon; P-T estimates of peak metamorphism are 785-820°C and >2.7 to 3.7 GPa. The gneisses show common amphibolite-facies mineral assemblages consisting of plagioclase, K-feldspar, muscovite, and quartz, with minor garnet, epidote (or zoisite), and biotite; coesite inclusions in zircon indicate that the gneisses together with eclogites were subjected to an early UHP metamorphism prior to amphibolite-facies retrogression. The Garnet peridotites show porphyroblastic textures, and consist mainly of garnet, clinopyroxene, orthopyroxene, and olivine with minor phlogopite. Garnet and clinopyroxene porphyroblasts show significant compositional zoning. Applying relevant geothermobarometers, core compositions of the minerals indicate P-T conditions of 6.0-7.0 GPa and 1100-1200°C. For the rim, similar or slightly higher P-T conditions compared to the eclogites were obtained. We suggest that the former represent crystallization conditions of garnet peridotite in the upper mantle, whereas the latter reflect reequilibrium conditions during incorporation of mantle rocks into the subducted slab. We conclude that the garnet peridotite may have been derived from the mantle wedge above the subduction zone. If so, these mantle-derived rocks were sandwiched between continental-derived country rocks.

Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite melange, Southwest Japan. The zircon porphyroblasts show pronounced zoning, and are characterized by both low Th/U ratios (0.2-0.8) and low Th and U abundances (Th = 1-81 ppm; U = 6-149 ppm). They contain inclusions of high-pressure minerals, including jadeite and rutile; such an occurrence indicates that the zircon crystallized during subduction-zone metamorphism. Phase equilibria and the existing fluid-inclusion data constrain P-T conditions to P 1.2 GPa at T < 350°C for formation of the jadeitite. Most U/Pb ages obtained by SHRIMP-RG are concordant, with a weighted mean 206Pb/238U age of 472 ± 8.5 Ma (MSWD = 2.7, n = 25). Because zircon porphyroblasts contain inclusions of high-pressure minerals, the SHRIMP U-Pb age represents the timing of jadeitite formation, i.e., the timing of interaction between alkaline fluid and ultramafic rocks in a subduction zone. Although this dating does not provide a direct time constraint for serpentinization, U-Pb ages of zircon in jadeitite associated with serpentinite result in new insights into the timing of fluid-rock interaction of ultramafic rocks at a subduction zone and the minimum age for serpentinization.

In the high-grade Cretaceous Sanbagawa high-pressure (HP) metamorphic belt, our new 1:5000 scale mapping of eclogitic mafic-ultramafic bodies and their surrounding epidote-amphibolite—facies schists has revealed a duplex structure formed by the subduction of the Izanagi-Pacific oceanic plate. Lithologies of the two largest mafic-ultramafic bodies in the Sanbagawa belt, the Iratsu eclogite and the Higashi-Akaishi peridotite, strike WNW-ESE and dip N; the upper boundary with the surrounding schist is a normal fault, whereas the lower boundary is a thrust. The Iratsu body is subdivided into at least two tectonic units; the unit boundary is subparallel to a lithological boundary. Protoliths of the upper unit are gabbro, basalt, minor quartz rock, and pelite, and those of the lower unit are pyroxenite, gabbro, basalt, chert, and marble, in ascending order. The lower unit is characterized by layers of alternating eclogitic metagabbro and pyroxenite. The layers are extensive at the bottom of the Iratsu eclogite, and transient toward the Higashi-Akaishi body. Eclogitefacies metapsammite is intercalated between the Iratsu and Higashi-Akaishi bodies. Our mapping has revealed the following: (1) a duplex structure of the mafic-ultramafic bodies indicating their accretionary complex origin; (2) reconstructed oceanic plate stratigraphy in ascending order of peridotite, gabbro, basalt, limestone, minor chert, and pelite, suggesting that different parts of the protolith were derived from a mid-oceanic topographic high, an oceanic island or plateau, and an overlying trench turbidite; and (3) a change in the convergent motion of the oceanic plate from NW to NE during the accretion of the large oceanic island or plateau.

The so-called Late Proterozoic snowball Earth disappeared at the beginning of Phanerozoic time: the surface of the Phanerozoic Earth was enveloped by an ocean and oxygen-rich atmosphere, and the planet had a warm climate. We propose that this dramatic change resulted from the return flow of seawater into the mantle beginning at 750 Ma, as a consequence of the cooling of the Earth and start of the main glacial epoch. Phase relations for minerals in the system MORB + H2O and peridotite + H2O suggest that old, cold subducting oceanic slabs would transport surface water through ultrahigh-P hydrous silicates to the mantle transition zone at depths of 410-660 km. This zone could store abundant H2O, inasmuch as hydrous β and γ phases of the olivine composition may contain up to 2 to 3 wt% water. Geotherms computed for rocks of ancient subduction zones from regional metamorphic belts around the world record such a warm-to-cold transition in and below the lithosphere at 750 Ma.The runaway cooling of the Earth's interior along consuming plate boundaries started at 750 Ma by propagation of a hydration front into the mantle from 70 km depth to 410-660 km through a narrow channel above the Wadati-Benioff plane, or directly by the descent of hydrated slab peridotites. Resultant hydration of the mantle wedge and adjacent regions caused a rapid sea-level drop during the interval of 750-600 Ma, and large landmasses appeared. This change was due to the increase of water content from 0.5 for amphibole peridotite to 6.5 wt% for antigorite peridotite in the wedge mantle, and to the popcorn effect to push up continental margins. The total seawater lost since then is estimated to have lowered sea level about 600 m.Introduction of seawater into the mantle drastically lowered the melting temperature and viscosity of mantle materials, both of which reactivated the plate and plume tectonic processes. Surface volcanism released mantle CO2, causing the greenhouse effect to melt global glaciers of the snowball Earth since about 750 Ma. The emergence of huge continental landmasses diversified surface environments; erosion and deposition of sediments became significant as in the present-day Earth. Extensive formation of sedimentary basins and accretionary complexes along consuming plate boundaries drastically increased free oxygen in the atmosphere by the burial of organic matter in those sediments, which was critical to the evolution of large multicellular organisms.