Kazuaki Okamoto

The high-pressure synthesis of lawsonite in the MORB+H2O system

Abstract Lawsonite is an important water reservoir in subducting oceanic crust below the amphibole dehydration depth ~70 km. To determine the maximum pressure stability of lawsonite in the MORB+H2O system, experiments were carried out using a 1000 ton uniaxial multi-anvil apparatus (SPI-1000). Mixtures of synthetic gel+2wt% H2O were used for the starting materials with the average MORB composition. Experimental P-T conditions were T = 700-900 8C and P = 5.5-13.5 GPa. Run durations were 12 and 24 h. Lawsonite was synthesized stably up to 10 GPa and T < 700 °C in the stishovite stability field, and <900 °C at 8 GPa and 750 °C at 5.5 GPa in the coesite stability field, with a steep positive slope for the lawsonite-out reaction. The lawsonite-out reaction in the coesite stability field changes to have a gentle negative slope in the stishovite stability field. The reaction leading to the disappearance of lawsonite is a continuous reaction due to the compositional enlargement of garnet toward the grossular end-member with increasing T and P. Lawsonite disappears when the tie line connecting grossular-rich garnet with omphacitic clinopyroxene reaches the bulk composition on the conventional AC(FM) ternary diagram.

Plate Tectonics at 3.8–3.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland

A 1:5000 scale mapping was performed in the Isukasia area of the ca. 3.8‐Ga Isua supracrustal belt, southern West Greenland. The mapped area is divided into three units bounded by low‐angle thrusts: the Northern, Middle, and Southern Units. The Southern Unit, the best exposed, is composed of 14 subunits (horses) with similar lithostratigraphy, bound by layer‐parallel thrusts. Duplex structures are widespread in the Isua belt and vary in scale from a few meters to kilometers. Duplexing proceeded from south to north and is well documented in the relationship between link‐ and roof‐thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, of greenstone with low‐K tholeiitic composition with or without pillow lava structures, chert/banded iron‐formation, and turbidites. The cherts and underlying low‐K tholeiites do not contain continent‐ or arc‐derived material. The lithostratigraphy is quite similar to Phanerozoic “oceanic plate stratigraphy,” except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present‐day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100°C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half‐space cooling at given parameters of surface and mantle temperatures of 100° and 1450°C, respectively, suggesting that the Archean oceanic lithosphere was rigid. These conclusions—rigidity and lateral plate movement—support the idea that the modern style of plate tectonics was in operation only 0.7–0.8 G.yr. after the formation of the Earth.

Supersilicic clinopyroxene and silica exsolution in UHPM eclogite and pelitic gneiss from the Kokchetav massif, Kazakhstan

Abstract Abundant exsolved quartz rods occur in matrix clinopyroxene of eclogite from the Kokchetav massif, Kazakhstan. These rocks are diamond-grade, ultrahigh-pressure (UHP) metamorphic rocks that recrystallized at P > 6 GPa and T > 1000 °C. Zircon is an excellent container, which effectively protects peak UHP metamorphic phases from retrogression. Therefore, to ascertain the pre-exsolution composition of the clinopyroxene, we analyzed clinopyroxene inclusions in zircon of the eclogite and a diamond-bearing biotite gneiss. Clinopyroxene in zircon has an excess of Fe3+ + VIAl over IVAl + Na + K, and calculated cation totals significantly less than 4.0 per six O atoms. The stoichiometry of these pyroxenes can be reconciled if the Ca-Eskola end-member (Ca0.5⃞0.5AlSi2O6) is considered. The zircon-hosted clinopyroxene in the eclogite contains up to 9.6 mol% of the Ca-Eskola component, and in the biotite gneiss up to 18.2 mol%, whereas the matrix clinopyroxene contains much less (1.3 mol%, on average). Recalculation of the composition of the matrix clinopyroxene prior to exsolution of quartz rods yields 6.8 mol% Ca-Eskola component, which is consistent with the composition of the clinopyroxene inclusions in zircon. We conclude that the Ca-Eskola component in the peak metamorphic clinopyroxenes broke down by a retrograde reaction: 2 Ca0.5⃞0.5AlSi2O6 → CaAl2SiO6 + 3 SiO2 resulting in exsolution of the quartz rods in the matrix clinopyroxene. Subducted crustal and supracrustal rocks form the Ca-Eskola clinopyroxene at high pressures and temperatures. The vacancy-containing clinopyroxene may have an important bearing on the physico-chemical properties of the subducted slab at upper mantle depth.

