The potential role of fluids during regional granulite-facies dehydration in the lower crust

High-grade dehydration of amphibolite-facies rocks to granulite-facies is a process that can involve partial melting, fluid-aided solid-state dehydration, or varying degrees of both. On the localized meter scale, solid-state dehydration, due to CO2-rich fluids traveling along some fissure or crack and subsequently outwards along the mineral grain boundaries of the surrounding rock, normally is the means by which the breakdown of biotite and amphibole to orthopyroxene and clinopyroxene occur. Various mineral textures and changes in mineral chemistry seen in these rocks are also seen in more regional orthopyroxene-clinopyroxene-bearing rocks which, along with accompanying amphibolite-facies rocks, form traverses of lower crust. This suggests that solid-state dehydration during high-grade metamorphism could occur on a more regional scale. The more prominent of these fluid-induced textures in the granulite-facies portion of the traverse take the form of micro-veins of K-feldspar along quartz grain boundaries and the formation of monazite inclusions in fluorapatite. The fluids believed responsible take the form of concentrated NaCl- and KCl- brines from a basement ultramafic magma heat source traveling upwards along grain boundaries. Additional experimental work involving CaSO4 dissolution in NaCl-brines, coupled with natural observation of oxide and sulfide mineral associations in granulite-facies rocks, have demonstrated the possibility that NaCl-brines, with a CaSO4 component, could impose the oxygen fugacity on these rocks as opposed to the oxygen fugacity being inherent in their protoliths. These results, taken together, lend credence to the idea that regional chemical modification of the lower crust is an evolutionary process controlled by fluids migrating upwards from the lithospheric mantle along grain boundaries into and through the lower crust where they both modify the rock and are modified by it. Their presence allows for rapid mass and heat transport and subsequent mineral genesis and mineral re-equilibration in the rocks through which they pass

The Bamble Sector, South Norway: A review

The Proterozoic Bamble Sector, South Norway, is one of the world's classic amphibolite- to granulite-facies transition zones. It is characterized by a well-developed isograd sequence, with isolated ‘granulite-facies islands’ in the amphibolite-facies portion of the transition zone. The area is notable for the discovery of CO2-dominated fluid inclusions in the granulite-facies rocks by Jacques Touret in the late 1960's, which triggered discussion of the role of carbonic fluids during granulite genesis. The aim of this review is to provide an overview of the current state of knowledge of the Bamble Sector, with an emphasis on the Arendal-Froland-Nelaug-Tvedestrand area and off shore islands (most prominantly Tromøy and Hisøy) where the transition zone is best developed. After a brief overview of the history of geological research and mining in the area, aspects of sedimentary, metamorphic and magmatic petrology of the Bamble Sector are discussed, including the role of fluids. Issues relevant to current geotectonic models for SW Scandinavia, directly related to the Bamble Sector, are discussed at the end of the review

The Varberg-Torpa Charnockite-Granite Association, SW Sweden: Mineralogy, Petrology, and Fluid Inclusion Chemistry

The Varberg-Torpa charnockite-granite association (Varberg, SW Sweden) consists of the magmatic Varberg charnockite (1399 +/- 6 Ma) and the Torpa granite (1380 +/- 12 Ma). The Torpa granite is both continuous and, based on its whole-rock geochemistry, synmagmatic with the Varberg charnockite. The granite body also contains a number of charnockite inliers. P-T estimation using garnet-clinopyroxene and orthopyroxene-clinopyroxene Fe-Mg exchange thermometry and garnet-orthopyroxene-plagioclase-quartz barometry gives temperatures and pressures (750-850 degrees C; 800-850 MPa) that most probably approximate the P-T conditions during emplacement of the charnockite compared with a lower crystallization temperature (650-700 degrees C) for the granite. The earliest recognized fluid inclusions in both the granite and charnockite consist of H2O-CO2 mixtures (H2O volume fraction 0 center dot 2-0 center dot 7). Fluid inclusions in the charnockite are characterized by high CO2 densities (up to 1 center dot 0 g cm(-3); 40-90% bulk CO2), of probable magmatic origin, and are best preserved in garnet, plagioclase, and fluorapatite (in order of decreasing CO2 densities), and sometimes also in clinopyroxene. Fluid inclusions with the highest CO2 densities (1 center dot 08-1 center dot 10 g cm(-3)) are found in quartz (T-h -31 to -36 degrees C) and may have originated under high P-T conditions during emplacement and cooling of the charnockite. Magmatic fluids in the granite correspond to aqueous-carbonic inclusions with an estimated bulk composition (mol %) of H2O 73%, CO2 25%, NaCl 2%. The salinity of the solutes in the granite (typically 14-20 wt % NaCl-eq.) is generally higher than for the charnockite (0-8 wt % NaCl-eq.). Field, petrographic, mineralogical, geochemical, and fluid inclusion evidence indicates that, compared with the H2O-rich granite, the magma responsible for the charnockite had a preponderance of CO2 over H2O, which lowered the H2O activity in the melt, stabilizing ortho- and clinopyroxene. This evidence also supports the idea that the granite and charnockite were derived from a common source magma (most probably a fluid-rich basalt at the base of the crust) as a result of fractional crystallization

