Acknowledgments

The Editorial Board would like to thank all the scientists who served as referees in 2020.

We present the first report of okhotskite in a lawsonite–blueschist–subfacies metachert of the Hakoishi subunit, Kurosegawa Belt, Kyushu, Japan, which was metamorphosed at peak temperatures and pressures of 200–300 °C and 0.6–0.8 GPa. This okhotskite–bearing assemblage is particularly notable because it formed at higher pressures than that of previously documented okhotskite with available pressure estimations. Textural relationships indicate that okhotskite formed during peak metamorphism in equilibrium with piemontite, Na pyroxene, magnesioriebeckite, braunite, and hematite. Okhotskite shows a significant variation in Fe:Mn ratio (Fe_tot/Mn_tot = 0.13–0.56) and a following average empirical formula; (Ca_7.62Mn²⁺_0.16)_Σ7.78(Mn²⁺_2.71Mg_1.29)_Σ4.00
(Mn³⁺_4.13Fe³⁺_2.26Al_1.36V³⁺_0.23Ti_0.02)_Σ8.00Si_11.86O_44.02(OH)_16.98. Raman spectra of okhotskite are reported for the first time and show characteristic peaks at 362, 480, and 563 cm⁻¹. The stability relationships between okhotskite and other Mn–bearing minerals, such as piemontite, sursassite, spessartine, braunite, and Mn–bearing lawsonite, are examined using a revised Schreinemakers’ analysis. The obtained petrogenetic grid provides tight constraints on the P–T relationship of natural mineral assemblages observed in Mn–bearing cherts within epidote–blueschist–grade and lawsonite–blueschist–grade. Furthermore, this petrogenetic grid predicts that the assemblage of okhotskite and Mn–bearing lawsonite should be stable at higher pressures. The higher–pressure stability suggests that highly oxidized Mn–bearing metacherts can transport water and buffer oxygen in the deeper parts of subduction zones, given that okhotskite and Mn–bearing lawsonite contain high water contents (6.9 and 11.3 wt% H₂O, respectively) and trivalent manganese.

Manguao Basalt is a Plio–Pleistocene basaltic lava flow located on the northeastern edge of Palawan Island, Philippines. The absence of active trenches surrounding the Palawan Continental Block (PCB) poses a challenge regarding the nature and origin of magmatism in the region. This study presents the petrographic and geochemical character of Manguao Basalt, as well as provides insights to the melt formation beneath the PCB. Manguao Basalt samples are olivine–phyric with minor pyroxenes, set in a plagioclase–dominated microcrystalline groundmass. Average bulk–rock major element composition of Manguao Basalt shows similarities to common olivine basalt tholeiite. Petrographic observations of the phenocrysts, however, show the unusual precedence of pyroxenes in the crystallization sequence. Calculated formation temperatures and pressures of the modal assemblage are consistent with this idea of early pyroxene formation. Simulations of mantle melting using the MELTS program show the formation of pyroxene–saturated primitive liquids. The evolution of these primitive liquids reaches similarities with Manguao Basalt composition at 1230–1260 °C. Simulations of equilibrium and fractional crystallization demonstrate the formation of olivine or orthopyroxene as the first crystals. However, the simulations done at equilibrium conditions are more consistent with the observed mineral chemistry of pyroxene phenocrysts in Manguao Basalt. Hence, maintaining the equilibrium between the source and melt is crucial for replicating the observed pyroxene chemistry. Magmatic underplating provides an excellent model for visualizing the melting and crystallization processes beneath the PCB. The model is also consistent with the narrative of other magmatic units in northern Palawan (e.g., Capoas Granite). The significant findings of this study contribute to the understanding of the tectonic evolution of the PCB.

The Tonaru epidote–amphibolite is one of the large metagabbro dominated bodies and occurs in schistose rocks of the Sanbagawa metamorphic belt, central Shikoku. This body locally retains mineral parageneses of eclogite facies equilibrium prior to the epidote–amphibolite facies stage. The lithologic boundary between the epidote–amphibolite and the surrounding schistose rocks was well observed along the Kokuryo River in the western part of the Besshi region in central Shikoku. Boundary zone of 1.5–2.5 m wide is developed between the epidote–amphibolite and pelitic schist. This zone is composed of a basic layer and alternating layers consisting of thin amphibole–rich and mica–rich bands, which occupy the epidote–amphibolite and pelitic schist sides, respectively. The basic layer has a chondrite–normalized rare–earth element (REE) pattern with slight enrichment of light REEs, which corresponds to epidote–amphibolite. By contrast, the amphibole–rich band has a flat REE pattern similar to the basic schist of the Sanbagawa belt.

