Tadao Nishiyama

Laser Raman microspectrometry of metamorphic quartz: A simple method for comparison of metamorphic pressures

Abstract A Laser Raman microspectrometry method was applied to metamorphic quartz in quartz-eclogite-, epidote-amphibolite-, and amphibolite-facies rocks to assess the quantitative correlation between the Raman frequency shift and metamorphic pressure. Quartz crystals sealed in garnet and other phases have a higher frequency shift than those in the matrix. Furthermore, the quartz inclusions show a frequency shift specific to the individual host crystals in eclogites (garnet ≈ kyanite > omphacite ≈ epidote). These observations imply that the residual pressures retained by quartz inclusions depend on elastic parameters of the host crystals, as discussed by previous researchers. The Raman frequency shift of quartz inclusions in garnet systematically increases with increasing peak metamorphic pressures from the amphibolite facies (0.30-0.55 GPa/470-570°C), through the epidote-amphibolite facies (0.8-1.1 GPa/470-635°C) to the quartz-eclogite facies (2.1-2.5 GPa/660-710°C). Calibrations based on experimental work suggest that the measured Raman frequency shifts signify residual pressures of 0.1-0.2, 0.4-0.6, and 0.8-1.0 GPa for these three groups of metamorphic rocks, respectively. Normal stresses (internal pressures) of quartz inclusions in garnet, numerically simulated with an elastic model, and inferred pressure-temperature conditions at peak metamorphic stage are compatible with the residual pressures estimated from the frequency shifts. Laser Raman microspectroscopic analysis of quartz is a simple and effective method for (1) comparison of pressure conditions in metamorphic rocks formed under various pressure-temperature conditions, and (2) detection of a higher-pressure signature in metamorphic rocks extensively recrystallized during the subsequent exhumation and hydration stage

Hf isotope and REE compositions of zircon from jadeitite (Tone, Japan and north of the Motagua fault, Guatemala): implications on jadeitite genesis and possible protoliths

Zircon separates from one jadeitite sample (JJ) from Tone, Japan and one from Guatemala (GJ) were studied for mineral inclusions, age dating, trace-element determination and Hf isotope analysis. These zircons can be categorized into two types. Type I (igneous) zircons are characterized by the presence of mineral inclusions, among others K-feldspar, which is not present in jadeitite matrix. They also show higher Th/U ratios, larger Ce anomalies and higher 176 Lu/ 177 Hf ratios. Type II (metasomatic/solution-precipitate) zircons contain omphacite/jadeite inclusions and exhibit lower Th/U ratios, smaller Ce anomalies and lower 176 Lu/ 177 Hf ratios. Both types of zircons display high eHf( t ) values, slightly lower than the depleted mantle evolution line. The JJ sample contains both type I and II zircons. SHRIMP and geochemical data indicate that this jadeitite sample was formed through the mechanism of whole-sale metasomatic replacement at ~80 Ma from an igneous protolith of juvenile origin with an age of 136 ± 2 Ma. The GJ sample contains only type II zircons and may have formed through a mechanism of, or close to, vein precipitation at 98 ± 2 Ma. The two samples therefore testify that both mechanisms may have been in operation during jadeitite formation. Based on Hf isotope composition of type I zircons and the back-calculated REE pattern of the presumed protolith, the geochemical characteristics of the protolith of the Tone jadeitite were shown to be similar to those of oceanic plagiogranites derived from partial melting of cumulate gabbros or subduction-zone adakitic granites originated from partial melting of subducted oceanic crust. The latter, however, is a more probable candidate because the former is known to be poor in K 2 O, which, in contrast, is a notable chemical component in Tone jadeitite. On the basis of the available data, it is also suggested that the protolith, the physicochemical conditions and the extent of jadeitization may all play a role in dictating the chemical variations of jadeitites.

