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We determined the mineralogical and petrological characteristics of ultramafic rocks dredged from two oceanic core complexes: the Mado Megamullion and 23°30′N non‐transform offset massif, which are located within the Shikoku back‐arc basin in the Philippine Sea. The ultramafic rocks are strongly serpentinized, but can be classified as harzburgite/lherzolite or dunite, based on relict primary minerals and their pseudomorphs. Strongly elongated pyroxene porphyroclasts with undulatory extinction indicate high‐temperature (≥700 °C) strain localization on a detachment fault within the upper mantle at depths below the brittle–viscous transition. During exhumation, the peridotites underwent impregnation by magmatic or hydrothermal fluids, lizardite/chrysotile serpentinization at ≤300 °C, antigorite crystallization, and silica metasomatism that formed talc. These features indicate that the detachment fault zones formed a fluid pathway and facilitated a range of fluid–peridotite interactions. [ABSTRACT FROM AUTHOR]

Relicts of deformed lithospheric mantle have been identified within serpentinites and weathered peridotites recovered from nine dredge sites and one submersible dive site from across the Godzilla Megamullion, which was emplaced at the now-extinct Parece Vela Rift in the Parece Vela Basin, a back-arc basin in the Philippine Sea. The serpentinites consist dominantly of lizardite ± chrysotile and magnetite with minor relict primary minerals that include pyroxene, spinel, and rare olivine. The weathered peridotites consist of pyroxene, spinel, lizardite ± chrysotile, and magnetite as well as weathering products of olivine. These rocks were classified in hand specimen into three types with different structures: massive, foliated, and mylonitic. In thin-section the serpentine minerals show no sign of deformation, whereas relict primary minerals show evidence of plastic deformation such as undulose extinction, kink bands, dynamic recrystallization, and weak to moderate crystallographic preferred orientations. Therefore, the serpentinites and weathered peridotites result from the static replacement and weathering of previously ductile-deformed peridotite. Given their location close to or on the detachment surface that exposed them, the relicts of peridotite provide evidence of deformation in the lithospheric mantle that could be related to the formation and emplacement of the Godzilla Megamullion in the Parece Vela Rift. [ABSTRACT FROM AUTHOR]

Abstract: Petit-spots, the late Miocene alkali basaltic volcanoes on the Early Cretaceous NW Pacific Plate, originate at the base of the lithosphere. The petit-spot volcanic rocks enclose fragments of tholeiitic basalt, dolerite, gabbro, and mantle peridotite, providing a unique window into the entire section of subducting oceanic lithosphere. We provide here the first direct observations on the deep structure of the Pacific lithosphere using microstructural analyses of a petit-spot peridotite xenolith. The xenolith is a lherzolite that consists mainly of coarse- and medium-grained olivine, orthopyroxene, and clinopyroxene, as well as fine-grained aggregates of spinel and orthopyroxene that probably represent replaced pyrope-rich garnet. A strong deformational fabric is marked by a parallel alignment of millimeter-sized elongate minerals and their crystallographic preferred orientation. The olivine displays a [010] fiber pattern with a girdle of [100] axes and a maximum of [010] perpendicular to the foliation, a pattern which is consistent with a transpressional deformation in high temperature conditions at the base of oceanic lithosphere. Our microstructural observations and seismic data indicate that the lower part of the NW Pacific lithosphere possess an early stage structure of mantle flow at the asthenosphere. This interpretation is compatible with a conventional model in which oceanic lithosphere is thickened during cooling and plate convection. A discrepancy between the weak anisotropy in the petit-spot peridotite and the strong azimuthal anisotropy from the seismic data in the NW Pacific plate implies the existence of a highly anisotropic component in the deep oceanic lithosphere. [ABSTRACT FROM AUTHOR]