Localized, solid-state dehydration associated with the Varberg charnockite intrusion, SW Sweden

The mineralogy, petrology, and fluid inclusion chemistry of two charnockite patches within a distance of 4-5 km of the Varberg magmatic charnockite intrusion, SW Sweden, are investigated and described utilizing SEM, EMPA, and fluid inclusion microthermometry. Garnet-clinopyroxene (890-930 degrees C), garnet-amphibole (600-800 degrees C), and garnet-biotite (670-860 degrees C) Fe-Mg exchange thermometry indicates high temperatures for charnockite Patch I compared to relatively lower garnet-orthopyroxene, garnet-amphibole, and garnet-biotite temperatures of 500 to 600 degrees C for charnockite Patch II. Plagioclase in the charnockitic patches tends to be more anorthitic and less albitic (X-An = 0.20, X-Ab = 0.76) than in the surrounding regional granitic gneiss (X-An = 0.13, X-Ab = 0.84). Replacement antiperthite is commonly found in unrelated plagioclase grains from either patch compared to the regional granitic gneiss where it is relatively rare. In either patch, K-feldspar is considerably less albitic (X-Kfs = 0.90-0.92, X-Ab = 0.05-0.10) compared to K-feldspar from the regional granitic gneiss. It can also be found as micro-veins along quartz grain rims. Both patches are dominated by clinopyroxene as opposed to orthopyroxene. Garnet, biotite, and amphibole and in both charnockite patches tend to have lower Fe and correspondingly higher Mg values compared with garnet, biotite, and amphibole from the surrounding regional granitic gneiss. Fluorapatite tends to be relatively enriched in Cl and depleted in (Y+REE) compared with fluorapatite from the regional granitic gneiss. Fluid inclusions in charnockite Patches I and II are dominantly carbonic similar to what is seen for the Varberg charnockite. In addition to quartz, relatively high-density carbonic inclusions are also preserved in garnet and in fluorapatite. It is presumed that pure carbonic fluids must have once coexisted with relic magmatic H2O-CO2-NaCl fluids at peak metamorphic conditions. The most likely scenario suggests that charnockite Patches I and II were formed during the later stages of crystallization of the Varberg charnockite magmatic body during which copious amounts of CO2-rich fluids with a brine (CaCl2-dominated) component were expelled into the country rock via pegmatoid segregations both within and in the immediate surroundings of the charnockite body. Patch I appears to represent the extension of a pegmatoid segregation, whereas Patch II appears to represent fluid-induced lower temperature, solid-state dehydration. Transport was facilitated via a system of tectonic fissures and fractures generated in the regional migmatized granitic gneiss during its emplacement. Within the scope of what is known, these two charnockite patches fall into the generally observed parameters for localized dehydration zones in general. (C) 2014 Elsevier B.V. All rights reserved

Baddeleyite formation in zircon by Ca-bearing fluids in silica-saturated systems in nature and experiment : resetting of the U–Pb geochronometer

Intergrowths of baddeleyite have been found in zircon grain interiors from amphibolite- and granulite-facies felsic rocks from southwest Greenland. The zircon grains are either close to or in direct contact with quartz. A series of experiments has been conducted using natural, unaltered zircon grains ± SiO2 in H2O–CaCl2, and H2O–Ca(OH)2 solutions with varying molar proportions of Ca to Si at 900 °C; 1000 MPa and 600 °C; 400 MPa for 4–50 days. Experimental results indicate that baddeleyite formed in the reacted zircon if the molar amount of Ca was close to or greater than Si in the system. The baddeleyite primarily takes the form of bead-like trails along the reaction front between the altered and unaltered zircon. Uranium, Th, and Y + REE were detected in both the newly formed baddeleyite and in the altered zircon, while Pb was effectively absent in both phases. Formation of baddeleyite from zircon in the silica-saturated rocks only appears to be possible when Ca saturates the system, such that the Si is tied up as CaSiO3 lowering the silica activity to < 1. This highly localized (µm to nm scale) effect in natural quartz-bearing rocks, where baddeleyite forms in the interiors of zircon grains in contact with quartz, implies that metastability in natural rock-forming systems can occur on a very small scale. Non-incorporation of Pb in the newly formed baddeleyite, or in areas of the zircon altered by fluids, implies that either could be used to date the metasomatic event responsible for their formation

Ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western Maine, USA

Ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic-inorganic interactions during diagenesis as well as of the devolatilization history and fluid/rock interaction during metamorphism. In this study, a collection of geochemically well-characterized biotite separates from a series of graphite-bearing Paleozoic greenschist- to upper amphibolite-facies metapelites, western Maine, USA, were analyzed for ammonium nitrogen (NH4+-N) contents and isotopic composition (δ15NNH4) using the HF-digestion distillation technique followed by the EA-IRMS technique. Biotite separates, sampled from 9 individual metamorphic zones, contain 3000 to 100ppm NH4+-N with a wide range in δ15N from +1.6‰ to +9.1‰ Average NH4+-N contents in biotite show a distinct decrease from about 2750ppm for the lowest metamorphic grade (∼500°C) down to 218ppm for the highest metamorphic grade (∼685°C). Decreasing abundances in NH4+ are inversely correlated in a linear fashion with increasing K+ in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect. Despite expected increasing δ15NNH4 values in biotite with nitrogen loss, a significant decrease from the Garnet Zones to the Staurolite Zones was found, followed by an increase to the Sillimanite Zones. This pattern for δ15NNH4 values in biotite inversely correlates with Mg/(Mg+Fe) ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between NH4+-NH3 or NH4+-N2, reflecting devolatilization history and redox conditions during metamorphism. © 2010 Elsevier Ltd

Experimental constraints on the relative stabilities of the two systems monazite-(Ce) - allanite-(Ce) - fluorapatite and xenotime-(Y) - (Y,HREE)-rich epidote - (Y,HREE)-rich fluorapatite, in high Ca and Na-Ca environments under P-T conditions of 200-1000 MPa and 450-750 A degrees C

The relative stabilities of phases within the two systems monazite-(Ce) - fluorapatite - allanite-(Ce) and xenotime-(Y) - (Y,HREE)-rich fluorapatite - (Y,HREE)-rich epidote have been tested experimentally as a function of pressure and temperature in systems roughly replicating granitic to pelitic composition with high and moderate bulk CaO/Na2O ratios over a wide range of P-T conditions from 200 to 1000 MPa and 450 to 750 A degrees C via four sets of experiments. These included (1) monazite-(Ce), labradorite, sanidine, biotite, muscovite, SiO2, CaF2, and 2 M Ca(OH)(2); (2) monazite-(Ce), albite, sanidine, biotite, muscovite, SiO2, CaF2, Na2Si2O5, and H2O; (3) xenotime-(Y), labradorite, sanidine, biotite, muscovite, garnet, SiO2, CaF2, and 2 M Ca(OH)(2); and (4) xenotime-(Y), albite, sanidine, biotite, muscovite, garnet, SiO2, CaF2, Na2Si2O5, and H2O. Monazite-(Ce) breakdown was documented in experimental sets (1) and (2). In experimental set (1), the Ca high activity (estimated bulk CaO/Na2O ratio of 13.3) promoted the formation of REE-rich epidote, allanite-(Ce), REE-rich fluorapatite, and fluorcalciobritholite at the expense of monazite-(Ce). In contrast, a bulk CaO/Na2O ratio of similar to 1.0 in runs in set (2) prevented the formation of REE-rich epidote and allanite-(Ce). The reacted monazite-(Ce) was partially replaced by REE-rich fluorapatite-fluorcalciobritholite in all runs, REE-rich steacyite in experiments at 450 A degrees C, 200-1000 MPa, and 550 A degrees C, 200-600 MPa, and minor cheralite in runs at 650-750 A degrees C, 200-1000 MPa. The experimental results support previous natural observations and thermodynamic modeling of phase equilibria, which demonstrate that an increased CaO bulk content expands the stability field of allanite-(Ce) relative to monazite-(Ce) at higher temperatures indicating that the relative stabilities of monazite-(Ce) and allanite-(Ce) depend on the bulk CaO/Na2O ratio. The experiments also provide new insights into the re-equilibration of monazite-(Ce) via fluid-aided coupled dissolution-reprecipitation, which affects the Th-U-Pb system in runs at 450 A degrees C, 200-1000 MPa, and 550 A degrees C, 200-600 MPa. A lack of compositional alteration in the Th, U, and Pb in monazite-(Ce) at 550 A degrees C, 800-1000 MPa, and in experiments at 650-750 A degrees C, 200-1000 MPa indicates the limited influence of fluid-mediated alteration on volume diffusion under high P-T conditions. Experimental sets (3) and (4) resulted in xenotime-(Y) breakdown and partial replacement by (Y,REE)-rich fluorapatite to Y-rich fluorcalciobritholite. Additionally, (Y,HREE)-rich epidote formed at the expense of xenotime-(Y) in three runs with 2 M Ca(OH)(2) fluid, at 550 A degrees C, 800 MPa; 650 A degrees C, 800 MPa; and 650 A degrees C, 1000 MPa similar to the experiments involving monazite-(Ce). These results confirm that replacement of xenotime-(Y) by (Y,HREE)-rich epidote is induced by a high Ca bulk content with a high CaO/Na2O ratio. These experiments demonstrate also that the relative stabilities of xenotime-(Y) and (Y,HREE)-rich epidote are strongly controlled by pressure