Evaluation of residual pressure in an inclusion–host system using negative frequency shift of quartz Raman spectra

Abstract Raman spectra of quartz inclusions in garnet hosts of low-pressure/temperature metamorphic rocks from the Yanai district in the Ryoke belt (around 0.1-0.3 GPa/500-600 °C), Southwest Japan, exhibit frequency (peak position) shifts toward lower wavenumbers as compared to those of a quartz standard measured at ambient conditions. The observed negative frequency shifts indicate that tensile normal stress is exerted on the quartz-garnet boundary and therefore, quartz inclusions are subjected to negative residual pressure. Elastic modeling that assumed the constant elastic properties of minerals cannot explain this negative residual pressure. This study estimated the residual pressure based on a new scheme of elastic modeling with equation of state (EOS) of quartz and garnet, which takes into account the pressure- and temperature-dependency of compressibility and expansivity. The calculated residual pressure was converted into frequency shifts of quartz Raman spectrum based on the experimentally determined relation. The results showed that the quartz inclusions in garnets retain residual pressure of about -0.3 GPa, and logically reproduced the observed frequency shifts in the direction of lower wavenumbers. The new elastic modeling also simulates positive frequency shifts retained by quartz inclusions in garnets of high-pressure metamorphic rocks from the Sambagawa metamorphic belt in Southwest Japan, and from the Motagua fault zone in Guatemala. The degree and direction of Raman frequency shifts of quartz inclusion in garnet depend on metamorphic conditions when the quartz was included in the host garnet. Conversely, the metamorphic conditions prevailing when a set of a quartz inclusion and garnet host was recrystallized can be inferred from Raman frequency shifts of quartz inclusion in garnet. The proposed Raman spectroscopic analysis should be a powerful and useful tool to decipher information at earlier stage of garnet growth even in samples of highly recrystallized matrix phases during exhumation and retrograde stages.

Jadeitites with metasomatic zoning from the Nishisonogi metamorphic rocks, western Japan: fluid–tectonic block interaction during exhumation

Petrography, isocon analysis and singular value decomposition analysis of jadeitites and metasomatic rocks surrounding them from the Nishisonogi metamorphic rocks revealed that the metasomatic rocks developed outside the stability field of jadeite + quartz after jadeitites had been included by a serpentinite melange in a subduction channel possibly during exhumation. The jadeitites occur at two localities, Tone and Mie in Nagasaki City, from a serpentinite melange in the Nishisonogi metamorphic rocks. The jadeitite mineral assemblage is jadeite/omphacite + paragonite + phlogopite + albite + clinozoisite/epidote ± muscovite ± analcime. Jadeite core with fine-grained quartz inclusions and the inclusion-free rim is partially replaced by albite. These jadeitites are surrounded by metasomatic zones of albitites and/or a muscovite rock. The mineral assemblage of the albitites is albite + clinozoisite/epidote ± muscovite ± omphacite ± phlogopite ± amphibole ± chlorite, and that of the muscovite rock is muscovite + clinozoisite + chlorite. Because these zones have no high-pressure minerals, they represent products in a P – T regime outside the stability field of jadeite + quartz. Isocon analyses between the jadeitites and the metasomatic zones reveal that K 2 O, H 2 O, Sr and Ba were added to jadeitite and SiO 2 , Na 2 O and Fe 2 O 3 were removed. REE-rich veinlets emanate from clinozoisite grains, suggesting REE mobility during the fluid–jadeitite interactions. The metasomatic zones developed by interaction between jadeitites and serpentinite via K-, Sr- and Ba-rich fluid, during exhumation from the stability field of jadeite + quartz through that of albite to that of analcime.

Coexistence of pyroxenes jadeite, omphacite, and diopside/hedenbergite in an albite-omphacite rock from a serpentinite mélange in the Kurosegawa Zone of Central Kyushu, Japan

Abstract An albite-omphacite rock, containing the three sodic pyroxenes of the jadeite-omphacite-diopside/ hedenbergite ternary system, has been investigated in detail to better characterize the phase relationships in this system. The rock is from a serpentinite mélange in the Kurosegawa Zone of Central Kyushu, Japan, and exhibits an omphacite overgrowth over diopside/hedenbergite and the further overgrowth of jadeite over omphacite, indicative of the successive replacement of pyroxenes during metamorphism. Partial replacement of omphacite by diopside/hedenbergite at the margins of K-feldspar veins suggests that all three pyroxenes were stable locally at the same stage of metamorphism. The peak temperature and pressure conditions are roughly estimated to be 350 °C and 5-10.8 kbar. Partitioning of Fe2+-Mg between pyroxene pairs is consistent with a recent thermodynamic model, indicating the attainment of local equilibrium for pyroxene pairs. The observed compositional gap is also consistent with the miscibility gap reported in many previous studies. The compositional field of omphacite in the ternary jadeite-aegirine-diopside/hedenbergite diagram extends from the jadeite-diopside/hedenbergite line toward the aegirine-rich field, maintaining approximately constant the jadeite content, resulting in an asymmetric phase diagram.