Mantle deformation processes leading to seafloor spreading are often difficult to infer due to the highly serpentinized and weathered state of most abyssal peridotites. We investigated the development of high-temperature crystal-plastic deformation and lower temperature mylonitization processes in relatively fresh (<50% modal serpentine) and ultra-fresh (<1% serpentine) mantle peridotites derived from the heterogeneous mantle in the sparsely magmatic zone of ultraslow-spreading Gakkel Ridge system by analyzing 12 peridotites from two dredge sites (<1 km apart). Microstructurally, these 12 peridotites consist of seven high- T deformed samples and five mylonites. Modally, the 12 samples include harzburgites, lherzolites, an olivine websterite, and a plagioclase-bearing lherzolite. Based on their mineral major and trace element compositions, the lherzolites, harzburgites, and olivine websterite are residual peridotites. The lherzolites containing clinopyroxenes with flat REE patterns likely underwent refertilization with a high influx of melt. The plagioclase-bearing lherzolites probably formed by subsolidus reaction after the partial melting process. Microstructural observations support that high- T crystal-plastic deformation (most likely at temperatures exceeding 1000 °C) was active in the peridotites of the high- T deformation group, accommodating mantle flow beneath the Gakkel Ridge. The identified melt refertilization process may have contributed to the formation of [010]-fiber olivine fabrics in these peridotites. Mylonitic microstructures, decreasing fabric strength and grain-size reduction of olivine suggest that mylonitization occurred under relatively low-temperature mantle conditions (~800 °C) and probably accommodated strain localization. Water did not greatly affect the peridotites during the development of the shear zones, although amphibole with "dusty" zones developed in one mylonitic peridotite after mylonitization, indicating that late-stage metasomatism occurred locally within the shear zone. This low- T mylonitization is likely to have affected mantle peridotites of this region independently of petrogenetic processes. The development of these deformation processes in Gakkel Ridge suggests a shift from flow in the uppermost mantle to shear zone formation in the rift valley walls. • Complex partial melting and refertilization developed in the heterogeneous mantle. • Peridotites later underwent mantle-flow to shear-zone plastic deformation. • Deformation in peridotites developed independent of petrogenetic processes. [ABSTRACT FROM AUTHOR]

A titanian clinohumite-bearing dunite was recently found in the Ulamertoq ultramafic body within the 3.0 Ga Akia Terrane of southern West Greenland. Titanian clinohumite occurs as disseminated and discrete grains. Titanian clinohumite contains relatively high amounts of fluorine, reaching up to 2.4 wt.%. The high-Fo content of olivine (Fo93) coupled with low Cr/(Cr + Al) ratio of orthopyroxene implies that the dunite host is not of residual origin after melt extraction by partial melting of the primitive mantle. Olivine grains are classified into two types based on abundances of opaque mineral inclusions: (1) dusty inclusion-rich and (2) clear inclusion-free olivines. Opaque inclusions in coarse-grained olivines are mainly magnetite. Small amounts of ilmenite are also present around titanian clinohumite grains. The observed mineral association indicates partial replacement of titanian clinohumite to ilmenite (+magnetite) and olivine following the reaction: titanian clinohumite = ilmenite + olivine + hydrous fluid. The coexistence of F-bearing titanian clinohumite, olivine, and chromian chlorite indicates equilibration at around 800–900 °C under garnet-free conditions (<2 GPa). Petrological and mineralogical characteristics of the studied titanian clinohumite-bearing dunite are comparable to deserpentinized peridotites derived from former serpentinites. This study demonstrates the importance of considering the effects of hydration/dehydration processes for the origin of ultramafic bodies found in polymetamorphic Archaean terranes. [ABSTRACT FROM AUTHOR]

For a better understanding of plate dynamics such as the opening of Japan Sea back-arc basins, it is critical to know why and how back-arc spreading/rifting occurs adjacent to convergent plate boundaries. Two models have been proposed for the formation of Japan Sea back-arc basins: the "slab rollback model" and the "plume model". We report here on 12 spinel peridotite xenoliths in the alkali basalt of the Cenozoic Kawashimo volcano in southwest Japan. These xenoliths are dominantly harzburgite with some lherzolite. The harzburgites and lherzolites show coarse-grained or porphyroclastic textures that record variable degrees of deformation. Olivine and spinel compositions indicate the samples are residual mantle peridotites formed under various degrees of partial melting. Olivine crystallographic preferred orientations in the xenoliths have orthorhombic patterns characterized by a strong [010] maximum, and this orthorhombic pattern implies that the olivine deformed by dislocation creep on the (010)[100] slip system. Using a sub-grain size piezometer, the maximum differential stress varies from 3 to 13 MPa in the samples. We also found that the peridotite xenoliths deformed under relatively high strain-rates, such as 10−13 to 10−10 s−1, using olivine flow laws and the obtained stress and grain size. The range of high strain-rates is comparable to that predicted using a thermomechanical model of back-arc spreading/rifting. Therefore, it is probable that the peridotite xenoliths preserve textures that formed during the mantle deformation that accompanied the Japan Sea back-arc spreading/rifting. We estimated an equilibrium temperature of 1238 ± 20 °C using the two-pyroxene geothermometer, and this temperature is higher than any temperature reported previously for peridotite xenoliths in southwest Japan. The obtained equilibrium P–T condition for the xenoliths implies that the hot mantle plume hypothesis best explains the mechanism of formation of Japan Sea back-arc basins. • Spinel peridotite xenoliths with coarse-grained or porphyroclastic textures were deformed at ∼1200 °C and ∼2 GPa. • Deformation involved dislocation creep and a relatively high strain rate (10−13 to 10−10 s−1). • The peridotites preserve deformation textures formed during the opening of Japan Sea back-arc basins. [ABSTRACT FROM AUTHOR]