Pacific-type orogeny (PTO) has long been recognized as a contrasting accretionary alternative to continent-continent collisional orogeny. However, since the original concept was proposed, there have many new developments, which make it timely to produce a new re-evaluated model, in which we emphasize the following new aspects. First, substantial growth of Tonarite–Trondhjemite–Granite (TTG) crust, and second the reductive effect of tectonic erosion. The modern analog of a Pacific-type orogen developed through six stages of growth exemplified by specific regions; initial stage 1: the southern end of the Andes; stage 2: exhumation to the mid-crustal level at Indonesia outer arc; stage 3: the Barrovian hydration stage at Kii Peninsula, SW Japan; stage 4: the initial stage of surface exposure of the high-P/T regional metamorphic belt at Olympic Peninsula, south of Seattle, USA; stage 5: exposure of the orogenic core at the surface at the Shimanto metamorphic belt, SW Japan; and stage 6: post-orogenic processes including tectonic erosion at the Mariana and Japan trench and the Nankai trough. The fundamental framework of a Pacific-type orogen is an accretionary complex, which includes limited ocean floor material, much terrigenous trench sediment, plus island arc, oceanic plateau, and intra-oceanic basaltic material from the ocean. The classic concept of a PTO stresses the importance of the addition within accreted rocks of new subduction-generated arcs and TTGs, which were added along the continental margins particularly during the Cretaceous. Besides the above additional or positive aspects of a PTO, here we emphasize the negative effects of previously little-considered tectonic erosion caused by subduction over time. The evaluation of such extensive tectonic erosion leads a prospect of the presence of huge quantities of TTG material in the lower transition zone, where many subducted slabs have ponded, as illustrated by mantle tomography. This is confirmed by density profiles of the mantle, which show that TTGs are abundant only along the bottom of the upper mantle accompanied by slab peridotite, lherzolite, and MORB. The major velocity anomaly in the lower transition zone is best explained by the predominance of SiO2 phases, hence TTG, and not by MORB or ultramafic rocks. Reasonable calculations indicate that at a depth range of 520-660 km TTG material amounts to 6-7 times more than the total mass of the surface continental crust. The traditional view is that the Japanese islands evolved since 520 Ma through five Pacific-type orogenies, which grew oceanward, thus creating a continuous accretionary complex ca. 400-500 km wide, with TTG growth at the continental side of each orogen. However, the subducting oceanic lithosphere has produced five times more TTG crust compared with the present TTG crust in the Japan islands. This is explained by the fact that over time tectonic erosion has dominated the increasing arc-TTG crust. Accordingly, Japan has lost four arc-TTG crusts to tectonic erosion. TTG material, such as trench sediment, arc crust, and continental margin crust, was fragmented by tectonic erosion and transported into the bottom of the upper mantle at depths of 520-660 km. Worldwide data suggest that tectonic erosion destroyed and fragmented most of the Pacific-type orogens.(View PDF for the rest of the abstract.)
Serpentinization of Al-depleted and Al-undepleted komatiites (and olivine for comparison) was experimentally characterized under high-temperature and high-pressure conditions of 300 °C and 500 bar to evaluate the H2 generation potential in komatiite-hosted hydrothermal systems in the early Earth. From the results, the steady-state H2 concentrations of fluids were estimated to be approximately 20 and 0.05 mmol/kg during the serpentinization reactions for Al-depleted and Al-undepleted komatiites, respectively (60 mmol/kg in the case of olivine). The H2 concentration of hydrothermal fluid generated from the serpentinization of Al-depleted komatiite is lower than that from olivine but is comparable to that of typical modern peridotite-hosted hydrothermal systems (~16 mmol/kg). The relatively low H2 concentration from Al-undepleted komatiite is similar to the levels in modern basalt-hosted hydrothermal fluids. Considering that the generation of Al-depleted komatiite melt requires a hotter mantle upwelling (plume) than the generation of Al-undepleted komatiite melt and that the temperature of the mantle has gradually decreased throughout Earth's history, Al-depleted komatiite may have constituted ultramafic volcanism in Hadean oceanic islands/plateaus. Furthermore, it seems unlikely that seafloor exposure of mantle peridotites occurred frequently in the Hadean because the oceanic crust of that time was presumably much thicker than the modern equivalent. Therefore, the serpentinization of Al-depleted komatiites may have been the main process that provided abundant H2-rich seafloor hydrothermal environments in the Hadean ocean, which potentially acted as a nursery for the prebiotic chemical evolution and the emergence and early evolution of life on Earth.
One of the important aims of metamorphic petrology is to unveil the physicochemical conditions of the Earth's interior. Decoding the record of metamorphism is a case of the inverse problem based on observations of stable equilibrium mineral assemblages and mineral compositions. Mineralogical forward-modeling is an alternative approach to estimate the metamorphic conditions of a rock. It enables a prediction of equilibrium composition of zoned minerals or mineral inclusions with the matrix phase assemblage. Thus, the forwardapproach tests the assumption of an equilibrium mineral assemblage, which is critical for inversion analysis.In this paper, we introduce thermodynamic forward-modeling in the field of metamorphic petrology. Then, we show an example of its application in estimating a prograde metamorphic P-T path of a whiteschist from the Kokchetav ultrahigh-pressure terrane in Kazakhstan. Garnet in the whiteschist shows prograde compositional zonation and contains mineral inclusions. We carried out geothermobarometric estimates using ilmenite-rutile composite inclusions in garnet, combined with an evaluation of the equilibrium mineral assemblage in a P-T pseudosection of the whiteschist in the K2O-CaO-MgO-FeO-Al2O3-SiO2-H2O system. The result yielded a counter-clockwise prograde P-T path for the rock. The amount and equilibrium composition of garnet were sequentially calculated for model P-T conditions along the P-T path, and the results were compared with a line-profile observation of the garnet. A consistency between the model and the observations was confirmed for XCa in garnet, however XFe and XMg had a large inconsistency. Both uncertainties in the equilibrium model and nonequilibrium effects during the crystal growth are possible reasons for such a discrepancy. Hence, it must be noticed that there are complexities in estimating the equilibrium composition of a zoned garnet and matrix minerals.