Interplay of irreversible reactions and deformation: a case of hydrofracturing in the rodingite–serpentinite system

The interplay of irreversible reactions and deformation during regional metamorphism was analyzed in the reaction zones between rodingite and serpentinite. Rodingites are leucocratic rocks found commonly in serpentinites and are considered metasomatic products of Ca-rich fluid. Rodingites occur ubiquitously in serpentinite from the Nomo metamorphic rocks, western Kyushu, a Cretaceous accretionary complex of greenschist to epidote–amphibolite facies condition. We used the singular value decomposition (SVD) method to analyze rodingitization reactions based on mineral compositions of a rodingite sample and a possible protolith (clinopyroxene gabbro). The resultant reaction implied that the rodingite was formed due to the addition of considerable amounts of CaO and H2O to the protolith, whereas other components such as SiO2, AF (Al2O3 + Fe2O3), and FM (FeO + MgO) were conserved. The Nomo rodingites are associated with reaction zones between serpentinite. A careful study on the compositional variations of diopside and chlorite in the rodingite, serpentinite, and the reaction zones between them showed a disequilibrium crystallization of diopside in the reaction zone. Diopside shows a serrated variation in composition across the reaction zone together with disequilibrium signature within grains (composite grain consisting of Fe-rich and Fe-poor parts). These features clearly indicate that diopside crystallized in a relatively short period compared to the metamorphism by irreversible reactions. Another conspicuous feature is that perovskite occurs in some reaction zones, whereas titanite is common in rodingites; this indicates that the reaction zone is poorer in SiO2. The tremolite veins are typically derived from the reaction zone into serpentinite; showing this is a good example of hydrofracturing associated with the formation of the reaction zone. All the reactions obtained by the SVD method that contribute to the formation of the reaction zones consumed CaO and evolved H2O, strongly suggesting that the hydrofracturing is caused by a rapid increment of fluid pressure during the progression of the reactions. The reactions consumed considerable amounts of SiO2 in the reaction zone, which is consistent with the occurrence of perovskite, and the conservation of AF and FM components.

Hydrothermal chloritization processes from biotite in the Toki granite, Central Japan: Temporal variations of of the compositions of hydrothermal fluids associated with chloritization

Abstract This paper describes the biotite chloritization process with a focus on mass transfer in the Toki granitic pluton, Central Japan, and also depicts the temporal variations in chemical characteristics of hydrothermal fluid associated with chloritization during the sub-solidus cooling of the pluton. Singular value decomposition (SVD) analysis results in chloritization reaction equations for eight mineral assemblages, leading to the quantitative assessment of mass transfer between the reactant and product minerals, and inflow and outflow of components through the hydrothermal fluid. The matrices for SVD analysis consist of arbitrary combinations of molar volume and closure component in the reactant and product minerals. The eight reactions represent the temporal variations of chemical characteristics of the hydrothermal fluid associated with chloritization: the progress of chloritization results in gradual increase of silicon, potassium, and chlorine and gradual decrease of calcium and sodium in the hydrothermal fluid with temperature decrease. The biotite chloritization involves two essential formation mechanisms: chlorite formation (CF) mechanism 1, small volume decrease from biotite to chlorite and large inflow of metallic ions such as Al3+, Fe2+, Mn2+, and Mg2+ from the hydrothermal fluid, and CF mechanism 2, large volume decrease and large outflow of the metallic ions into hydrothermal fluid. Chlorite produced with CF mechanism 1 dominates over that of CF mechanism 2, resulting in the gradual decrease of the metallic components in the hydrothermal fluid with chloritization progress. The chloritization reactions also give the temporal variations in physicochemical parameter of the hydrothermal fluid: a gradual decrease of pH and a gradual increase of redox potential in the hydrothermal fluid as chloritization proceeds. The combination of continuous reactions based on compositional variations in chlorite together with corresponding continuous AlIV variations gives an indication of the temporal variations in rates of decreasing and increasing concentration of chemical components in the hydrothermal fluid associated with chloritization. The biotite chloritization and resultant temporal variations of chemical and physicochemical characteristics in hydrothermal fluid act as a trigger for the successive dissolution-precipitation process of a granitic rock.

