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Aluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD.

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage.

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.

AbstractRaman 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.

Indentation testing of natural single-crystal α-quartz parallel to the crystallographic c axis, using a triangular pyramidal diamond indenter at a maximum load of 500 mN, produced a very small residual volume of less than 1 μm3 in which α-quartz is highly stressed. Laser Raman microspectroscopy across the indentation at room temperature and pressure revealed a shift in the Raman bands, interpreted to reflect the residual stress field generated within the α-quartz. Based on the observed Raman shift, we identified a steep gradient in the residual non-hydrostatic stress field after complete unloading in quartz near the impression formed by indentation. At the center of the indentation, the maximum compressive stress and tensile stress were inferred to be higher than 2.2 and 0.1 GPa, respectively

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

Chromian dissakisite-(Ce), a Cr-enriched Mg analogue of allanite-(Ce), is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. Major minerals of the lherzolite are olivine [XMg= Mg/(Mg + Fe) = 0.92], pyrope-rich garnet (Prp70.1-73.2Alm11.3- 13.4Grs6.9-8.3Uvr3.6-5.1Anr0-0.4Sps0.7-1.7), orthopyroxene (XMg= 0.92-0.93), and clinopyroxene (XMg = 0.92-0.95). The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains. It is partly metamict, and there is intensive radial cracking in the surrounding clinopyroxene. Its composition is represented by the averaged formula: (Ca0.86Na0.03)0.89(REE0.86Th0.04)0.90(Mg0.98Fe2+ 0.20Fe3+0.21Cr0.36Al1.39Ti0.04)Si3.03O12(OH), with Ce2O3, MgO and Cr2O3contents up to 13.1, 7.3, and 5.4 wt%, respectively. Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and Fe2+, Fe3+and Cr can only occupy the smaller M1 site. Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt, Japan, which contains up to 0.547 Cr atoms per formula unit (apfu). Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members, “CeCa(Mg,Fe)CrAlSi3O12(OH),” for the epidote-group minerals, which may form solid solution with Ca2Al3Si3O12(OH) through coupled substitutions of REEMgCa-1Al-1, REEFeCa-1Al-1, and CrAl-1. The rare earth elements (REE), Ca, and Na total 1.79 apfu, implying that Mn and Fe2+are also present on the nine-coordinated A1 site. The high REE abundance, the presence of minor Na, and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crustderived melt/fluid at UHP conditions. Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene. Dissakisite- (REE) in subducted ultramafic rocks can transport rare earth elements into the Earth’s mantle.

Chromian dissakisite-(Ce), a Cr-enriched Mg analogue of allanite-(Ce), is found in a garnet lherzolite from the Su-Lu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. Major minerals of the lherzolite are olivine [XMg= Mg/(Mg + Fe) = 0.92], pyrope-rich garnet (Prp70.1-73.2Alm11.3- 13.4Grs6.9-8.3Uvr3.6-5.1Anr0-0.4Sps0.7-1.7), orthopyroxene (XMg= 0.92-0.93), and clinopyroxene (XMg = 0.92-0.95). The chromian dissakisite-(Ce) occurs as a single-crystal inclusion in a clinopyroxene neoblast that also contains several irregularly shaped olivine grains. It is partly metamict, and there is intensive radial cracking in the surrounding clinopyroxene. Its composition is represented by the averaged formula: (Ca0.86Na0.03)0.89(REE0.86Th0.04)0.90(Mg0.98Fe2+ 0.20Fe3+0.21Cr0.36Al1.39Ti0.04)Si3.03O12(OH), with Ce2O3, MgO and Cr2O3contents up to 13.1, 7.3, and 5.4 wt%, respectively. Because the M3 site in the chromian dissakisite-(Ce) is fully occupied by Mg and Fe2+, Fe3+and Cr can only occupy the smaller M1 site. Similar site occupancy is noted for a chromian allanite from the Sanbagawa metamorphic belt, Japan, which contains up to 0.547 Cr atoms per formula unit (apfu). Such compositional characteristics of the chromian dissakisite/allanite and existing polarized absorption data on Cr-bearing epidote suggest the existence of possible end-members, “CeCa(Mg,Fe)CrAlSi3O12(OH),” for the epidote-group minerals, which may form solid solution with Ca2Al3Si3O12(OH) through coupled substitutions of REEMgCa-1Al-1, REEFeCa-1Al-1, and CrAl-1. The rare earth elements (REE), Ca, and Na total 1.79 apfu, implying that Mn and Fe2+are also present on the nine-coordinated A1 site. The high REE abundance, the presence of minor Na, and the Mg and Cr-enriched characteristics of the mineral suggest that it was formed as a result of reaction between the host lherzolite and crustderived melt/fluid at UHP conditions. Textural relations suggest that the chromian dissakisite-(Ce) was unstable in the garnet lherzolite and reacted with olivine to form clinopyroxene. Dissakisite- (REE) in subducted ultramafic rocks can transport rare earth elements into the Earth’s mantle.

