Amphibole Zoning Research Articles

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Amphibole record of the 1964 plinian and following dome-forming eruptions of Shiveluch volcano, Kamchatka

Shiveluch is one of the most active explosive volcanoes worldwide. During the last рlinian eruption in 1964 and the following (1980-current time) dome-forming eruptions Shiveluch has produced andesites and dacites (SiO2 ~ 60–64 wt%) containing variably zoned, compositionally and texturally diverse amphibole phenocrysts. In this work, we attempt to decode the complex zoning of the amphibole crystals in the 55-year series of pumice, dome rocks and mafic enclaves in order to reconstruct the most recent evolution of the volcano plumbing system.The amphibole zoning in Shiveluch andesites reveals correlation with the style and date of eruption. High-Al cores mantled by low-Al rims in amphiboles from the 1964 plinian eruption record a drastic decrease of pressure and rapid magma ascent from the lower crust to the shallow magma chamber. Typically unzoned and often opacitized low-Al crystals from the early dome-building episodes in 1980–1981 and 1993–1995 reflect magma crystallization in the shallow magma chamber. Complexly zoned amphiboles from andesites erupted in 2000s indicate replenishment of the shallow magma chamber with mafic magma and syn-eruptive mixing processes. Amphibole-based barometric calculations obtained by different approaches indicate that the Shiveluch plumbing system is complex and comprises two, mafic and silicic magma storage zones at ~15–20 km and ~ 5–6 km depths. We suggest that both episodes of the plinian eruption in 1964 and the extensive dome growth in 2001–2016 were driven by influx of mafic magma in the shallow storage zone beneath Shiveluch. The mafic replenishment likely preceded the 1964 plinian eruption and repeatedly occurred during the period of extensive dome growth in 2001–2016. The variable styles of the recent Shiveluch eruptions may be controlled by the relative volume of the mafic recharges and their thermal and viscosity effects on the efficiency of magma mixing.

Biotite in olivine gabbros from Atlantis Bank: Evidence for amphibolite-facies metasomatic alteration of the lower oceanic crust

Gabbroic rocks recovered from deep holes drilled in the ocean floor provide us with valuable information about in-situ alteration processes of the lower oceanic crust. We found that the occurrence of biotite is widespread in gabbroic rocks recently drilled from IODP Hole U1473A at Atlantis Bank, near the Southwest Indian Ridge. Biotite is rare in oceanic gabbros, thus we analyzed textural and compositional details of biotite and associated minerals to better understand the conditions governing their formation.In olivine gabbros from Hole U1473A and from nearby ODP Hole 735B, biotite occurs mainly in coronitic aggregates mantling olivine. It also forms monomineralic veins or occurs in biotite-chlorite-amphibole veins within plagioclase grains that contact the coronitic aggregates. The coronitic aggregates typically have an outer biotite-dominated zone and an inner zone mostly made up of Al-poor calcic amphibole. The biotite and the calcic amphibole zones frequently include Al-rich calcic amphibole (± cummingtonite) and talc, respectively. Plagioclase in direct contact with Al-rich calcic amphibole has 65–90 mol% anorthite. The coronitic aggregates also frequently have an outermost zone composed of submicron-scale biotite-chlorite mixtures, which show intermediate optical and chemical characteristics between biotite and chlorite, and a composite pattern of Raman-shift spectra. Most biotite-rich coronitic aggregates occur in proximity to felsic veins or to biotite or alkali feldspar microveins branched from felsic veins, whereas most biotite-chlorite coronas are connected to biotite-, chlorite- or amphibole-bearing microveins. Chlorite coronas around olivine, though rare in Atlantis Bank gabbros, occur in contact with chlorite-bearing microveins and show no relationship with felsic veins. Based on the plagioclase-amphibole equilibrium, we evaluated temperature of 750–850 °C for the formation of the biotite-rich coronitic aggregates.From the modes of occurrence, compositions of minerals, and thermodynamic modeling, we conclude that the biotite coronas formed at higher temperatures and higher SiO2 and/or K+/H+ activities than chlorite coronas typically found in olivine gabbros from other mid-ocean ridge localities. The coronitic biotite-chlorite mixtures formed in response to lower SiO2 and/or K+/H+ activities, and possibly lower temperature, than the biotite coronas. Such a difference in physical and chemical conditions for corona formation probably reflects the distance from felsic vein/microvein or is related to the relative timing of reactions. The high-temperature metasomatic alteration of lower oceanic crustal gabbros shown in this study is most likely characteristic of oceanic core complexes from ultraslow-spreading ridges.

