Attenuated total reflection infrared (ATR–IR) spectroscopy allows measurements to be made directly from the surface of one–sided, diamond polished thin sections of geological samples. This method greatly reduces the sample preparation time when compared to other IR spectroscopy methods and opens the possibility of using infrared spectroscopy to study thin–section scale microstructures. ATR–IR spectroscopy of antigorite, chrysotile, and lizardite in samples from the Mt. Shiraga serpentinite body, central Shikoku, SW Japan, reveals clear spectral differences in the 650–1250 cm−1 region associated with the vibration of the Si–O bonds in SiO4 tetrahedra and in the 3300–3750 cm−1 region associated with the vibration of the O–H bond in MgO2(OH)4 octahedra. A data–processing algorithm developed in this study allows the absorbance intensity and wavenumber of a particular absorbance peak to be used to create serpentine mineral phase maps based on the highest intensity Si–O absorbance bands for antigorite, chrysotile, and lizardite. Our methodology can be used to map serpentinite microstructures in thin sections illustrating the potential of ATR–IR as a relatively un–explored analytical tool in petrological studies. A combination of ATR–IR and electron microprobe data shows that for antigorite the wavenumber of the O–H absorbance band is correlated with the Fe content. Metamorphic reactions of serpentine minerals play a key role in the hydrodynamics of the earth’s lithosphere, and the new information on serpentine mineral hydroxyl group behavior obtained by applying the technique outlined in this study are of great potential interest to researchers in a wide range of different fields.
Early Paleozoic serpentinite melanges in Japan preserve the oldest high–P metamorphic rocks in the circum–Pacific orogenic belt. To understand the tectonic regime at the subduction initiation of the proto–Japan convergent plate boundary, whole–rock geochemistry, and zircon U–Pb geochronology were investigated for amphibolite blocks in the Omi serpentinite mélange, central Japan. The studied amphibolites from two different localities have the mineral assemblage of albite + clinozoisite + amphibole ± rutile ± titanite, which characterize epidote–amphibolite facies metamorphism. Whole–rock trace element concentrations of the amphibolites suggest that gabbroic protoliths formed possibly in an oceanic setting. The zircon U–Pb weighted mean ages obtained from two amphibolite samples indicate that the protolith was formed in the Cambrian. The protolith ages of the studied amphibolites are comparable with those of reported Early Paleozoic ophiolite and high–pressure rocks in Paleozoic serpentinite mélanges in Japan. This fact implies that the young hot oceanic crust was subducting into the East Asian convergent plate margin during the Cambrian.
We conducted textural and chemical analyses of melt inclusions and their host plagioclase crystals in the scoria of the Izu–Omuroyama monogenetic volcano, erupted at ~ 4 ka in the Higashi–Izu monogenetic volcanic field, Japan. The groundmass melt was andesitic with ~ 59–61 wt% SiO2, and it contained abundant microphenocrysts of olivine and plagioclase. In contrast, ~ 59% of the plagioclase–hosted melt inclusions have rhyolitic compositions with ~ 70–75 wt% SiO2. The host plagioclase phenocrysts have cores with An# of 44.7 ± 4.2 [An# = 100Ca/(Ca + Na) in mol] and rims with An# of 68–78, and the calcic rims have compositions similar to the microphenocrysts. The cores of the host plagioclase phenocrysts have FeO* and K2O contents that are in equilibrium with the rhyolitic melt inclusions. Using the plagioclase–melt geohygrometers and assuming temperatures of 790–850 °C, we estimated the H2O contents of the rhyolitic melt inclusions to be ~ 4.4–10.2 wt%, indicating H2O–saturation depths of >4.5 km. Our results suggest that an inhibited reservoir of plagioclase–bearing rhyolitic melt existed beneath the monogenetic volcano at the time of the scoria eruption, which was ~ 800 years earlier than the first rhyolitic eruption in the volcanic field. Plagioclase content in the silicic reservoir is estimated to be less than 35.8%, suggesting the magma was eruptible. Our results demonstrate the potential usefulness of plagioclase–hosted melt inclusions for indicating the existence of such an inhibited silicic magma.
