Jacques L.R. Touret

Le facies granulite en Norvege Meridionale: II. Les inclusions fluides

Abstract Liquid inclusions in quartz found in rocks of amphibolite and of granulite facies were studied. In the former water is predominant, in the latter CO2 (probably juvenile). Estimates of PT conditions during metamorphism based on the study of inclusions gave values (800°C, 8 kb) comparable to those obtained by the petrological study (7–800 °C, 6–8 kb) presented in part I.

Fluids in metamorphic rocks

Abstract Basic principles for the study of fluid inclusions in metamorphic rocks are reviewed and illustrated. A major problem relates to the number of inclusions, possibly formed on a wide range of P – T conditions, having also suffered, in most cases, extensive changes after initial trapping. The interpretation of fluid inclusion data can only be done by comparison with independent P – T estimates derived from coexisting minerals, but this requires a precise knowledge of the chronology of inclusion formation in respect to their mineral host. The three essential steps in any fluid inclusion investigation are described: observation, measurements, and interpretation. Observation , with a conventional petrographic microscope, leads to the identification and relative chronology of a limited number of fluid types (same overall composition, eventually changes in fluid density). For the chronology, the notion of GIS (Group of synchronous inclusions) is introduced. It should serve as a systematic basis for the rest of the study. Microthermometry measurements , completed by nondestructive analyses (mostly micro-Raman), specify the composition and density of the different fluid types. The major problem of density variability can be significantly reduced by simple considerations of the shape of density histograms, allowing elimination of a great number of inclusions having suffered late perturbations. Finally, the interpretation is based on the comparison between few isochores, representative of the whole inclusion population, and P – T mineral data. Essential is a clear perception of the relative chronology between the different isochores. When this is possible, as illustrated by the complicated case of the granulites from Central Kola Peninsula, a good interpretation of the fluid inclusion data can be done. If not, fluid inclusions will not tell much about the metamorphic evolution of the rocks in which they occur.

Le facies granulite en norvege meridionale: I. Les associations minéralogiques

Abstract The basement of Southern Norway is a genetically homogeneous migmatite complex in which old supracrustals produced granitic and granodioritic ‘neosome’ during a late Precambrian (10 9 years ago) regional metamorphism. Observed isograds of critical minerals: muscovite, cordierite, orthopyroxene indicate that the Telemark amphibolite facies in the North passes into the coastal Bamble granulite facies in the South. The data of expeimental petrology suggest that the transition, which took place at temperatures of 700–800°C, is essentially due to a decrease of the partial water pressure. P H 2 O was equal to total pressure P s in the amphibolite facies and lower than 2 kb in the granulite facies.

Fluid inclusions in granulites, granulitized eclogites and garnet clinopyroxenites from the Dabie–Sulu terranes, eastern China

Minor granulites (believed to be pre-Triassic), surrounded by abundant amphibolite-facies orthogneiss, occur in the same region as the well-documented Triassic high- and ultrahigh-pressure (HP and UHP) eclogites in the Dabie–Sulu terranes, eastern China. Moreover, some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation. Granulitized HP eclogites/garnet clinopyroxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775–805 °C and ≥15 kbar, followed by granulite- to amphibolite-facies overprint of ca. 750–800 °C and 6–11 kbar. The presence of (Na, Ca, Ba, Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions. Metamorphic mineral assemblages and P–T estimates for felsic granulite at Huangtuling and mafic granulite at Huilanshan indicate peak conditions of 850 °C and 12 kbar for the granulite-facies metamorphism and 700 °C and 6 kbar for amphibolite-facies retrograde metamorphism. Cordierite–orthopyroxene and ferropargasite–plagioclase coronas and symplectites around garnet record a strong, rapid decompression, possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites. Carbonic fluid (CO2-rich) inclusions are predominant in both HP granulites and granulitized HP/UHP eclogites/garnet clinopyroxenites. They have low densities, having been reset during decompression. Minor amounts of CH4 and/or N2 as well as carbonate are present. In the granulitized HP/UHP eclogites/garnet clinopyroxenites, early fluids are high-salinity brines with minor N2, whereas low-salinity fluids formed during retrogression. Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz matrices (UHP eclogite). For HP granulites, a limited number of primary CO2 and mixed H2O–CO2(liquid) inclusions have also been observed in undeformed quartz inclusions within garnet, orthopyroxene, and plagioclase which contain abundant, low-density CO2±carbonate inclusions. It is suggested that the primary fluid in the HP granulites was high-density CO2, mixed with a significant quantity of water. The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metasomatic reactions (“giant myrmekites”) occurring at quartz–feldspar boundaries. Compared with the UHP eclogites in this region, the granulites were exhumed in the presence of massive, externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids, probably derived from the surrounding gneisses.