Transition from dehydration to hydration during exhumation of the Sanbagawa metamorphic belt, Japan, revealed by the continuous P–T path recorded in garnet and amphibole zoning

To investigate the water budget in deep accretionary prisms, we undertook a detailed petrologic analysis of garnet-bearing amphibolites in the subduction-origin high-pressure Sanbagawa metamorphic belt, Japan. We obtained a continuous P–T path of exhumation by the application of differential thermodynamics to compositional zoning in both garnet and amphibole. The P–T path is characterized by three stages of decompression: (1) heating decompression from 1.5 GPa and 550 °C to 1.1 GPa and 600 °C; (2) isothermal decompression toward 0.8 GPa and 580 °C; and (3) cooling decompression to 0.3 GPa and 400 °C. Comparison with the results of numerical modeling of the thermal structures of subduction zones suggests that the peak-pressure condition is warmer than the slab surface temperature in a typical subduction zone, but is consistent with subduction of a young slab during the Sanbagawa metamorphism. The heating decompression just after the peak-pressure condition is explained by heating from the mantle wedge during ascent of the metamorphic body in the accretionary prism; heating stops when the metamorphic body reaches crustal depths. Mass balance calculations for the mineral reactions in an amphibolite reveal that the dehydration associated with the growth of Ca-poor garnet takes place during heating decompression, whereas hydration takes place during cooling decompression by the breakdown of garnet and pargasite to form chlorite and actinolite. Such a transition from dehydration to hydration in the exhuming metabasite body in an accretionary prism generates fluids between the continental Moho and the slab surface and may explain the seismic activity above the subduction plate boundary, such as supra-slab earthquakes.

Mineral chemistry of high-Mg diorites and skarn in the Han-Xing Iron deposits of South Taihang Mountains, China: Constraints on mineralization process

The Han-Xing region is located in the south Taihang Mountains (TM) in the central part of the North China Craton, and is an important iron producing area. The iron deposits in this region are of skarn type, related to an Early Cretaceous high-Mg diorite complex, including gabbro diorite, hornblende diorite, diorite, diorite porphyrite, and monzonite. In this study we report the detailed mineral chemistry of the high-Mg diorites and skarn rocks. The olivine in the gabbro diorite shows chemical composition similar to that in mantle peridotite xenoliths. Clinopyroxene in the gabbro diorite is dominantly augite, with only minor diopside, whereas the clinopyroxenes in the diorite and monzonite are diopside. Amphiboles in the high-Mg diorites show compositional range from magnesiohornblende to magnesiohastingsite, with minor pargasite and tschermakite. Most plagioclase in the high-Mg diorite is andesine and oligoclase. The magnesio-biotite in gabbro diorites shows chemical characteristics of re-equilibrated primary biotites and those in calc-alkaline rocks. In the diorite and diorite porphyrite, plagioclase shows complex chemical zoning. Clinopyroxene and garnet in skarn rocks show varying FeO contents, the former containing low FeO (<9wt.%) and occurring as the major skarn mineral in large-scale iron deposits, and the latter within small-scale iron deposits with high FeO (mostly >25wt.%) content. We computed the pressure, temperature, oxygen fugacity and water contents based on the mineral chemistry of amphibole and biotite. Based on the results, the magma crystallization can be divided into two stages, one within the deep magma chamber, forming clinopyroxene, amphibole and plagioclase phenocrysts; the other after emplacement, forming the rim of phenocrysts and matrix minerals. The magma during the early stage shows high temperature (~900°C–950°C), pressure (~300MPa–500MPa), relatively high logfO2 (NNO–NNO +2), and H2O content in melt (4%–8%). During the late stage, the magma temperature dropped to about 750°C, and pressure came down to less than 100MPa, with the logfO2 rising to NNO +1–NNO +2.The zoning of amphibole and plagioclase records the process of magma mixing and crystallization, with injection of mafic magma into the felsic magma chamber. The relatively high logfO2 and H2O content inhibited partitioning of iron into mafic minerals and favored concentration of Fe in the melt. Iron ore precipitation occurred when the magma was emplaced at shallow level, and was principally controlled by the chemical composition of carbonate wall rocks. The high logfO2, Fe3+ rich ore-forming fluid generated andradite and clinopyroxene when it reacted with limestone and dolomitic limestone respectively.