Fluid-metapelite interaction in an ultramafic mélange: implications for mass transfer along the slab-mantle interface in subduction zones

The slab-mantle interface in subduction zones is a site of tectonic mixing of crustal and mantle rocks. It is the interface for fluid flow of slab-derived components into the mantle wedge. To assess the fluid-rock interaction along the slab-mantle interface, we studied the bleaching of pelitic schist in an ultramafic mélange. The Nishisonogi metamorphic rocks in Kyushu, Japan, comprise ultramafic mélanges intercalated with epidote-blueschist facies schists. The ultramafic mélange consists of tectonic blocks of various lithologies and a matrix of chlorite-actinolite schist and serpentinite. Along the contact with the mélange matrix, pelitic schist blocks are bleached mainly due to the modal increase of albite and the consumption of carbonaceous material. The bleaching is probably attributed to infiltration of Na-rich external fluid from the mélange matrix. Mass balance analysis indicates losses of C, Rb, K2O, Ba, Pb, and SiO2 from the bleached pelitic schist, although Al2O3, TiO2, Sc, Y, Zr, Nb, La, Ce, and Nd remain immobile. This suggests fractionation of large-ion lithophile elements (LILE) and Pb from the high-field-strength elements and rare earth elements during the bleaching. If this ultramafic mélange is analogous to the slab-mantle interface, similar infiltration metasomatism will promote liberation of C, Si, LILE, and Pb from subducting metapelites and enhance metasomatism of the mantle wedge.

Retrograde strontium metasomatism in serpentinite mélange of the Kurosegawa Zone in central Kyushu, Japan

Abstract Strontium-rich epidote, including epidote-(Sr) and epidote with major amounts of Sr (i.e. epidote containing up to 17.3 wt.% SrO), was found in pumpellyite schist and epidote blueschist in a tectonic block in the serpentinite mélange of the Kurosegawa Zone, central Kyushu, Japan. The tectonic block is 20 m wide and made primarily of lawsonite blueschist, with subordinate amounts of pumpellyite schist and epidote blueschist. The pumpellyite schist typically occurs at the edge of the block and is composed mainly of pumpellyite with subordinate amounts of strontium-poor epidote, albite and chlorite, and thin veins of fine-grained calcite and clinopyroxene. Epidote-(Sr) forms rims around strontium-poor epidote, fills fractures in strontium-poor epidote and also occurs interstitially between pumpellyite aggregates and along the boundaries between pumpellyite and calcite-clinopyroxene veins. The epidote blueschist is found between the pumpellyite schist and lawsonite blueschist, and consists mainly of sodic amphibole, epidote and titanite, with albite veining. Strontium-rich epidote occurs as rims, replacing Sr-poor epidote near the albite vein. The bulk strontium contents of the rocks are as follows: lawsonite blueschist (200 ppm), epidote blueschist (2800 ppm) and pumpellyite schist (~10,700 ppm). The chemical and petrological characteristics of the Sr-rich epidote-bearing metabasites suggest that the infiltration of a metamorphic fluid promoted extensive Sr metasomatism during the later stages of high-pressure metamorphism.

Origin of omphacitites from the Nishisonogi metamorphic rocks, western Kyushu, Japan: Comparison with jadeitites

Two types of omphacitites are distinguished in ultramafic melanges that are intercalated with epidote–blueschist facies schists of the Nishisonogi metamorphic rocks in western Kyushu, Japan. One is an omphacitite layer in a metabasite block, and the other is an omphacitite lens in an albitite block. These omphacitites are principally composed of omphacite with minor amounts of winchite/magnesiohornblende, epidote, albite and titanite. Although the petrographic features of both types are similar, the omphacitite layer is depleted in light rare earth elements (LREE; La, Ce and Nd) compared with the omphacitite lens. In the case of jadeitites, those interpreted as being formed by direct precipitation from aqueous fluids are LREE-depleted, while those interpreted as being formed by metasomatic replacement from protoliths are LREE-enriched. Thus, by analogy, the omphacitite layer may have directly precipitated from aqueous fluids, and the omphacitite lens may be metasomatic in origin. The omphacitite layer has amphibole-rich selvages, which are likely formed by the reaction between the omphacitite-forming fluid and the host metabasite. Mass-balance analysis using the isocon method indicates that the formation of the selvages involves additions of large ion lithophile elements (LILE; Rb, K and Ba), Na and Si to the host metabasite. These components were likely introduced via fluid infiltration. Although the source of the omphacitite-forming fluid is not clear, the LILE-rich composition suggests participation of fluids derived from metapelitic rocks.