Tatsuki Tsujimori

Global UHP Metamorphism and Continental Subduction/Collision: The Himalayan Model

Continental crust (density ~2.8 g·cm-3) resists subduction into the earths mantle (~3.3 g·cm-3) because of buoyancy. However, more than 20 recognized ultrahigh-pressure (UHP) terranes have been documented; these occurrences demonstrate that not only is continental crust subducted to depths as great as 150 km, but also that some supracrustal rocks were then exhumed to the earths surface. UHP terranes are composed of mainly supracrustal rocks that contain minor amounts of minerals such as coesite or diamond, indicative of P > 2.5 GPa. In general, quartzofeldspathic units are thoroughly back reacted, and only mafic eclogite lenses and boudins retain scattered UHP phases. These index minerals are restricted to micron-scale inclusions in chemically and mechanically resistant zircon, garnet, and a few other strong container minerals, and are difficult to identify by conventional petrologic studies. The continental rocks were subjected to UHP metamorphism at T ranging from ~700 to 950°C and P > 2.8 to 5.0 GPa, corresponding to depths of ~100 to 150 km. These UHP units were subsequently exhumed to crustal depths and subjected to intense hydration and amphibolite-facies overprint. Widespread Barrovian-type metamorphism in many collisional orogens may mask an earlier, higher-pressure metamorphic history. We suspect that coesite-bearing UHP rocks were once generated in the majority of exhumed collisional orogens. The recent finding of coesite inclusions in rare Himalayan eclogites and country rock gneisses is a typical example. We use the Himalayan model to illustrate UHP metamorphism and subduction of continental crustal rocks to mantle depths and later Barrovian-type overprint during exhumation. Himalayan UHP eclogites and adjacent gneisses were formed at mantle depths > 100 km at 46 to 52 Ma. These rocks were exhumed to crustal depths and subjected to Barrovian amphibolite- to granulite-facies metamorphism; associated magmatism occurred at 30 to 15 Ma. The Himalayan metamorphic belt was domally uplifted and the mountain-building process initiated since 11 Ma, when underthrusting of the Indian tectosphere beneath the Lesser Himalayas occurred.

Cenozoic and Mesozoic metamorphism in the Longmenshan orogen: Implications for geodynamic models of eastern Tibet

New zircon U-Pb and mica 40Ar/39Ar dating combined with structural studies in the Longmenshan orogen confirm that most of the upper crustal deformation in the eastern margin of Tibet is Mesozoic. However, at lower structural levels, apatite U-Pb and monazite electron microprobe dating reveals a previously unknown domain of Cenozoic (ca. 65 Ma) Barrovian-type metamorphism and deformation. This discovery shows that the crust in the eastern margin of Tibet was already a substantial thickness around the time of the India-Asia collision. Associated deformation has a N-S-oriented stretching lineation, implying that deformation was not driven by topographic gradients in the Tibetan Plateau. The observed moderate amounts of distributed postmetamorphic E-W shortening can probably explain the present thickness of the continental crust in the area. These results do not support models of crustal thickening caused by solid-state lateral flow of midcrustal metamorphic rocks.

U-Pb dating of large zircons in low-temperature jadeitite from the Osayama serpentinite melange, southwest Japan: insights into the timing of serpentinization

Crystals of zircon up to 3 mm in length occur in jadeitite veins in the Osayama serpentinite mélange, 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.

Petrogenetic relationships between jadeitite and associated high-pressure and low-temperature metamorphic rocks in worldwide jadeitite localities: a review

Jadeitite-bearing serpentinite-matrix melange is distributed in the Caribbean (Guatemala, Cuba, and Dominican Republic), circum-Pacific (Japan, Western USA, and Papua New Guinea), Alpine-Himalayan (Italy, Iran, Greece, and Myanmar), and Caledonian (Russia and Kazakhstan) orogenic belts, and always contains high-pressure, low-temperature (H P -L T ) metamorphic rocks. There are also jadeitite xenoliths in kimberlitic pipes in the Colorado Plateau (USA). The oldest occurrences of jadeitite are Early Paleozoic in Japan, Russia, and Kazakhstan, suggesting subduction-zone thermal structures evolved the necessary high pressure/temperature conditions for jadeitite formation since Early Paleozoic; the youngest occurrence is a xenolith from the Colorado Plateau. Major occurrences consist principally of fluid precipitates (P-type) that infiltrated the mantle wedge; fewer occurrences document metasomatic replacement (R-type) of plagiogranite, metagabbro and eclogite, and both types may be possible in the same occurrence or system. The P-T conditions for jadeitite formation can be extended beyond the previously argued limits of blueschist-facies conditions. Some jadeitite formed at epidote amphibolite and others at eclogite facies conditions. Available geochronological data of both jadeitite and associated H P -L T rock show temporal discrepancies between jadeitite formation and H P -L T metamorphism at some localities. The close association between older jadeitite and younger H P -L T rock in a single melange complex implies different histories for the subduction channel and jadeitite-bearing melange. Jadeitite-bearing serpentinite melange can stay at the mantle wedge for a considerable time and, as a result, experience multiple fluid-infiltration events. The subduction channel can occasionally incorporate overlying serpentinized mantle wedge material due to tectonic erosion. With time, the disrupted mantle wedge containing jadeitite veins is mixed with younger blueschists, exhumed eclogites and various fragments of suprasubduction-zone lithologies. Consequently, recrystallization and re-precipitation of jadeitite are reactivated along a slab–mantle wedge interface. All these possible scenarios and their combinations yield a complicated petrological record in jadeitite. With further investigation, the rock association of jadeitite–H P -L T metamorphic rocks–serpentinite has the potential to yield a greater understanding of subduction channels and overlying mantle wedge.