Spinel group minerals in metamorphosed ultramafic rocks from Río de Las Tunas belt, Central Andes, Argentina

In the Rio de Las Tunas belt, Central Andes of Argentina, spinel group minerals occur in metaperidotites and in reaction zones developed at the boundary between metaperidotite bodies and their country-rocks. They comprise two types: i) Reddish-brown crystals with compositional zonation characterized by a ferritchromite core surrounded by an inner rim of Cr-magnetite and an outer rim of almost pure magnetite. ii) Green crystals chemically homogeneous with spinel (s.s.) and/or pleonaste compositions. The mineral paragenesis Fo+Srp+Cln+Tr+Fe-Chr and Fo+Cln+Tr+Tlc±Ath+Fe-Chr observed in the samples indicate lower and middle grade amphibolite facies metamorphic conditions. Nonetheless, the paragenesis (green)Spl+En+Fo±Di indicates that granulite facies conditions were also reached at a few localities. Cr-magnetite and magnetite rims in zoned reddish-brown crystals and magnetite rims around green-spinel/pleonaste grains are attributed to a later serpentinization process during retrograde metamorphism. The chemical composition of spinel group minerals in the ultramafic reaction zones is determined by the mineral zone where they crystallize. Green pleonaste occurs in the chlorite zone, ferritchromite predominates in the amphibole zone, whereas Crmagnetite and magnetite are more common in the carbonate zone. The mineral paragenesis of the Rio de Las Tunas metaperidotites together with the chemical characteristics of the spinel group minerals support a clockwise P-T path evolution for the ultramafic protoliths during the Paleozoic regional metamorphic cycle of this area

Repeated recharge, assimilation, and hybridization in magmas erupted from El Chichón as recorded by plagioclase and amphibole phenocrysts

Compositional and isotopic zoning patterns in plagioclase and amphibole phenocrysts from El Chichón record multiple cycles of country rock assimilation, magma injection, hybridization, and mixing. Laser ablation ICP-MS and electron microprobe analyses of plagioclase crystals from 7 eruptions spanning 3100 years reveal four types of zoning. These compositional and isotopic zones are often associated with textural changes observed in the crystals in thin section (e.g. sieved or patchy regions). Amphiboles are frequently zoned in Al and Si, and, in two magmas, may have clinopyroxene rims. Interestingly, most plagioclase show multiple and repeated zoning patterns. Moreover, all magmas contain all zoning patterns and textures, and crystals with substantially different sequences of zones occur within mm of one another. The most reasonable explanation for the origin of these textures is a frequently recharged chamber. Plagioclase zones with increasing anorthite contents ( An) and decreasing 87Sr/ 86Sr ratios record injection by a hotter, possibly wetter, and more primitive magma (lower 87Sr/ 86Sr ratio). Zones with decreasing An and increasing 87Sr/ 86Sr ratios record assimilation of country rock and/or hybridization of the host and injected magmas; injection of hot magma may provide the heat for country rock assimilation. Changes in An without corresponding changes in 87Sr/ 86Sr ratio likely record slight variations in pressure or temperature during crystallization, or the far-field thermal effects of magma injection. Variations in 87Sr/ 86Sr ratio unaccompanied by Anzoning record assimilation of country rock. Amphibole zoning patterns also record periodic heating events; amphibole with clinopyroxene rims record episodes where the magma was heated beyond the amphibole stability field. Bulk compositional homogeneity and the juxtaposition of many crystals with disparate zoning patterns in single pumice require the magmatic system to be well mixed. Strontium diffusion rates indicate that the plagioclase zoning patterns cannot have persisted at magmatic pressures and temperatures for more than ~ 500 years, thus cycles of injection and assimilation occur on timescales equal to or shorter than the eruption recurrence interval. Long-term compositional and isotopic homogeneity indicate that there is a balance between recharge, assimilation, and crystallization.

