Keiko Hattori

Exhumation Processes in Oceanic and Continental Subduction Contexts: A Review

Although the exhumation of high pressure (HP) and ultrahigh pressure (UHP) rocks is an integral process in subduction, it is a transient process, likely taking place during the perturbation in subduction zones. Exhumation of HP to UHP rocks requires the weakening of a subduction channel and the decoupling of the exhumed slice from the rest of the slab. Considering more than 60 occurrences of HP to UHP units of Phanerozic ages, we propose three major types of subduction zones:

Volcanic fronts form as a consequence of serpentinite dehydration in the forearc mantle wedge

The release of fluids from subducting slabs is considered to result in partial melting of the mantle wedge and arc magmatism. By contrast, we propose that the breakdown of serpentinites, which acted as a sink for water and fluid-soluble elements released from underlying slab in the mantle wedge, most likely leads to arc magmatism at volcanic fronts. Serpentinites exhumed from mantle wedges in Himalayas, Cuba, and the Alps are enriched in elements that are fluid soluble at low temperatures, such as As, Sb, and Sr. The downward movement of the serpentinite layer by mantle flow transports these elements to deeper, hotter levels in the mantle. Eventual dehydration of serpentinite discharges water and fluid-soluble elements, leading to partial melting of the overlying mantle wedge, thus accounting for the observed enrichment of these elements in magmas at the volcanic front.

Evidence of hydration of the mantle wedge and its role in the exhumation of eclogites

Serpentinite samples from the Indus suture zone, representing a shallower part of a paleo-subduction zone, show lowgrade metamorphic recrystallization (chrysotile+magnetite ˛ magnesite ˛ talc). They are cumulates of melts formed in the uppermost mantle or the base of the Nidar intra-oceanic arc. Serpentinite samples associated with the Tso Morari eclogitic unit, representing the more deeply subducted portion of a paleo-subduction zone, exhibit high-grade metamorphic recrystallization (antigorite+magnetite ˛ forsterite ˛ talc) and the trace element chemistry of these samples suggests a strongly depleted mantle wedge origin. Nd concentrations and ONd values show that fluids responsible for hydration of the mantle wedge were derived from subducting clastic sediments overlying Tethyan oceanic crust. The exhumation of eclogites requires a mechanically weak zone at the interface between the subducting plate and the mantle wedge. We suggest that serpentinites associated with the Tso Morari eclogites acted as a lubricant for the exhumation of the eclogitic unit. Geophysical data suggest common occurrences of hydrated ultramafic rocks about 10 km thick along the interface between the mantle wedge and the subducting plate. We propose that such a low-viscosity zone played an important role for the exhumation of eclogitic rocks. fl 2001 Elsevier Science B.V. All rights reserved.

Geochemical character of serpentinites associated with high- to ultrahigh-pressure metamorphic rocks in the Alps, Cuba, and the Himalayas: Recycling of elements in subduction zones

Serpentinites associated with eclogitic rocks were examined from three areas: the Alps, Cuba, and the Himalayas. Most serpentinites have low Al/Si and high concentrations of Ir-type platinum group elements (PGE) in bulk rock compositions, indicating that they are hydrated mantle peridotites. A few samples contain high Al/Si and low concentrations of Ir-type PGE, suggesting that they are ultramafic cumulates. Among the hydrated mantle peridotites, we identified two groups, primarily on the basis of Al/Si and Mg/Si ratios: forearc mantle serpentinites and hydrated abyssal peridotites. Forearc serpentinites occur in the Himalayas and along a major deformation zone in Cuba. All serpentinites in the Alps and most serpentinites in Cuba are hydrated abyssal peridotites. Himalayan serpentinites have low Al/Si and high Mg/Si ratios in bulk rock compositions, and high Cr in spinel; they were serpentinized by fluids released from the subducted Indian continent and enriched in fluid-mobile elements, and show high 87Sr/86Sr, up to 0.730, similar to the values of rocks of the subducted margin of the Indian continent. Although Himalayan serpentinites have a similar refractory geochemical signature as the Mariana forearc serpentinites, the former contain markedly high concentrations of fluid-mobile elements and high 87Sr/86Sr compared to the latter that were hydrated by subducted Pacific Ocean crust. The data indicate that the enrichment of fluid-mobile elements in forearc serpentinites depends on the composition of subducted slabs. Alpine serpentinites and most Cuban serpentinites show moderate Al/Si similar to abyssal peridotites. Hydration of peridotites near the seafloor is supported by micro-Raman spectra of earlier formed lizardite, high δ34S (+11 to +17‰) of sulphides, and elevated 87Sr/86Sr, ranging from 0.7037 to 0.7095. The data support the contribution of S and Sr from seawater and sediments. These serpentinites are not highly enriched in fluid-mobile elements because serpentinization occurred at a high water/rock ratio. Alkali elements are conspicuously unenriched in all serpentinites. This lack of alkali enrichment is explained by slab retention of alkalis. This is also consistent with the observation of relatively low alkali concentrations in volcanic front magmas, since partial melting related to the volcanic fronts is triggered by dehydration of serpentinites.