Retrograde methane-dominated fluid inclusions from high-temperature granulites of Rogaland, southwestern Norway

Non-aqueous inclusions in the high-grade (800-1,000C; 4kbar) metamorphic Rogaland region, southwestern Norway, consist of mixtures of CO{sub 2}-CH{sub 4}-N{sub 2}. In particular the fluid inclusions in quartz veins in migmatites are characterized by high CH{sub 4} concentrations and they were re-equilibrated at temperatures below 500C during isobaric cooling. Observations by microthermometry demonstrated several complicated sequences of phase transitions, including partial and metastable homogenization (at lower temperature than melting), and S-L or S-V transitions. The phase behavior reflects a wide variation in composition and molar volume. Fluid compositions were measured by Raman microspectrometry. By this method, also small amounts of graphite have been detected in CO{sub 2}-CH{sub 4} inclusions. The instantaneous formation of graphite in a CO{sub 2}-CH{sub 4} inclusion by induction of the argon laser beam has been observed which points to the metastability of the CO{sub 2}-CH{sub 4} mixture. The calculated densities of the observed fluid mixtures are relatively low, necessitating a revision of the earlier interpretation of these inclusions as containing pure high-density fluids. Inclusions in quartz, trapped after the peak of metamorphism, record fluid heterogeneity which may present evidence for fluid-deficient metamorphism during the retrograde M2-M3 metamorphism.

Magmatic CO2 and saline melts from the Sybille Monzosyenite, Laramie Anorthosite Complex, Wyoming

There are three populations of fluid inclusions in quartz from the Sybille Monzosyenite: early CO2, secondary CO2, and rare secondary brines. The oldest consist of low density CO2 (ρ≅0.70) inclusions that appear to be co-magmatic. The densities of these inclusions are consistent with the inferred crystallization conditions of the Sybille Monzosyenite, namely 3 kilobars and 950–1000° C. The other types of inclusions are secondary; they contain CO2 (ρ≅0.50) and secondary brine inclusions that form trains radiating out from a decrepitated inclusion. The sites of these decrepitated inclusions are now marked by irregularly shaped fluid inclusions and solid inclusions of salt and carbonate. Rather than fluid inclusions, feldspar contain abundant solid inclusions. These consist of magmatic minerals, hedenbergite, hornblende, ilmenite, apatite, and graphite, intimately associated with K, Na chlorides. We interpret these relations as follows: The Sybille Monzosyenite formed from a magma that contained immiscible droplets of a halide-rich melt along with a CO2 vapor phase. The salt was trapped along with the other obvious magmatic minerals during growth of the feldspars. CO2 may have also been included in the feldspars but it probably leaked later during exsolution of the feldspars and was not preserved. Both the saline melt and the CO2 vapor were trapped in the quartz. The melt inclusions in the quartz later decrepitated, perhaps due to progressive exsolution of fluids, to produce the secondary H2O and CO2 inclusions. These observations indicate that the Sybille Monzosyenite, which is a markedly anhydrous rock, was actually vapor-saturated. Rather than being H2O, however, the vapor was CO2-rich and possibly related to an immiscible chloride-rich melt.

