Official journal of Japan Association of Mineralogical Sciences (JAMS), focusing on mineralogical and petrological sciences and their related fields. Journal of Mineralogical and Petrological Sciences (JMPS) is indexed in the ISI database (the Thomson Corporation), the Science Citation Index-Expanded, Current Contents/Physical, Chemical & Earth Sciences, and ISI Alerting Services.
Lawsonite, pumpellyite and epidote coexist in the chlorite zone pelitic schists of the Sanbagawa metamorphic belt. As these pelitic schists contain calcite ubiquitously, the phase relations of hydrous Ca-Al silicate minerals are examined by mixed volatile equilibria in the K2O-Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O system with excess quartz, calcite and a fluid phase. XCO2 of the stability field of pumpellyite is lower than that of lawsonite. An idealized geometry of an isobaric T-XCO2 diagram including mixed volatile reactions shows that the stable hydrous Ca-Al mineral changes from lawsonite through pumpellyite to epidote for pelitic schists in the chlorite zone with progressive increase in both temperature and XCO2 of the fluid phase, though ferric iron expands the stability of epidote throughout the chlorite zone. The fluid phase released by the metamorphic reactions in the chlorite zone is high in H2O content even in the presence of CaCO3. The proposed mixed volatile reactions, instead of the pure dehydration reactions, are applicable directly to the phase relations of CaCO3-bearing Sanbagawa basic schists and to the interpretation of the mineral sequence of the hydrous Ca-Al silicates of other high P/T metamorphic belts.
Experiments on metasomatism in a system comprising huebnerite and aqueous CaCl2 solution were performed at 130, 150, and 170 °C. SEM observations revealed the formation of euhedral crystals of scheelite replacing huebnerite in all experiments. The replacement texture of huebnerite by scheelite was classified as rim replacement texture. Although the molar volume ratio of replacing mineral to replaced mineral was 1.12 for the present study, huebnerite was replaced by scheelite. This suggests that the influence of molar volume ratio is small on the metasomatism of huebnerite to scheelite, which has not been expected in previous studies. Our results also suggest that the metasomatism of huebnerite to scheelite can be achieved if the CaCl2 concentration in the solution and the reaction temperature are sufficiently high. The apparent rate constant k for huebnerite dissolution in the metasomatism is approximately 10-9-10-6 mol l-1 s-1; the activation energies are 9.6, 20.7, and 31.2 kJ/mol for CaCl2 concentrations of 0.01, 0.10, and 1.00 mol/l, respectively. The precipitation of scheelite can be the rate-determining process of the metasomatism during the initial stage when the W concentration in the solution is high.
We report the finding of sapphirine + quartz equilibrium assemblage in an orthopyroxene-sapphirine granulite from Ganguvarpatti in southern India, providing diagnostic evidence for ultrahigh-temperature (UHT) metamorphism in the central Madurai Block. The sapphirine-quartz-bearing rock is composed of coarse-grained (∼ 3 mm) porphyroblastic orthopyroxene and sapphirine which were probably stable phases during the peak UHT condition within the stability field of sapphirine + quartz. The rock also contains corona textures such as orthopyroxene + sapphirine + cordierite and orthopyroxene + cordierite symplectites formed by post-peak decompression. The porphyroblastic sapphirine contains rare inclusions of quartz. The equilibrium sapphirine + quartz reported here is regarded as a diagnostic indicator of peak metamorphism at T > 1000 °C and confirms the formation and exhumation of ultra-hot orogens during the accretionary history of the Madurai Block related to the Pacific-type orogeny and final collisional assembly of the Gondwana supercontinent in the Late Neoproterozoic-Cambrian.
A Ni-Fe alloy (Ni3.8Fe0.1Co0.1) was found in olivine-poor gabbro recovered from the Integrated Ocean Drilling Program (IODP) Hole U1309D, Atlantis Massif, 30°N on the Mid-Atlantic Ridge. Ni-Fe alloys are one of the unique minerals formed during serpentinization; however, our finding establishes that Ni-Fe alloys are also formed in the gabbroic crust possibly associated with extremely reduced magmatic fluids. The occurrence of the Ni-Fe alloy suggests that oceanic lower-crustal gabbros as well as serpentinites may be potential reservoirs of abiogenic CH4.
A petrographic and mineralogical study of Ningqiang reveals that a significant proportion of the Ca-Al-rich inclusions (CAIs) consist entirely of fine-grained porous aggregates, and melilite in them has been extensively replaced by nepheline. Small fragments of nepheline-bearing inclusions and fine nepheline grains are widely scattered in the matrix. We found evidence suggesting that those fine-grained porous inclusions were formed by replacing primary Type A CAIs and spinel-pyroxene type CAIs. The results suggest that Ningqiang has experienced extensive Na-metasomatism on its parent body. In that process, part of CAIs became fine-grained porous, and during subsequent brecciation, they were disaggregated and segregated from the original portion in the parent body. We infer that these processes may be responsible for the low abundances of CAIs and bulk refractory lithophiles that are characteristic of Ningqiang.