Na and K distribution in agpaitic pegmatites

Composition and zoning of amphibole in agpaitic pegmatites of the 1.16 Ga Ilímaussaq complex, South Greenland record the chemical evolution of the final stages of an already extremely fractionated melt. Our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites, albeit with some significant modifications: the dominating exchange mechanism of Na + Si ⇔ Ca + Al in the amphiboles of the magmatic stage changes to K + Si ⇔ Ca + Al and K ⇔ Na in some pegmatitic samples. Na/K ratios in amphiboles, which generally increase in the course of the Ilímaussaq fractionation, partly display a reversal during the crystallization of the most differentiated amphiboles. The alkali trends are probably related to the buffering of Na +and K +activity by the co-crystallization of albite and microcline. This buffering favors Na +in cooling fluids. This mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization, e.g. due to locally elevated H 2O activity. Analcime does not incorporate significant amounts of K and accordingly, amphibole incorporates more K in analcime-bearing assemblages. The Na–K variation in amphiboles in the Ilímaussaq pegmatites allow a detailed view into the late-stage evolutionary trends of a textbook agpaitic complex. The transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from Na to K. Only in the very latest stage, virtually K-free mineral assemblages in analcime–aegirine veins support the existence of a Na-dominated aqueous fluid.

Interaction between basic and acid magmas during the latest stages of the post-collisional Variscan evolution: Clues from the gabbro–granite association of Ota (Corsica–Sardinia batholith)

The intrusive association of Ota (post-Variscan Corsica–Sardinia batholith) consists of mafic rocks and granites displaying mingling relations. The mafic rocks are olivine gabbros to quartz-diorites, and the granites are subsolvus and characterised by biotite as the only mafic phase. Minor amounts of amphibole-bearing granodiorites to granites develop along the contacts between mafic rocks and granites. The rather high initial ε Nd (+ 4.5) of the most primitive mafic sample indicates that parental basic melts had an extensive contribution from a depleted mantle source. In the mafic rocks, the decrease of initial ε Nd (to + 2.1) is associated with an increase of Th and U concentrations in clinopyroxene, thus indicating that the Ota basic melts underwent a process of crustal contamination. Associated biotite granites have low initial ε Nd (+ 0.4) and high Th and U concentrations. The chemical evolution of Ota basic melts most likely occurred through fractional crystallisation and concomitant contamination with the acid magma that gave rise to associated biotite granites. The incompatible element signature of late-stage anhedral amphibole from variably evolved mafic rocks is similar, thus suggesting that the whole Ota mafic sequence recorded the contamination process, possibly by percolation within the gabbroic crystal mush of a melt contaminated by the acid magma. Hybridisation between basic and acid magmas is also documented by contact amphibole-bearing granitoids. In particular, major and trace element zoning of amphibole from granodiorites reveals that such a hybridisation was associated with reactions between crystals from the basic magma (amphibole and clinopyroxene) and the acid melt. The occurrence of chemically homogeneous amphiboles in associated granites suggests that “contact” hybridisation comprised also a process of gradient-induced chemical diffusion.