Late Triassic–Late Cretaceous accretion/subduction in the Taiwan region along the eastern margin of South China – evidence from zircon SHRIMP dating

To examine the tectonic history of the Taiwan segment of the eastern margin of South China, six rock samples from the Tailuko belt, the metamorphic basement of Taiwan, were selected for zircon SHRIMP dating. The aim was to identify evidence shedding light on the timing of the change from passive to active tectonics for this part of the continental margin since South China separated from the supercontinent of Rodinia. The results lead to two age groups, 190–200 and 88–90 Ma. These age groups, augmented by the previously published age data, suggest that they could have resulted from two Mesozoic accretion/subduction events. In addition, this mid‐late Mesozoic Tailuko belt might have also been reactivated and structurally complicated by the late Cenozoic collision/accretion of the Luzon arc with the Eurasian continent. Records of older tectonic events, such as those derived from the Japanese Islands, are absent in this metamorphic basement. An important finding of this study is the existence of the 191±10 Ma Talun metagranite, the oldest granitic intrusion ever reported in the Taiwan region and along the eastern coast area of South China. In spite of a large age uncertainty, the occurrence of this metagranite is not consistent with the apparent younging trend of Jurassic‐Cretaceous igneous activity toward the coastline in South China, and should be taken into consideration by future studies.

Accretionary Complex Origin of the Mafic-Ultramafic Bodies of the Sanbagawa Belt, Central Shikoku, Japan

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.

Exhumation Tectonics of the Sanbagawa High-Pressure Metamorphic Belt, Southwest Japan—Constraints from the Upper and Lower Boundary Faults

We have determined the exhumation process of the Sanbagawa Belt based on kinematic analyses of the upper and the lower boundary faults. The Sanbagawa Belt is tectonically intercalated as a thin subhorizontal sheet between overlying, weakly metamorphosed Jurassic and underlying Cretaceous accretionary complexes (e.g., Maruyama et al., 1996). On the lower boundary in Kii Peninsula, pumpellyite—actinolite facies metabasites have undergone semibrittle deformation, indicating a top-to-the south sense of thrusting. The upper boundary in central Shikoku is north-vergent, and indicates a top-to-the north sense of shear; this suggests that the original normal fault on the boundary was warped by later doming. These results support a model of selective exhumation of the Sanbagawa Belt from a depth of 30 km, and its juxtaposition against the over-and underlying accretionary complexes by orogen-orthogonal movements.

Zircon-Inclusion Mineralogy of a Diamond-Grade Eclogite from the Kokchetav Massif, Northern Kazakhstan

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.

SHRIMP age of detrital zircon from the rock of Taiwan

The macroscale morphology of Archean stromatolites has been used as evidence of early microbial ecosystems. But as Archean stromatolites only rarely contain fossil microbes, their biogenicity is tacitly assumed on the basis of macroscopic morphological comparisons with modern structures. Biogenetic definitions, however, require microscopic examination of suspected stromatolites. We obtained an unique collection of pristine samples from a diamond drillhole that intersected the 2.7Ga Tumbiana Formation, Australia (Pilbara Drilling Project, Van Kranendonk et al., 2006). We report the occurrence of micronsized globules of organic carbon intimately associated with the host micritic carbonate. Scanning Transmission X-ray Microscopy (STXM) analysis revealed that these organic globules are composed of organic carbon with aromatic, aliphatic and carboxyl functional groups. High Resolution Transmission Electron Microscopy (HRTEM) analysis revealed that the organic material occurs in intimate association with clustered, 50-200nm rounded bodies of aragonite. These nano-aragonite aggregates show striking similarities with nano-carbonate spheroids associated with microbial cells and polymers in modern microbialites. Our results indicate that Tumbiana stromatolites were likely formed via in-situ microbial lithification. They also extend the geologic record of aragonite back more than 2,300 million years, with profound implications on the environmental conditions prevailing on Early Earth.