Olivine is a major mineral in ultramafic and mafic rocks. Due to the higher Mg/Fe partition coefficient of olivine than the coexisting phases, the occurrences of Mg–poorer olivine grains, especially with Mg#[= Mg/(Mg + Fe²⁺)] = 0.2–0.6, are rarely reported, and thus, their spectroscopic data are limited. Fe–rich olivine phenocrysts showing compositional zoning with Mg# = ~ 0.5 (core) and ~ 0.3 (rim) and microphenocrysts with Mg# = ~ 0.4 (core) and ~ 0.2 (rim) occur in basaltic trachyandesite of Miocene Shitara volcanic rocks in central Japan. These olivine grains were investigated by Raman spectroscopy. Combining our data with the published values, we have revised the equation for Mg#–Raman spectrum relationship proposed by Mouri and Enami (2008) as follows: Mg# = 0.005446ω ² − 0.20259ω + 1.8442 (correlation coefficient r ² = 0.984), where ω is the difference between the doublet peak positions (κ₂ − κ₁).

This study presents first report of the sulfur isotopic compositions of carbonatites from the Mesoproterozoic Newania complex of India along with their stable C and O isotope ratios. The δ³⁴S_V–CDT (−1.4 to 2‰) and Δ³³S (−0.001 to −0.13‰) values of these carbonatite samples (n = 7) overlap with the S isotope compositions of Earth’s mantle. Additionally, the δ¹³C_V–PDB and δ¹⁸O_V–SMOW values of these carbonatites also show overlapping compositions to that of Earth’s mantle. Based on these mantle–like stable isotopic compositions of carbonatites along with their higher crystallization temperature (~ 600 °C) compared to a hydrothermal fluid (<250 °C), we suggest that the sulfide minerals in these carbonatites were formed under a magmatic condition. The mantle like signatures in the δ³⁴S, δ¹³C– δ¹⁸O, and ⁸⁷Sr/⁸⁶Sr values of these carbonatites rule out possible crustal contamination. Coexistence of the sulfide phase (pyrrhotite) with magnesite in these carbonatites suggests that the sulfide phase has formed early during the crystallization of carbonatite magmas under reducing conditions. Overall restricted variability in the δ³⁴S values of these samples further rules out any isotopic fractionation due to the change in the redox condition of the magma and reflect the isotopic composition of the parental melts of the Newania carbonatite complex. A compilation of δ³⁴S of carbonatites from Newania and other complexes worldwide indicates limited variability in the isotopic composition for carbonatites older than 400 Ma, which broadly overlaps with Earth’s asthenospheric mantle composition. This contrasts with the larger variability in δ³⁴S observed in carbonatites younger than 400 Ma. Such observation could suggest an overall lower oxidation state of carbonatite magmas emplaced prior to 400 Ma.

Ferriprehnite (IMA2020–057), ideal formula Ca₂Fe³⁺(AlSi₃)O₁₀(OH)₂, is a new mineral that was found as secondary mineral in druses developed in the hydrothermal altered dolerite from Kouragahana, Shimane Peninsula, Japan. Ferriprehnite is an Fe analogue of prehnite. The crystals consisting of ferriprehnite and prehnite occur as a radial aggregate. The tabular crystals are up to 300 µm long, 100 µm wide, and 50 µm thick. Ferriprehnite is colorless to pale green with white streak and vitreous luster. It has a Mohs hardness of 6½. Its cleavage is good on {100}. The calculated density is 2.97 g/cm³. The empirical formula of ferriprehnite on the basis of 10O + 2OH using the result obtained by electron microprobe analysis is Ca_1.99(Fe³⁺_0.66Al_0.34)_Σ1.00(Al_1.02Si_2.98)_Σ4.00O₁₀(OH)₂. Structure refinement converged to R1 = 4.64%. Its space group is orthorhombic Pma2 with unit–cell parameters a = 18.6149(10) Å, b = 5.4882(3) Å, c = 4.6735(3) Å, and V = 477.46(1) ų. The determined site occupancy at the octahedral M site is Fe_0.637(9)Al_0.363 indicating that the M site is predominantly occupied by Fe. <M–O> increases with increasing Fe content leading to isotropic expansion of MO₆ octahedra. The a– and c– dimensions of ferriprehnite are longer than those of prehnite due to Fe substitution for Al at the M site.

Quantitative and semi–quantitative procedures using a portable energy dispersive X–ray fluorescence spectrometer (portable XRF) were applied to geochemical studies in fault rocks, lake sediments, and soils, including paleotsunami deposits. The results obtained are as follows:

1) Correlation coefficients (r²; measured values by portable XRF versus reported values) for working curves of major elements obtained using powdered reference standard materials were 0.70–0.99 with p < 0.01, except for Mg, and r² values for the trace elements were 0.72–0.99 with p < 0.05, except for Cl, Ba, and U.

2) In the fault rocks from Central Japan (e.g., Shiraki–Nyu fault, Atera faults; powdered and nonpowdered rock fragment samples), measured values of K, Ca, Ti, Mn, Fe, Rb, Sr, Zr, and Th contents obtained using portable XRF are consistent with the reported values (r² = 0.47–0.90, with p < 0.01). Clear Fe and Mn enrichments and K and Rb depletions were observed within the gouges of the Shiraki–Nyu fault from Central Japan.

