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

Thermochemistry of minerals in the system silver sulfide-copper(I)sulfide-antimony trisulfide-arsenic trisulfide

In this study phase relations in the Ag\sb2S-Cu\sb2S-Sb\sb2S\sb3-As\sb2S\sb3 system are investigated via a series of Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) at temperatures ranging from 75 to 350\sp\circC. Phases investigated include polybasite-pearceite (Ag,Cu)\sb{16}(Sb,AS)\sb2S\sb{11}, pyrargyrite-proustite (Ag,Cu)\sb3(Sb,As)S\sb3, miargyrite-smithite (Ag,Cu)(Sb,As)S\sb2, skinnerite (Cu,Ag)\sb3SbS\sb3, chalcostibite CuSbS\sb2, sinnerite Cu \sb6As\sb4S\sb9 and Ag\sb2S-Cu\sb2S sulfide phases stable above 120\sp\circC. As-Sb unmixing in polybasite-pearceite is experimentally documented at 75\sp\circC. Utilizing this Ag-Cu exchange data, As-Sb unmixing data, relevant crystallographic data (both actual and inferred), and calorimetric data for Ag\sb2S and Cu\sb2S, internally consistent Ag-Cu and As-Sb solution models are calibrated for polybasite-pearceite, pyrargyrite, proustite, skinnerite, miargyrite, body centered cubic-(Ag,Cu)\sb2S, face centered cubic-(Ag,Cu)\sb2S, and hexagonal close packed-(Cu,Ag)\sb2S. All ordering is assumed to be nonconvergent. The Gibbs energy of formation for Cu\sb{16}Sb\sb2S\sb{11} polybasite (5.3 $\pm$ 1.14 kJ/gfw) and for Ag\sb{16}Sb\sb2S\sb{11} polybasite ($-$31.1 $\pm$ 0.7 kJ/gfw) from the simple sulfides is estimated using these solution models and previously estimated Gibbs energies of formation for skinnerite and pyrargyrite

Titaniferous accessory minerals in very low-grade metamorphic rocks, Keweenaw Peninsula Michigan, USA

Titanite, TiO(2), and pseudorutile are associated with primary igneous Fe-oxide grains in basalt and rhyolite clasts from the Keweenaw Peninsula, Michigan, USA which were metamorphosed to the prehnite-pumpellyite facies. Pseudorutile occurs with titanite + TiO(2) in broad lamellae within titaniferous magnetites It also occurs as intergrowths with titanite and Fe-oxide in embayments within primary oxide grains and in composite Fe-oxide-titanite grains that appear to represent nearly complete replacement of original Ti-bearing Fe-oxides Thin {111} lamellae in titaniferous magnetite grains contain Marine. titanite + TiO(2) and titanite + TiO(2) + Fe-oxide TiO(2) also occurs by itself in networks of closely spaced small elongated lenses in Fe-Ti oxides Concentrations of CaAlSiO(4)F + CaAlSiO(4)(OH) in marine range from 0 to 30% The large variation in marine compositions suggests that equilibrium was not achieved except on a very local scale a conclusion also supported by local variations in the assemblages of Ti-bearing secondary minerals. Textures and mineral assemblages indicate that ulvospinel lamellae were altered to titanite and/or TiO(2) polymorphs while ilmenite was altered to pseudorutile and titanite +/- TiO(2) The relative proportions of TiO(2) and titanite appear to reflect local variations in the composition of the metamorphic fluid phase that may be linked to the degree of interaction with a hydrothermal fluid Titanite-rich regions may indicate a greater degree of fluid flushing than TiO(2)-rich regions. Textures and mineral assemblages cannot distinguish between models in which pseudorutile and a TiO(2) polymorph formed during weathering or diagenesis followed by conversion of TiO(2) to Marine during metamorphism and models in which pseudorutile formed, together with Marine and TiO(2), during metamorphism In either case, pseudorutile was able to persist through prehnite-pumpellyite metamorphism suggesting that it can be a significant Ti-rich accessory mineral in very low-grade metamorphic rocks

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