Space environment of an asteroid preserved on micrograins returned by the Hayabusa spacecraft

Records of micrometeorite collisions at down to submicron scales were discovered on dust grains recovered from near-Earth asteroid 25143 (Itokawa). Because the grains were sampled from very near the surface of the asteroid, by the Hayabusa spacecraft, their surfaces reflect the low-gravity space environment influencing the physical nature of the asteroid exterior. The space environment was examined by description of grain surfaces and asteroidal scenes were reconstructed. Chemical and O isotope compositions of five lithic grains, with diameters near 50 μm, indicate that the uppermost layer of the rubble-pile-textured Itokawa is largely composed of equilibrated LL-ordinary-chondrite-like material with superimposed effects of collisions. The surfaces of the grains are dominated by fractures, and the fracture planes contain not only sub-μm-sized craters but also a large number of sub-μm- to several-μm-sized adhered particles, some of the latter composed of glass. The size distribution and chemical compositions of the adhered particles, together with the occurrences of the sub-μm-sized craters, suggest formation by hypervelocity collisions of micrometeorites at down to nm scales, a process expected in the physically hostile environment at an asteroid’s surface. We describe impact-related phenomena, ranging in scale from 10-9 to 104 meters, demonstrating the central role played by impact processes in the long-term evolution of planetary bodies. Impact appears to be an important process shaping the exteriors of not only large planetary bodies, such as the moon, but also low-gravity bodies such as asteroids.

Epidote-rich talc-kyanite-phengite eclogites, Sulu terrane, eastern China: P-T-fo2 estimates and the significance of the epidote-talc assemblage in eclogite

Abstract Eclogites interlayered with gneiss and minor quartzite in the Qinglongshan near Donghai are characterized by unusually abundant (15.40 vol%) hydrous phases including talc, phengite, and epidote; many also contain kyanite. Garnet hosts both prograde (paragonite, amphibole, epidote) and peak stage (omphacite, epidote, phengite, kyanite) mineral inclusions. Several eclogites contain talc rimmed by barroisite; optically and compositionally similar coarse-grained amphibole in other samples indicates that the reaction Omp + Tlc = Amp has completely consumed talc. Estimated peak conditions of 30.35 kbar, 600.700 °C, are consistent with polycrystalline quartz pseudomorphs after coesite included in garnet, omphacite, epidote, and kyanite, and up to 3.6 Si pfu (11 O atom basis) in phengite. Garnet-epidote oxygen barometry on the peak metamorphic assemblage indicates oxygen fugacities above the Hem-Mag buffer, consistent with the epidote + talc assemblage and 5.20 mol% aegerine component in omphacite. The high oxygen fugacity calculated in this study as well as previously documented negative oxygen isotope values recorded by these rocks may both reflect alteration by oxidizing, meteoric water in a hydrothermal system. Oxidized conditions during peak metamorphism may explain the extreme scarcity of microdiamond in this area. The Ep + Tlc assemblage is stabilized by high oxygen fugacity, and demonstrates that talc-bearing eclogites are not restricted only to unusually Mg-rich bulk compositions.

Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala

Abstract Coexisting jadeite and omphacite were found as retrograde minerals in a jadeite-bearing lawsonite- eclogite from the Motagua Fault Zone, Guatemala. The lawsonite-eclogite is characterized by the occurrence of garnet porphyroblasts up to 2.5 cm in size, and the eclogite-facies parageneses, almandine- rich garnet + impure jadeite + lawsonite + rutile + quartz; garnet contains inclusions of impure jadeite, phengite, ferroglaucophane, chlorite, lawsonite, rutile, ilmenite, and quartz. Textural relations and parageneses and compositions of minerals indicate that the lawsonite-eclogite experienced two stages of metamorphism: prograde eclogite-facies stage (M1) and retrograde stage (M2). The impure jadeite (Jd-I) of the M1 eclogite-facies occurs in both the matrix and as inclusions in garnet, and contains considerable amounts of augite and aegirine components (Jd61-75Aug16-24Ae0-18). It is partly recrystallized to retrograde M2 jadeite (Jd-II) (Jd74-87Aug9-16Ae0-11) and omphacite (Jd42-50Aug36-46Ae7-16); some of these two sodic pyroxenes may have crystallized from fluids. Both M2 jadeite and omphacite show textural equilibrium and are believed to have grown concurrently. Based on the observed compositions and the phase relations of sodic pyroxenes from Carpenter (1980), the M1 impure jadeite (Jd-I) may have had a disordered C2/c symmetry at T = ca. 450 °C and P = ca. 1.8.2.4 GPa, and was subsequently crystallized into jadeite (Jd-II) plus ordered P2/n omphacite during retrogression with infiltration of fluids at T < ca. 300 °C and P = ca. 0.7 GPa (M2). The extreme low-T conditions during retrogression may have prevented reaction between eclogitic jadeite and adjacent minerals. Instead, eclogitic impure jadeite (plus fluid) has recrystallized into the retrograde jadeite + omphacite pair with a wide compositional gap.