Progress of actinolite‐forming reactions in mafic schists during retrograde metamorphism: an example from the Sanbagawa metamorphic belt in central Shikoku, Japan

AbstractHydration reactions are direct evidence of fluid–rock interaction during regional metamorphism. In this study, hydration reactions to produce retrograde actinolite in mafic schists are investigated to evaluate the controlling factors on the reaction progress. Mafic schists in the Sanbagawa belt contain amphibole coexisting with epidote, chlorite, plagioclase and quartz. Amphibole typically shows two types of compositional zoning from core to rim: barroisite → hornblende → actinolite in the high‐grade zone, and winchite → actinolite in the low‐grade zone. Both types indicate that amphibole grew during the exhumation stage of the metamorphic belt. Microstructures of amphibole zoning and mass‐balance relations suggest that: (1) the actinolite‐forming reactions proceeded at the expense of the preexisting amphibole; and (2) the breakdown reaction of hornblende consumed more H2O fluid than that of winchite, when one mole of preexisting amphibole was reacted. Reaction progress is indicated by the volume fraction of actinolite to total amphibole, Yact, with the following details: (1) reaction proceeded homogeneously in each mafic layer; (2) the extent of the hornblende breakdown reaction is commonly low (Yact &lt; 0.5), but it increases drastically in the high‐grade part of the garnet zone (Yact &gt; 0.7); and (3) the extent of the winchite breakdown reaction is commonly high (Yact &gt; 0.7). Many microcracks are observed within hornblende, and the extent of hornblende breakdown reaction is correlated with the size reduction of the hornblende core. Brittle fracturing of hornblende may have enhanced retrograde reaction progress by increasing of influx of H2O and the surface area of hornblende. In contrast to high‐grade rocks, the winchite breakdown reaction is well advanced in the low‐grade rocks, where reaction progress is not associated with brittle fracturing of winchite. The high extent of the reaction in the low‐grade rocks may be due to small size of winchite before the reaction.

Amphibole zoning, a record of progressive oxidation during crystallization of mafic microgranular enclaves in the Kurobegawa Granitic Pluton

Sulfide, oxide, and amphibole in MME (mafic microgranular enclaves) in the young Kurobegawa Granitic Pluton (2 Ma) indicate that their textural and mineralogical features are characteristic of progressively oxidized magnetite-series granitoids. Hydrous mafic magma intruded into the chamber from the bottom and fragmented to form MME, subsequently vesiculated and floated through the chamber. The MME generally have sharp fine chilled margins and show no in situ mixing between the MME and the host granite. Amphibole, an early phase together with sulfides and plagioclase, began to crystallize just after the mafic magma intruded into the chamber bottom (10 km in depth) and then continued to crystallize as the MME floated. Amphibole shows strongly patchy zoning from brownish green pargasitic core to pale green magnesio-hornblende rim, with a systematic decreasing in Fe2+/(Fe2++Mg) with AlIV decreasing toward the rim. Early sulfide precipitation followed by late -stage magnetite formation and the simultaneous decreasing in Fe2+ during amphibole crystallization were resulted from progressive oxidation of iron in minerals. Transformation of sulfur species from SO2 to H2S probably oxidized iron in the minerals in this hydrous magma on cooling, as suggested in previous models for large sulfur supplies and magma oxidation in subduction related volcanisms.

Intermediate-P/T type Archean metamorphism of the Isua supracrustal belt: Implications for secular change of geothermal gradients at subduction zones and for Archean plate tectonics

The Isua supracrustal belt (ISB) rocks are dated at about 3.8 Ga and constitute the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of over 1,500 rock samples in ISB enabled us to estimate the extent of regional metamorphism, petrotectonic environment and subduction- zone geothermal gradient in the Archean. The following lines of evidence indicate the first discovery of progressive, prograde metamorphism from greenschist (Zone A) through Ab-Ep-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeast part of the Isua supracrustal belt: (1) systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) absence of any vestige of high-grade metamorphism even in the lowest metamorphic zone. Geology and chronological constraints of ISB indicate that the regional metamorphism was related to the subduction of Archean lithosphere. Metamorphic pressures and temperatures of the metamorphism are estimated to be 5 to 7 kbar from Grt-Hbl-Pl-Qz geobarometry and 380° to 550°C from the Grt-Bt geothermometry in Zones B to D. These P-T estimates indicate that ISB was affected by progressive metamorphism of an intermediate P/T ratio metamorphic facies series, and that it records a much higher geothermal gradient of a subduction zone in the Archean than is known from the Phanerozoic. The high geothermal gradient may have resulted from the subduction of young lithosphere and a high potential temperature of mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin oceanic plate, creating many huge granitic (tonalite, trondhjemite, and granodiorite) batholiths. The slab melting changed the oceanic crust (density = 3.07) into a denser Grt-bearing residue (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics. In addition to the preservation of the oldest accretionary complex, this suggests that Precambrian-type plate tectonics, whose driving force is slab-pull due to densification of the residue of oceanic crust as a consequence of slab melting, was already operating in the Early Archean. The transition from Precambrian-type to Phanerozoic-type plate tectonics may be caused by thinning of oceanic crust and thickening of oceanic lithosphere in the late Archean, due to decrease of mantle temperature.