Mantle wedge serpentinization and exhumation of eclogites: Insights from eastern Ladakh, northwest Himalaya

In eastern Ladakh, northwest Himalaya, serpentinite layers occur in close association with eclogites. The occurrence of metamorphic olivine and talc in serpentinites suggests that the serpentinization and eclogitization took place under similar conditions (600 °C, 20 kbar). The serpentinites and eclogites show similar deformation, including the direction of normal shearing. The highly refractory nature of the serpentinite protolith, as shown by the composition of bulk rocks and chromite and the concentrations of Re and platinum group elements, indicates their derivation from mantle wedge. We propose that the serpentinites formed by hydration of the mantle wedge as a result of dewatering of the subducted slab. The serpentinites then facilitated exhumation of the subducted rocks by acting as a lubricant. At shallow depths, sediments are generally considered to be the lubricant for the exhumation, but serpentinites may commonly take over this role at greater depths. Under sediment-poor conditions, serpentinites may contribute to the exhumation even at shallower depths. This may explain the close spatial association of serpentinites and partially hydrated peridotites with many well-known high-pressure to ultrahigh-pressure metamorphic belts worldwide.

High-sulfur magma, a product of fluid discharge from underlying mafic magma: Evidence from Mount Pinatubo, Philippines

Beneath Mount Pinatubo in the Philippines, a hot mafic melt ascended, releasing supercritical fluids rich in SO 2 into an overlying semisolidified dacitic magma. The SO 2 was reduced to H 2 S in the cool, wet dacite, causing oxidation of this magma. H 2 S thus formed was initially precipitated in the dacite as sulfides, which were high in Cu, Cd, Zn, and Se/S, elements also introduced by the fluids. Continued influx of SO 2 and oxidation of the dacite led to an increase in the S solubility of the melt, causing partial resorption of sulfide minerals. Further addition of SO 2 then led to excess S, which, in part, was precipitated as anhydrite. High S contents and the oxidized nature of the eruption products were due to the conjunction of an overlying, cool dacitic magma and ascending hot mafic melt. The 1982 eruption products of El Chichon (Mexico) and those from the 1985 eruption of Nevado del Ruiz (Colombia) have features similar to Pinatubo, suggesting that these high-S magmas may have formed by a similar process.

Magma evolution recorded in plagioclase zoning in 1991 Pinatubo eruption products