Omphacite-diopside vein in an omphacitite block from the Osayama serpentinite melange, Sangun-Renge metamorphic belt, southwestern Japan

Abstract Omphacite (Jd46.1-52.0Ae0-8.4Aug48.0-51.2) and diopside (Jd4.3-6.3Ae0-0.4Aug93.6-95.6) coexist in a vein cutting an omphacitite block in a serpentinite melange of the Oeyama ophiolite, central Chugoku Mountains. The compositional gap between omphacite and diopside is significantly wider than for other omphacite- diopside pairs reported in the literature. The intergrowth texture of the omphacite-diopside vein suggests that the clinopyroxene pair was contemporaneously crystallized in the fracture from a Ca-bearing, alkali-rich fluid in a sub-solvus condition. Such a fluid may have been introduced from the surrounding serpentinized clinopyroxene-bearing harzburgite. The stability of omphacite and Al-rich pumpellyite in the matrix and the omphacite-diopside vein indicate that the earlier matrix recrystallization and later fracture filling may have both taken place under high-P-T condition during the melange-forming stage.

Coexisting chromian omphacite and diopside in tremolite schist from the Chugoku Mountains, SW Japan: The effect of Cr on the omphacite-diopside immiscibility gap

Abstract Chromian clinopyroxenes with exsolution textures were found in high-pressure (HP) tremolite schist from the Osayama serpentinite melange in the Chugoku Mountains, Southwestern Japan. Chromian omphacite [jadeite (Jd)23.3-41.4 diopside (Di)46.7-60.7 kosmochlor (Ko)4.5-19.6 aegirine (Ae)<7.0] with up to 6.6 wt% Cr2O3 occurs as neoblastic crystals in a foliated tremolite-rich matrix, or as pseudomorphs after relict chromian spinel, and contains irregular-shaped thin lamellae of chromian diopside (Jd3.8-18.2Di72.5- 93.6Ko1.1-13.9Ae<3.4). Chromian omphacite contains roughly constant jadeite plus kosmochlor components at 38.2-51.6 mol%; this is equivalent to the jadeite component of ordered P2/n omphacite. Systematic analyses of coexisting chromian pyroxenes yield a clear immiscibility gap between “omphacite” and “diopside.” The compositional gap becomes much narrower with increasing Ko component; addition of only 10 mol% Ko component narrows the omphacite-diopside gap by an order of magnitude. Such an effect is similar to, but more effective than, the introduction of Fe3+ on the omphacite-diopside immiscibility gap. Chromian pyroxenes replacing relict chromian spinel are associated with other chromian silicates including phengite, chlorite, and pumpellyite. The wide compositional gap of chromian pyroxenes and the presence of chromian pumpellyite and Si-rich chromian phengite indicate T < 300-400 °C and P > 0.8 GPa. This P-T estimate is consistent with parageneses of minerals in the host serpentinite. The variation of Cr content in chromian silicates reflects the extent of Cr ↔ Al substitution, and may be related to a chemical heterogeneity of the Cr-bearing fluid.

Plate tectonic gemstones

The gemstones jadeite and ruby generally form as a result of the plate tectonic processes subduction and collision. Jade made of jadeite (jadeitite) forms when supercritical fluids released from subducting oceanic crust condense in the overlying mantle wedge, 20–120 km deep in the Earth. Jadeitite deposits thus mark the location of exhumed fossil subduction zones. Ruby, the red gem variety of corundum, forms during amphibolite- and granulite-facies metamorphism or melting of mixed Al-rich and Si-poor protoliths, 10–40 km deep in the crust. Suitable conditions generally exist where passive-margin carbonates and shales are involved in continental collision. Most ruby deposits formed during Ediacaran-Cambrian (ca. 550 Ma) collisions that produced the East African–Antarctic orogen and the supercontinent Gondwana, or during Cenozoic collisions in south Asia. Ruby is thus a robust indicator of continental collision. As a result of these diagnostic properties, we propose the term “plate tectonic gemstones” (PTGs) for jadeitite and ruby. The PTGs are a new type of petrotectonic indicator that are mostly found in Neoproterozoic and younger rocks. The PTGs as petrotectonic indicators that form deep in the Earth have the added advantage that their record is unlikely to be obliterated by erosion, although the possibility of destruction via retrogression needs to be further assessed. Recognition of the PTGs links modern concepts of plate tectonics to economic gemstone deposits and ancient concepts of beauty, and may aid in exploration for new deposits.