Regional variation in exhumation and strain rate of the high‐pressure Sambagawa metamorphic rocks in central Shikoku, south‐west Japan

AbstractRegional variation in the P–T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P–T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P–T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P–T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low‐grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio ≥ 4.0) than in the northern part (aspect ratio&lt; 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate.

Application of differential thermodynamics (Gibbs' method) to amphibole zonings in the metabasic system

Sodic, calcic and subcalcic amphiboles are expressed by the combination of tremolite, edenite, tschermakite, glaucophane, magesioriebeckite and Fe-tremolite. Because such complex compositions of amphiboles provide enough chemical variables to constrain equilibrium conditions of the metabasic system, amphibole composition makes it possible for Gibbs' method to be applied to this system. We made a framework for estimating quantitative P–T paths from compositions of a zoned amphibole in metabasite consisting of amphibole, chlorite, epidote, plagioclase, quartz and water in the system Na2O–CaO–MgO–FeO–Fe2O3–Al2O3–SiO2–H2O. For deriving total derivatives simply, mole fractions of cations in each site were introduced as the variables of amphibole composition. In obtaining non-ideality terms of amphibole activity, we used the regular solution models with 15 Margules parameters derived from published data. Application of the differential thermodynamics to a natural basic schist from the Sanbagawa metamorphic belt in central Shikoku provides both prograde and retrograde paths. The ranges of calculated P–T conditions are in good agreement with the previous study, and calculated compositions of coexisting minerals are consistent with the values from analyses of minerals in the sample. Activity models for amphiboles, especially non-ideality of tremolite–tschermakite (tr–ts) and tremolite–edenite (tr–ed) joins, provide significant effects on the calculated P–T paths. The initial P–T conditions given at the amphibole rim affect the calculated P–T paths slightly, but the shapes and lengths of the paths almost do not change. If compositional changes in each step of the amphibole zoning are small enough, Gibbs' method is able to transform the change of amphibole chemistry into changes of P–T conditions that are independent of the histories of the compositional changes of amphibole. This implies that all amphibole chemistry can be translated into the specific P–T condition, considering the relative compositional change from the reference amphibole, as long as the same mineral assemblage is preserved.

Archean Regional Metamorphism of the Isua Supracrustal Belt, Southern West Greenland: Implications for a Driving Force for Archean Plate Tectonics

The Isua supracrustal belt (∼3.8 Ga) constitutes the oldest accretionary complex in the world. Petrochemical and geothermobarometric studies of more than 1500 rock samples of the Isua belt have enabled us to estimate the extent of regional metamorphism, the petrotectonic environment, and the subduction-zone geothermal gradient in the Archean. The following line of evidence indicates progressive, prograde metamorphism from greenschist (Zone A) through albite-epidote-amphibolite (Zone B) to amphibolite facies (Zones C and D) in the northeastern part of the Isua supracrustal belt: (1) the systematic change of mineral paragenesis in metabasites and metapelites; (2) progressive change of the composition of major metamorphic minerals, including plagioclase, amphibole, chlorite, epidote, and garnet; (3) normal zoning of amphibole and garnet; and (4) the absence of relict minerals of high-grade amphibolitic metamorphism even in the lowest metamorphic zone. Metabasites of the Isua belt vary extremely in Mg#, causing a complex mineral paragenesis throughout the area. For example, a high FeO content of metabasites expands the stability field of hornblende to both lower and higher grades. The compositional and mineralogical characteristics above also indicate that the Isua supracrustal belt underwent a single regional metamorphic event, involving minor contact metamorphism and mylonitization; however, weak ocean-floor metamorphism and low-grade regional metamorphism during accretion cannot be ruled out. Metamorphic pressures and temperatures are estimated to be 5–7 kbar from garnet-hornblende-plagioclase-quartz geobarometry and 380–550°C from garnet-biotite geothermometry in Zones B to D. These P-T estimates indicate an intermediate P/T ratio metamorphic facies series. Geological investigations and chronological constraints of the Isua metamorphic belt indicate that the regional metamorphism was related to the subduction of Archean lithosphere, and records a geothermal gradient for the Archean subduction zone that is much higher than geotherms for Phanerozoic subduction zones. The high geothermal gradient may have resulted from the young age of subducted lithosphere and high potential temperature of the mantle. The Archean high geothermal gradient led to melting of thick oceanic crust in a thin, descending oceanic plate, creating many huge granitic (tonalite-trondhjemite-granodiorite [TTG]) batholiths. Slab melting changed the oceanic crust (density = 3.07) to denser garnet-bearing assemblages (density = 3.55), implying that TTG melt extraction provided a potential driving force for Archean plate tectonics.