Abstract Plagioclase, the most abundant phenocryst at Mount Pinatubo, displays varying textures and compositions within the 1991 eruption products. In June 7-14 dome-forming andesite, plagioclase phenocrysts show prominent rims with higher MgO (0.04-0.06 wt%), Fe2O3T (0.6-0.8 wt%), and K2O at given An than the interiors. The compositions of the rims are identical to those of microlites, which are abundant in the groundmass glass. White dacitic pumice, the most voluminous product of the June 15 eruption, contains partly corroded plagioclase phenocrysts but no prominent rims and no microlites. The interiors of phenocrysts from the dacitic pumice and the dome-forming andesite are remarkably similar in terms of textures and compositions. They show oscillatory zoning (mostly An35-60), low MgO (<0.02 wt%) and Fe2O3T (0.10-0.30 wt%), and similar K2O at given An. This similarity indicates that the two types of plagioclase phenocrysts formed in the same rhyolitic melt. The oscillatory zoning likely formed by temperature fluctuations in the convecting magma and incorporation and degassing of external fluids. A portion of the felsic magma (~800 °C) mixed with a mafic melt (~ 1000-1100 °C) to become an andesitic magma that extruded to form the June 7-14 dome. All plagioclase phenocrysts in the andesite were derived from the felsic magma. The mixing caused de stabilization of phenocrysts, forming sieve textures, dusty zones, and partial resorption. Extrusion of the mixed magma resulted in overgrowths on once-resorbed phenocrysts and the nucleation of plagioclase microlites in the groundmass glass. In the unmixed, remaining portion of the felsic magma, some plagioclase underwent partial resorption but did not develop overgrowths. This lack of overgrowths and the absence of microlites in the ground- mass glass of the June 15 dacitic pumice indicate rapid magma ascent during the eruption and a short time span between the injection of a mafic melt and the cataclysmic eruption, supporting the linkage between the two.

Osmium-isotope ratios of platinum-group minerals associated with ultramafic intrusions : Os-isotopic evolution of the oceanic mantle

Abstract Osmium-isotope ratios were determined by an ion microprobe on the individual platinum-group minerals (PGM) from placers, which are associated with ultramafic intrusions of late Precambrian to Tertiary age. Unlike Os-isotope ratios in large layered mafic intrusions, these 187 Os/ 186 Os ratios are low, and within a narrow range from 0.99 to 1.12, which is attributed to the occurrences of the intrusions. There was no opportunity to incorporate old crustal Os because of the small sizes of the intrusions and the mode of emplacement into the upper crustal level. In addition, the interaction with the host volcanic rocks of similar age, if any, would not have seriously affected the 187 Os/ 86 Os ratios of the peridotites. While different phases of PGM in one grain have similar 187 Os/ 186 Os ratios, there is a significant variation in a given district. The variation is attributed to a long-term heterogeneity in Re/Os ratios of the oceanic upper mantle. The lowest value in each area is lower than the value expected from the evolution of bulk Earth composition. The lowering may be due to primordially low Re/Os ratios in the mantle or preferential removal of Re by partial melting to form the continental crust. The former model is rejected because most chondrites have higher Re/Os ratios than type C1 and the core-mantle separation would not have lowered Re/Os ratios. The low 187 Os/ 186 Os ratios are, therefore attributed to the extraction of continental crust by preferential removal of Re from the mantle through partial melting. The model is consistent with the depleted nature of oceanic peridotites (positive e Nd , negative e Sr , and low Re/Os ratios). Calculations of 187 Os/ 186 Os ratios of the mantle residue suggest that the observed data are in accordance with a model involving the extraction of ∼ 2% melt by fractional fusion from the mantle of C1 chondritic composition at ∼ 2.0 Ga. If the bulk Earth has higher Re/Os ratios, as proposed by Martin [1], then the observed data require much larger degrees of partial melting or older mean age of partial melting at ∼ 3.0 Ga. The variation in Re/Os ratios in each area is ascribed to the various degrees of mixing of the depleted mantle source and more fertile material. This may be due to local heterogeneity in the mantle, “marble-cake” like mantle [2] or replenishment of Re and Os from a more fertile source by sulphide ( + CO 2 ) in the mantle. Generally higher 187 Os/ 186 Os ratios found in this study compared with those from the southern African sub-continental lithosphere [3] may be attributed to the nature of the latter which is more isolated than the oceanic mantle from the rest of the mantle. The occurrence of S in the mantle may influence Re Os isotope variations and the behavior of S may decouple the Re Os system from other lithophile radiogenic isotope systems.