Effects of small volume fractions of insoluble particles on grain growth rates of polycrystalline ice were examined at 270 K. The presence of approximately 1 vol% of insoluble particles significantly inhibits the grain growth, and this inhibition effect strongly depends on the particle size. The observed grain growth behavior of ice can be reasonably interpreted on the basis of the Zener pinning effect. Our preliminary results suggest that low viscosities due to small grain sizes required for convective instability in ice shells of icy satellites are achieved by the presence of very small fractions of fine silicate dust particles.
The influence of the local stress state and crystallinity of graphite on the graphite-diamond transformation process was investigated using a laser-heated diamond anvil cell and two types of graphite samples with contrasting crystalline characteristics. The samples recovered from the experiments using highly oriented graphite (HOG) have layered structures composed of lonsdaleite and diamond arranged in a coaxial relation, indicative of their martensitic formation from graphite. The compression of HOG perpendicular to the graphite c axis results in a partial fragmentation of the original layered structure, but no clear evidence of diamond formation by the diffusion process is found even in the fragmented regions. On the other hand, the sample recovered from the experiment using mechanically milled graphite powder consists of euhedral diamond nanocrystals (size: 20-50 nm), indicating that the diamond is formed by the diffusion process. The results of the present study suggest that the graphite-diamond transformation process and the resulting microstructures are controlled mainly by the initial structure characteristics (such as crystallinity and crystallite size) of the graphite, while the influence of the local stress state in the graphite under compression is likely less significant for the formation process and the microtexture.
Hydrous peridotites from the Happo-O'ne area in central Japan, which form a serpentinite mélange in the high-P/T Renge metamorphic belt, vary from depleted lherzolites to harzburgites with subordinate dunite that contains primary chromian spinel with Cr# = 0.72 on average. Systematic variations of major and trace elements of bulk-rock compositions possibly resulted from various degrees of partial melting experienced by their protoliths (15% to <30% fractional melting). Some hydrous peridotites from the tremolite zone are representatives of the mantle peridotite facies with Ti-rich chromian spinel (up to 5.7 wt% TiO2), and these peridotites have the same compositions as other peridotites except for enrichment in Na2O, Ba, and Ti. Their bulk rocks show U-shaped PM (primitive mantle)-normalized REE (rare earth elements) patterns (0.05-0.5 times PM) and are enriched with Cs, Pb, Sr, Ba, and Rb (0.2-20 times PM), indicating metasomatism by fluids derived from the subducting slab. The hydrous peridotites contain low amounts of Zr, Hf, Ta, and Nb (HFSEs), which possibly indicate that metasomatizing melts are not involved. These results are confirmed by in-situ analysis of tremolites that show U-shaped REE patterns (0.1-3 times PM) and are highly enriched with fluid-mobile elements (B, Li, Cs, Sr, and Pb; 1-100 times PM) and Sc relative to HFSEs.
We report rare högbomite in a sapphirine + quartz bearing Mg-Al rock from the Palghat-Cauvery Suture Zone (PCSZ), the trace of the Cambrian Gondwana suture in southern India. Fine-grained (∼ 0.8 mm) högbomite occurs mantled by sapphirine and spinel with or without rutile, and the entire assemblage is included within poikiloblastic garnet. The microstructure suggests the progress of a possible prograde reaction; Grt + Hög → Spr + Spl + Rt + H2O. As the garnet hosting högbomite also includes equilibrium sapphirine + quartz, the högbomite probably underwent T > 1000 °C peak metamorphism, in which case this the first report of this mineral formed under ultrahigh-temperature conditions. On the other hand, coarse-grained (∼ 12 mm) högbomite, which is slightly richer in Fe + Mg (6.7-6.9 pfu) and poor in Al (14.5-14.7 pfu) than the former type (6.4-6.6 and 14.7-15.0 pfu, respectively), occurs in the matrix around spinel and/or sapphirine. The texture suggests retrograde formation of the latter högbomite.
Ethane- and hydrogen-bearing carbonic fluid inclusions are found within quartz in a leucocratic garnet granulite obtained from the Neoarchean Limpopo Belt in South Africa. The trapped fluids are present as secondary inclusions and show melting temperatures in the range of -60.9 to -56.9 °C, suggesting a dominantly CO2-rich composition. The wide range of estimated CO2 densities (0.611-1.020 g/cm3) indicates entrapment of the fluids at 5 kbar at 700 °C and significant subsequent density decrease due to post-peak decompression. Laser Raman analysis of the inclusions at room temperature confirmed that the trapped fluids are CO2-rich (>95 mol%) with very minor CH4 (2.5-4.6 mol%), N2 (0.1-0.4 mol%), and C2H6 (0.01-0.02 mol%) constituents. A thin carbon film precipitated on the cavity wall, not visible optically, was also identified by Raman spectroscopy. We therefore infer the following carbon-forming reaction during decompression and/or cooling below 700 °C and 5 kbar; CO2 + CH4 → 2C + 2H2O. When the inclusions were heated to 150 °C to homogenize all the fluid phases, H2 (0-0.05 mol%) and H2O (1.7-0.3 mol%) were detected instead of C2H6. Although the processes of the formation of C2H6 and H2 are not known, the presence of CH4, C2H6 and H2 in CO2-dominant fluid is considered as a possible product of a series of reactions in the C-O-H system within the inclusion cavity during decompression/cooling.