Simple Models of CO Release from Metacarbonates with Implications for Interpretation of Directions and Magnitudes of Fluid Flow in the Deep Crust

Simple one-dimensional models of coupled advection–hydrodynamic dispersion–reaction are used to investigate processes of CO2 release from metacarbonate beds during deep crustal (~8 kbar) Acadian prograde metamorphism in New England, USA. Two broad models in which reaction progress is controlled by gradients in H2O–CO2 fluid composition between different rock types are presented. In the first, diffusional exchange of volatiles across lithologic contacts is significant. CO2 generated during prograde temperature (T) rise is transported away from metacarbonate layers to surrounding (1) metapelitic layers which generate H2O by dehydration and/or (2) flow conduits (e.g. permeable layers or fractures) for externally derived, elevated XH2O/XCO2 fluids. H2O is transported from the surroundings into the metacarbonate layers and drives further mineral reaction. In the second model, reaction in metacarbonate layers is driven mostly by layer-parallel flow of external fluids with elevated XH2O/XCO2 derived from, for example, dehydrating schists or outgassing magmas. For both models, the XCO2 of the fluid within metacarbonate layers is generally predicted to increase with increasing grade from the Ankerite–Oligoclase to the Amphibole zones, and then decrease in the Diopside zone—key relationships that are commonly observed in the field. The slow reaction progress in metacarbonates driven by progressive dehydration of surrounding metapelite from greenschist to amphibolite facies probably requires time scales of fluid–rock interaction comparable to the duration of the Acadian orogeny (~106−107 my). Intense episodes of fluid flow through conduits such as fractures may produce veins and alteration selvages over fluid–rock interaction times as short as 103–104 years. Model results emphasize that the accuracy of field-based fluid flux estimates depends critically on correct identification of mass transport processes. The modeling suggests that reactive transport of volatiles between metacarbonate layers and their lithologically heterogeneous surroundings can account for basic T–fluid composition–reaction progress relationships observed in much of the Acadian orogen of New England. The results provide an alternative to up-T flow scenarios that account for these relationships by large, pervasive, horizontal fluid fluxes up regional T gradients.

A17 雲仙岳 1991-1995 年噴出物中の角閃石斑晶の累帯構造 : 特に塩素の挙動とマグマの脱ガスについて

A17 雲仙岳 1991-1995 年噴出物中の角閃石斑晶の累帯構造 : 特に塩素の挙動とマグマの脱ガスについて

Cl incorporation into successively zoned amphiboles from the Ramnes cauldron, Norway