3) The depth profiles of chemical components in lake sediments (powdered) from the Noto Peninsula in Central Japan measured using portable XRF methods were similar to those measured using the stationary–type XRF (r² = 0.61 in Ti and r² = 0.40 in Zr contents, with p < 0.05).

4) For paleotsunami deposits on the Pacific coast of northeast Japan, the chemical components and titanium–normalized values (Si/Ti, Ca/Ti, Sr/Ti, and As/Ti) in powdered samples measured using portable XRF correlated well with those in a previous study measured using stationary–type XRF (r² = 0.84–0.98, with p < 0.01). Cluster analyses of geochemical data conducted using portable XRF were useful to characterize event deposits on the north Sendai Plain.

In each kind of sample, the element concentration values (SiO₂, TiO₂, Fe₂O₃, MnO, CaO, K₂O, S, V, Cr, Cu, Zn, As, Rb, Sr, Zr, Pb, and Th) obtained via quantitative analyses using portable XRF were consistent with the reported values obtained via other methods, such as stationary–type XRF and inductively coupled plasma–mass spectrometry.

The scale precipitation is a major issue at geothermal power plants as it reduces the production rate of geothermal energy. The scale precipitates at different physical and chemical conditions in each geothermal power plant as a result from a fluid–rock interaction for the specific conditions at each plant. Thus, it is important to understand the chemical characteristics and precipitation process of the scale from geothermal fluid. The information on the process of transportation of metals and its precipitation from hydrothermal fluid in general will be useful to understand the formation of hydrothermal ore deposit. In this study, we have examined the chemical characteristics of silica scaling from the Onuma geothermal power plant at Akita Prefecture, Japan. The scale consists of mainly amorphous silica and trace amounts of smectite, kaolinite, and euhedral pyrite. Chemical composition of silica scale indicates that Fe content scale shows positive correlation with Pb, Cu, and REE. These elements probably incorporate into pyrite in silica scale. The texture of pyrite suggests that pyrite is possible to crystalize prior to the growth of amorphous silica. Silica scale gradually changes its chemical composition from the production well toward the reinjection well. Concentrations of SiO₂, Fe₂O₃, MgO, and MnO in silica scale significantly decrease toward to the reinjection well from the production well, and those of Al₂O₃, LOI, and alkali and alkali earth elements (Na₂O, K₂O, and CaO) increase toward to the reinjection well. Most of trace elements including REE in silica scale also significantly decrease toward to the reinjection well, and furthermore HREE decreases more extensively than LREE though alkali and alkali earth elements (Be, Rb, Sr, Cs, and Ba) increase toward to the reinjection well. The change of element concentration in silica scales can be utilized to understand the physical and chemical conditions in the pipes at the geothermal power plant.

Oxidation states within the planetary interior are intrinsically linked with the broad scale tectonism; however, it is difficult to estimate the actual oxidation conditions. Orthopyroxene–magnetite symplectite formed by olivine oxidation may provide a significant clue into such oxidation events. Here we report detailed mineralogical and petrological synthesis of such orthopyroxene–magnetite symplectites from olivine gabbros of Oman Ophiolite (Hole GT2A, ICDP Oman Drilling Project). In order to understand how oxidation affects different olivine compositions, we employed a phase equilibria approach and computed several temperature–composition diagrams at a fixed pressure (1 kbar). Our experiments predict the coexistence of olivine with Fo_75–76 and Fo₇₁ with the orthopyroxene (En₇₉ and En₇₆), respectively, which is remarkably similar to the mineral chemistry obtained from the Oman lower crustal gabbros. From the magnetite content, we also infer that the symplectite formation may have taken place over a range of temperatures (600–1000 °C) via subsolidus olivine oxidation and/or melt (oxidizing)–olivine interaction. The latter is more probable, considering the partial occurrence of orthopyroxene and clinopyroxene rim adjacent to the symplectites.

Magnetite veins are commonly observed in serpentinized peridotite, but the mobility of iron during serpentinization is poorly understood. The completely serpentinized ultramafic rocks (originally dunite) in the Taishir Massif in the Khantaishir ophiolite, western Mongolia, contain abundant antigorite + magnetite (Atg + Mag) veins, which show an unusual distribution of Mag. The serpentinite records multi–stage serpentinization in the order: (1) Atg + lizardite (Lz) with a hourglass texture (Atg–Lz); (2) thin vein networks and thick veins of Atg; (3) chrysotile (Ctl) that cuts all earlier textures. Mg# values of the Atg–Lz (0.94–0.96) are lower than those of the Atg (~ 0.99) and chrysotile (~ 0.98). In the Atg–Lz regions, magnetite occurs as arrays of fine grains (<50 µm) around the hourglass texture, and magnetite is absent in the thin Atg vein networks replacing Atg–Lz. Magnetite occurs as coarse grains (100–250 µm) in the center of some thick Atg veins. As the volume ratio of thin Atg veins to Atg–Lz increases, both the modal abundance of Mag and the bulk iron content decrease. These features indicate that hydrogen generation occurred mainly during Atg–Lz formation, and that the Mag distribution was largely modified by dissolution and precipitation in response to the infiltration of the higher temperature fluids associated with the Atg veins. The transport of iron during redistribution of Mag in the late–stage of serpentinization is potentially important for ore deposit formation and modifying the magnetic properties of ultramafic bodies.