Asthenospheric upwelling, oceanic slab retreat, and exhumation of UHP mantle rocks : Insights from Greater Antilles

[1] Exhumation of garnet-bearing peridotites has been associated with low density continental rocks. This association does not explain the occurrence of garnetbearing peridotites in the oceanic subduction zone of the Greater Antilles in Hispaniola. We use numerical models of intra-oceanic subduction to explain exhumation of garnet peridotites without involvement of buoyant continental crust. We demonstrate that rheological weakening of the mantle wedge takes place due to its strong hydration during subduction of serpentinized slow spreading ridge. This weakening triggers upwelling of the hydrated peridotites and partially molten peridotites followed by upwelling of hot asthenosphere and subsequent retreat of the subducting slab. According to numerical modelling of P-T paths this process can explain exhumation of UHP (4GPa) rocks in an intra-oceanic setting. Citation: Gorczyk, W., S. Guillot, T. V. Gerya, and K. Hattori (2007), Asthenospheric upwelling, oceanic slab retreat, and exhumation of UHP mantle rocks: Insights from Greater Antilles, Geophys. Res. Lett., 34, L21309, doi:10.1029/ 2007GL031059.

Seafloor hydrothermal clay alteration at Jade in the back-arc Okinawa Trough: mineralogy, geochemistry and isotope characteristics

Seafloor hydrothermal activity at Jade has resulted in extensive alteration of the host epiclastic sediments and pumiceous tuffs, forming mica, kaolins (kaolinite and halloysite), Mg-rich chlorite, talc, montmorillonite, and a mixed-layer mineral of dioctahedral chlorite and montmorillonite (Chl/Mont). Clay mineral assemblages show a vertical variation, which reflects variable amounts of cold seawater incorporated into hot hydrothermal fluids in subsurface sediments and tuff. However, mixing alone cannot explain the occurrence of abundant kaolin minerals at Jade. The formation of kaolin minerals requires much more acidic fluid than expected from simple mixing of hydrothermal fluids and cold seawater. Low pH values are likely attained by oxidation of H2S either dissolved in the hydrothermal fluid or released from the fluid during decompression. The fluid reaching the seafloor is discharged into cold seawater, which caused precipitation of sulfides close to vents and native sulfur and barite at the margins of the vent areas. Halloysite, barite and anhydrite show Sr isotope compositions similar to marine Sr, indicating the derivation of marine Sr directly from seawater or by the dissolution of calcareous nannoplanktons. The isotopic compositions of kaolinite (δ18O = +7.4‰, δD = −23‰), Chl/Mont (δ18O = +7.0‰, δD = −32‰), and mica (δ18O = +5.4 to +9.9‰, δD = −30 to −26‰) suggest fluids of a heated seawater origin. The O isotopic data yielded formation temperatures of 170°C for kaolinite, 61 to 110°C for halloysite, and 145 to 238°C for mica. Barite δ34S values (+21.0 to +22.5‰) are very similar to the marine sulfate value, confirming that the barite formation took place due to mixing of Ba-bearing hydrothermal fluids and sulfate-rich seawater. Native sulfur shows a large variation in δ34S in one hand specimen probably because of rapid disequilibrium precipitation of S during fluid exhalation on the seafloor. Sulfur in hydrothermal fluids is usually consumed to form metal sulfides. Therefore, abundant native sulfur at Jade suggests high H2S/metals ratios of the hydrothermal fluids. The alteration assemblages and isotopic data of hydrothermal minerals from Jade are very similar to those of Kuroko-type barite deposits of middle Miocene age, which formed from fluids of high S/metals ratios at less than 200°C. At Jade, there is only one black smoker actively discharging high temperature (∼320°C) fluid, but there are many fossil sulfide chimneys and mounds in the area. The mineralogy and high Au and Cu in these precipitates suggest highly metalliferous hydrothermal activity in the past. These activities likely resulted in discharge of hydrothermal plumes and fall-outs of sulfides and sulfates on the seafloor. These fall-outs were incorporated in sediments far from the vent areas. They are now recorded as high metal contents in sediments with no petrographic and mineralogical evidence of in-situ hydrothermal activity. Some are high as 8,100 ppm for Cu, 12,500 ppm for Zn, 1,000 ppm for As, 100 ppm for Ag and 21,000 ppm for Pb. Detrital grains of montmorillonite in such sediments are coated with Fe-oxyhydroxides during the suspension in seawater before settling on the seafloor. The depths of such metal anomalies in sediments suggest high levels of metalliferous hydrothermal activities from 1,800 to 300 ybp.