Amphibole from alkali granites in the Ramnes cauldron in the Oslo rift was altered hydrothermally by corrosion and growth through multiple events of fluid circulation. The alteration developed successive zones of amphibole at the crystal margins, resulting in a (1) brownish ferro-edenitic hornblende core (FE), (2) deep bluish-green hastingsitic hornblende zone (HH), and (3) light-greenish Fe-rich actinolite rim (FA). The edenitic core preserves the original igneous amphibole composition. The Cl content of amphibole strongly increases from the FE (0.78‐0.82 wt%) to the HH zone (2.07‐2.96 wt%) and abruptly decreases in the FA rim (0.01‐0.36 wt%). In the Cl-rich HH amphibole zone, amphibole has characteristically high Cl content [2.96 wt%, 0.82 atoms per formula unit (apfu)] and high concentrations in both [4] Al (1.73 apfu) and A-site occupancy (0.86 apfu) with a large K/(Na 1 K) value of 0.47. Both [4] Al and A-site occupancy increase systematically with positive correlation with Cl content throughout the three amphibole zones. On the other hand, Fe 21 content is not so simply correlated to the Cl content. Based on crystal structure considerations on Cl-rich amphiboles, the cation substitutions are illustrated by structural (geometrical) constraints for [4] Al and by a chemical constraint for Fe 21 . These contributions for Cl incorporation

Ca-Al silicates in low-grade metavolcanic rocks from southern Tanzawa Mountains, Central Japan.

The metavolcanics of the Miocene Tanzawa Group of the southern Tanzawa Mountains can clearly be divided into six mineral zones ; stilbite, laumontite, prehnite-pumpellyite, epidote, epidote-amphibole and amphibole zones. Four phases of mafic phyllosilicates in the five lower grade zones are identified by systematic X-ray diffraction studies; (1) smectite in the stilbite and laumontite zones, (2) randomly mixed-layer chlorite/smectite mineral which is ubiquitous in the stilbite, laumontite and prehnite-pumpellyite zones, (3) regularly mixed-layer chlorite/smectite in the stilbite zone, (4) chlorite occurs first in the laumontite zone. With increasing metamorphic grade, smectite layers progressively transform to chlorite. Chlorite is the predominant phyllosilicate in the prehnite-pumpellyite zone and is a unique phyllosilicate in the epidote and epidote-amphibole zones. Variations in Fe/(Fe+Mg) of chlorite are positively correlated with Fe/(Fe+Mg) ratios of whole rocks, but the other chemical variations are clearly unrelated to whole rock compositions. The content of Si decreases with increasing metamorphic grade.

Amphibole zoning in the Garland Peak Syenite, Red Hill complex, New Hampshire: camptonitic parental magmas and differentiation to silica-oversaturated syenites

The Garland Peak Syenite (GPS) of the Red Hill complex, New Hampshire, consists predominantly of amphibole, oligoclase, perthite, and quartz; amphiboles have homogeneous kaersutite cores with strongly zoned rims ranging in composition from pargasite to hastingsite to hornblende. The thin section scale association of kaersutite, an amphibole that typically crystallizes in silica-undersaturated magmas, with quartz suggests that the GPS magma experienced substanital changes in magmatic composition, including silica activity, during its crystallization history. Kaersutite-bearing camptonites are also associated with the Red Hill complex. The camptonite amphiboles are very similar in composition to the core kaersutites in the GPS, suggesting that the earliest GPS liquid may have had camptonitic affinities. In order to elucidate the process where-by silica-undersaturated magmas differentiate to saturation, amphiboles in these rocks were analyzed by electron and ion microprobe techniques. Amphiboles show a progressive increase in REE abundances from the camptonites to the GPS kaersutite cores to the GPS pargasite/hastingsite/hornblende rims. The systematic change in REE concentrations, and the variations in V, Ti, Sr versus Zr, Eu/Eu0 and La/Yb versus Ce, suggest a possible differentiation relationship for the amphiboles and imply that the GPS was derived from magmas similar to camptonites. Rimward depletions in Sr, Ti, V, and Eu/Eu0, and the increase in La/Yb values suggest that parental camptonites fractionated plagioclase, magnetite, and amphibole to produce the silica-oversaturated GPS. Bulk-rock modelling agrees with the trace element record preserved in the amphiboles, that plagioclase, magnetite, and amphibole fractionation caused silica saturation. Minor pegmatitic patches occur in the GPS. Ferrohornblendes in the pegmatites have REE abundances distinct from the other GPS amphiboles, and this difference may be due to open system processes.