 The basic layer in the boundary zone and epidote–amphibolite have composite–zoned garnet, showing a compositional discontinuity between the core and mantle parts, similar to that in the Sanbagawa eclogite unit. Garnet in the amphibole–rich and mica–rich bands of the alternating layers and pelitic schist shows simple normal zoning, which commonly occurs in the Sanbagawa non–eclogite unit. Sodic plagioclase occurs as inclusions in the mantle part of the composite–zoned garnet and normally zoned garnet as well as in a matrix phase. These lithologies belong to the oligoclase–biotite zone with equilibrium pressure/temperature conditions of 1.1–1.2 GPa/595–625 °C; discontinuity of metamorphic grade is not detected throughout the outcrop for the epidote–amphibolite facies stage.

 These data suggest that (1) the basic layer is the fractured part of the epidote–amphibolite and (2) the tectonic boundary between the eclogite and non–eclogite units corresponds to the lithologic boundary between the basic layer and alternating layers of thin amphibole–rich and mica–rich bands in the boundary zone.

CO₂ distribution in M¹² and M¹⁴ cages of CO₂–rich melanophlogite from Fortunillo, Tuscany, Italy was studied using synchrotron powder X–ray diffraction. Original and two heat–treated samples at 500 and 1000 °C were studied at room temperature. The diffraction patterns of these samples can be indexed as a cubic cell (Pm3n). For the non–heated sample, CO₂ occupancy for M¹⁴ cage is close to unity, whereas about 0.85 for M¹² cage. For the 500 °C–heated sample, the occupancies for M¹⁴ and M¹² cages are reduced to 0.79 and 0.57, respectively. Present study showed that CO₂ has preference to M¹⁴ cage, but substantial CO₂ occupies M¹² cage. The electron distributions obtained by MEM analysis clearly reveal orientationally ordered CO₂ distribution in M¹² and M¹⁴ cages. Present result is also used to clarify a recently proposed interpretation for the splitting of CO₂ Raman vibrational peak for the heat–treated melanophlogite.

Fedorovskite was found as aggregates in crystalline limestone associated with gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Fedorovskite occurs as gray to dark gray aggregates of anhedral translucent crystals up to 0.8 mm across in association with shimazakiite, uralborite, vimsite, cuspidine, fluorite, and calcite. An electron microprobe analysis of fedorovskite gave an empirical formula Ca_2.013(Mg_1.901Fe_0.072Mn_0.023Zn_0.002Co_0.001Ni_0.001)_Σ2.000(B_3.852Si_0.104)
_Σ3.956O_7.000(OH_5.421F_0.579)_Σ6.000 based on O = 7 and OH + F = 6. The mineral is orthorhombic, and the unit cell parameters refined from X–ray diffraction are a = 8.915(7), b = 13.086(16), c = 8.295(9) Å, and V = 967.7(18) ų. The calculated density is 2.692 g cm⁻³. The fedorovskite from the Fuka mine was probably formed as a secondary mineral from calcium borates such as shimazakiite in a reaction with magnesium–bearing late hydrothermal solution.

A mixture of metavivianite, vivianite, and spheniscidite was obtained from an aqueous solution of FeSO₄·7H₂O and (NH₄)₂HPO₄ at temperatures between 50 and 60 °C. The oxidation sequence of partially oxidized vivianite toward santabarbaraite or a solid solution between β–Fe₂(PO₄)O and Fe₄(PO₄)₃(OH)₃, without passing through metavivianite, was also demonstrated.

Index to Journal of Mineralogical and Petrological Sciences Vol. 115 No. 1-6

Author Index

Keyword Index

Journal of Mineralogical and Petrological Sciences

Volume 115, Number 1-6

February-December, 2020