Coexisting minerals of the hollandite-cryptomelane series, piemontite-group, and abswurmbachite-braunite series occur as a small nodule in a quartzose schist (metachert) from the epidote-amphibolite facies area of the Sanbagawa metamorphic belt, central Shikoku, Japan. Stron­tium, Ba. K. and Pb contents of the hollandite-cryptomelane minerals are 0.00-0.42, 0.07-0.99, 0.00-0.75 and 0.00-0.05 apfu (atoms per formula unit: O = 16), respectively. The piemontite-group minerals can be divided into common piemontite (Ca > Sr in the ten-coordinated A2-site) and strontiopiemontite (Sr > Ca). Strontium. Ba and Pb contents of the strontiopiemontite reach 0.71, 0.26 and 0.20 apfu (O = 12.5), respectively, suggesting the existence of two possible new end- members for epidote-group minerals: “Ba-piemontite [BaCa(Mn3+, Fe3+, Al)3Si3O12(OH)]” and "Pb- piemontite [PbCa(Mn3+, Fe3+, Al)3Si3O12(OH)]”. The abswunnbachite-braunite series minerals (Cu = 0.30-0.53 apfu: O = 12) contain up to 0.34 apfu Mg, indicating the exchange Mg(Mn2+, Cu)-1. Distribution coefficients for Sr between the hollandite-cryptomelane series and the piemontite-group minerals are up to 20. implying that the hollandite-type structure with its large open tunnel is more suitable for incorporation of Sr than the A2-site of the piemontite-group minerals. A synthetic alumi­nosilicate analogue of the hollandite-cryptomelane compound is stable under P > 8-10 GPa. and suggests the hollandite-type aluminosilicate may be a reservoir of Sr and other large-ion-lithophile elements in the upper mantle.

Coexisting minerals of the hollandite-cryptomelane series, piemontite-group, and abswurmbachite-braunite series occur as a small nodule in a quartzose schist (metachert) from the epidote-amphibolite facies area of the Sanbagawa metamorphic belt, central Shikoku, Japan. Stron­tium, Ba. K. and Pb contents of the hollandite-cryptomelane minerals are 0.00-0.42, 0.07-0.99, 0.00-0.75 and 0.00-0.05 apfu (atoms per formula unit: O = 16), respectively. The piemontite-group minerals can be divided into common piemontite (Ca > Sr in the ten-coordinated A2-site) and strontiopiemontite (Sr > Ca). Strontium. Ba and Pb contents of the strontiopiemontite reach 0.71, 0.26 and 0.20 apfu (O = 12.5), respectively, suggesting the existence of two possible new end- members for epidote-group minerals: “Ba-piemontite [BaCa(Mn3+, Fe3+, Al)3Si3O12(OH)]” and "Pb- piemontite [PbCa(Mn3+, Fe3+, Al)3Si3O12(OH)]”. The abswunnbachite-braunite series minerals (Cu = 0.30-0.53 apfu: O = 12) contain up to 0.34 apfu Mg, indicating the exchange Mg(Mn2+, Cu)-1. Distribution coefficients for Sr between the hollandite-cryptomelane series and the piemontite-group minerals are up to 20. implying that the hollandite-type structure with its large open tunnel is more suitable for incorporation of Sr than the A2-site of the piemontite-group minerals. A synthetic alumi­nosilicate analogue of the hollandite-cryptomelane compound is stable under P > 8-10 GPa. and suggests the hollandite-type aluminosilicate may be a reservoir of Sr and other large-ion-lithophile elements in the upper mantle.