The Cretaceous granitoid batholith is characterized by sporadically occurring small mafic bodies. Some of these mafic bodies show high–Mg diorite (HMD) compositions derived from a high–Mg andesite (HMA) magma. One of the mafic bodies, the Shaku–dake body, can be divide into two groups: Two–pyroxene diorite (TPD), diorite (Do), porphyritic fine–grained tonalite (PFT), and clinopyroxene granodiorite (CG) belong to the Group–1, but hornblende–biotite granodiorite (HBG) and fine–grained biotite granite (FBG) can be found in the Group–2. The Group–1 is influenced by the assimilation and fractional crystallization process during the ascent and emplacement of magma, whereas the Group–2 changes its whole–rock compositions via fractional crystallization. Discrimination diagrams of HMA indicate that the TPD shows geochemical signatures similar to those of the Sanukitic HMA, where the TPD is defined as Sanukitoid. On the other hand, the Do is plotted as the composition range of island arc calc–alkaline basalts and tholeiite. The Cretaceous magma activities in northern Kyushu were led by the highly thermal structure of the wedge mantle at that time, it was the primary heat source of the voluminous igneous activities during the Cretaceous in Southwest Japan and the Korean Peninsula.
Mukundpura is a carbonaceous chondrite (CM2) recently fell in Rajasthan, India (June 6, 2017). A typical fine–grained, clast–dominant matrix contains a few isolated forsterite and FeO–rich olivine grains. In this study, forsterite–rich olivines were investigated using color cathodoluminescence (CL) and Raman spectroscopy in order to explain the primitive stages of asteroidal aqueous alteration. Isolated forsterite (Fo99) in Mukundpura emits bright CL of varying color and shows CL zonation in different patterns accounting the structural defects and chemical inhomogeneity. Blue luminescence (also distinguished by enriched CaO and TiO2) is common in cores of the relict forsterite attributing refractory nature of the olivine. Electron Probe Micro Analyzer (EPMA) line scan across the CL–active forsterite grains shows minor elements zonation especially for activator elements and thus provides a correlation of color of the emitted luminescence with diffusible ions. The red CL zonation (also characterized by enriched FeO, Cr2O3, and MnO content) is common along the majority of forsterite rims suggesting aqueous activity in the parent asteroid. The strongest doublet Raman peaks corresponding to 821 and 854 cm−1 are due to SiO4 tetrahedral vibrational modes, and other peaks are often related to infer pure crystalline state of the forsterite. Thus, a combination of CL imaging and Raman spectroscopy is useful to explain the chemical–structural properties of luminescent pure forsterite and also helps in understanding the aqueous alteration of CM chondrite.
The Urgamal eclogite in western Mongolia, described here for the first time, occurs in the Altai allochthon (also known as the Urgamal subzone) in the Zavkhan terrane of the Central Asian Orogenic belt. The eclogite consists of garnet, omphacite, white mica, quartz, epidote, rutile, and barroisite. We measured zircon U–Th–Pb ages in a barroisite–rich sample of the eclogite using an electron probe microanalyzer, which yielded an age of 1113 +31/−52 Ma. This suggests that the eclogite protolith is slightly older than the recorded metamorphic and igneous activity in the Zavkhan autochthon (880–780 Ma). To constrain the timing of eclogite facies metamorphism, K–Ar geochronology was applied to white mica separates, yielding an age of 395 ± 7.9 Ma. This Devonian age is much younger than the time at which the Lake terrane and the Zavkhan terrane collided (545–525 Ma). After the collisional event, the Zavkhan terrane was subjected to extensional tectonics, resulting in alkaline volcanism until the beginning of the Silurian (440 Ma). The Urgamal eclogite records a subduction or collision event at 440–395 Ma, which has not been recognized before. This convergent tectonic environment may have been the result of collision between the Zavkhan terrane and the eastern Tarvagatai terrane. During collision, part of the Altai allochthon may have been subducted beneath the Zavkhan autochthon, thereby forming the Urgamal eclogite.