Ephemeral carbonate melts in the upper mantle: carbonate-silicate immiscibility in microveins and inclusions within spinel peridotite xenoliths, La Gomera, Canary Islands

Carbonate droplets containing mafic silicate glass ± CO 2 occur within mineral inclusions and late microveins in spinel-bearing ultramafic xenoliths (dunite, dunite-werhlite and pyroxenite) from La Gomera, Canary Islands. Primary carbonates are Mg-calcite (X Ca 0.89–0.93) and dolomite (X Ca 0.46–0.54), with low Na 2 O (≤ 0.1 wt.%) and variable MnO (0.2–8 wt.%). The mafic glass has high MgO (24–35 wt.%), FeO (1–18 wt.%) and SiO 2 (40–55 wt.%), with low Al 2 O 3 , TiO 2 , CaO and alkalis. Mafic glass contains high amounts (> 10 wt %) of volatiles ( i.e. , H 2 O). The composite carbonate droplets represent a quenched liquid, resulting from unmixing within the inclusions of a carbonate-rich melt into separate carbonate- and silicate-rich phases. Modelling of initial bulk compositions suggests dolomitic melts, with high silica (≈ 10 wt.%) and H 2 O, but low alkali contents. If not protected within inclusions, these melts are ephemeral, unstable in the P-T field of spinel peridotites (10–18 kbar; 900–1000°C). Mafic glass remnants in microveins represent a residual degassed hydrous mafic silicate fraction after decarbonation.

An empirical phase diagram for a part of the N2CO2 system at low temperature

Abstract Microthermometric and chemical (Raman analysis) data for 23 inclusions from Triassic dolomites and evaporites (northern Tunisia) are used to generate a T-X section of the N2CO2 binary system. Most observed features, including some metastable homogenizations, are consistent with a simple topology well known in many binary and notably in the CO2CH4 system.

Anomalously Low Temperature Orthopyroxene, Spinel, and Sapphirine Occurrences in Metasediments from the Bamble Amphibolite‐to‐Granulite Facies Transition Zone (South Norway): Possible Evidence for Localized Action of Saline Fluids: A Reply

The question has been raised whether isolated granulite facies “islands,” which occur throughout the amphibolite facies of the regional amphibolite‐to‐granulite facies transition zone in the central part of the Bamble sector, southern Norway are the result of prograde or retrograde metamorphism. We have studied one of these islands, the sapphirine granulites at Hauglandsvatn. Despite the preponderance of amphibolite facies assemblages in the surrounding rocks and the small scale of distribution of the granulite facies assemblages, the current assemblage is considered to be prograde, and has developed at the expense of biotite in the regional main foliation, leaving relict biotite separated from quartz by a continuous corona of orthopyroxene and K‐feldspar. Other high temperature, low aH2O reactions resulted in the formation of spinel ± corundum + albite/oligoclase symplectites at the expense of biotite and sillimanite. The symplectites have partially been replaced by symplectites of tiny blue sapphirine ±...

Magmas as a Source of Heat and Fluids in Granulite Metamorphism

Three diverse modes of granulite formation, CO2-streaming, partial melting, and recrystallization of originally anhydrous rocks, can be aspects of the same process: movement of magmas through the lower crust. CO2-saturated silicic and mafic magmas can exsolve enough CO2 to dehydrate a volume of country rock approximately equal to 20%that of the magma itself. Consequently, movement of magmas through the crust can provide both the heat and the CO2 necessary for granulite metamorphism. Furthermore, silicic magmas emplaced into the deep crust are likely to produce anhydrous pyroxene-bearing cumulates (ie. charnockites) while more hydrous portions of the magma would be forced to migrateto shallower, cooler levels before they could crystallize to theH2O-saturated liquidus. Thus, magmas may form conduits by which CO2 of mantle origin is transported into the lower crust while H2O is extracted from the lower crust and moved to shallower levels. Evidencesupporting this hypothesis lies in the abundance of CO2 fluid inclusionsin clearly igneous charnockitic rocks, in the elevated geotherms suggested by P-T conditions of some granulites, and in the relict igneous features found in the highest grade areas of some granulite terranes. This theory implies that some felsic rocks with high K/Rb ratios may be cumulates, and that such K/Rb ratios are not diagnostic of CO2-fluxing.