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation vein in metabasite, and another, a metabasite, both from Ward Creek, Cazadero area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highestgrade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are very heterogeneous and cannot be ascribed to crystallization under conditions of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz

Sodian and yttrian (Na-Y) garnet occurs in orthogneisses of the Su-Lu terrane of eastern China with major quartz, microcline, and albite and subordinate biotite, titanite, hematite, and zircon. Acmite-rich pyroxene, muscovite, epidote, allanite, and hastingsitic amphibole occur in a few of the samples. The Na- Y garnet can be divided into almandine-spessartinegrossular [Al > 1.6 per formula unit (pfu) for O = 12, hereafter denoted as Al-garnet], and andradite-spessartine (Fe3+> 1.3 pfu, hereafter denoted as Fe3+-garnet) groups. Average composition of the Al-garnet in each sample ranges from Alm35Sps13Prp1Grs49Adr2to Alm21Sps67Prp0Grs5Adr7. The Fe3+-garnet contains a calderite component (Cdr) and its composition is Alm4-7Sps10-21Prp1-2Adr59-81Cdr3-12. Na, Y, and Yb contents of the Al-rich garnet reach 0.37, 1.46, and 0.42 oxide wt%, respectively, and Na/(Y + Yb) = 0.99 ± 0.16. The Fe3+-garnet contains up to 0.11,2.1, and 0.48 wt% of Na2O, Y2O3, and Yb2O3, respectively, and its Na/(Y + Yb) value (0.17 ± 0.1) is lower than that of the Al-garnet. We conclude that Na, Y, and Yb are incorporated in the Al-rich garnet by the [8]Na[8](Y,Yb)(Ca,Mn)-2substitution, whereas in the Fe3+-garnet, Y and Yb are dominantly incorporated by the [8](Y,Yb)[4](Al,Fe3+)(Ca,Mn)-1Si-1substitution with up to 3 mol% Y + Yb for Ca + Mn. Comparison ofNa/(Y + Yb) of garnets in the orthogneisses and in some pegmatites implies that increasing pressure favors incorporation of Na with Y and Yb, and that under the albite-epidote amphibolite facies or higher-pressure conditions, Y and Yb probably substitute into the garnet structure by [8]Na[8](Y,Yb)(Ca,Mn)-2, instead of the [8](Y,Yb)[4](Al,Fe3+)(Ca,Mn)-1S-1-type substitution effective under lower-pressure conditions.

Sr-bearing zoisite and epidote are common constituents of eclogites and associated paraschists throughout the Su-Lu ultra-high pressure (UHP) province, eastern China. The SrO content of prograde zoisite and epidote reaches 3.2 wt% in crystal cores and generally decreases toward crystal margins. Retrograde epidote is poorer in SrO (<0.1 wt%). Preliminary rare earth element (REE) analyses of epidote give La2O3(up to 2.9 wt%), Ce2O3(5.9 wt%), and Nd2O3(3.0 wt%). REE contents of zoisite are distinctly lower (La2O3up to 0.16 wt%, Ce2O3up to 0.26 wt%, and Nd2O3up to 0.16 wt%) than coexisting epidote. Apatite is always more depleted in SrO (0.10-0.59 wt% on average) than coexisting zoisite and epidote, and Sr-Ca partition coefficients for zoisite and epidote and apatite [(Sr/Ca)zo/ep-ap] range from 5 to 20. SrO content of K-white mica (0.012-0.044 wt%) is an order of magnitude lower than that of apatite. An evaluation of the SrO content in zoisite and epidote and their modal abundances in seven samples indicates that >70% of the whole-rock SrO is contained in these minerals. Apatite and K-white mica are only minor reservoirs for SrO in these rocks. Zoisite and epidote are thus regarded as the most important Sr reservoirs at UHP conditions where calcic plagioclase and titanite are unstable.