J. Ben H. Jansen

Carbon, hydrogen, and oxygen isotope studies of the regional metamorphic complex at Naxos, Greece

Abstract At Naxos, Greece, a migmatite dome is surrounded by schists and marbles of decreasing metamorphic grade. Sillimanite, kyanite, biotite, chlorite, and glaucophane zones are recognized at successively greater distances from the migmatite dome. Quartz-muscovite and quartz-biotite oxygen isotope and mineralogie temperatures range from 350 to 700°C. The metamorphic complex can be divided into multiple schist-rich (including migmatites) and marblerich zones. The δ 18 O values of silicate minerals in migmatite and schist units and quartz segregations in the schist-rich zones decrease with increase in metamorphic grades. The calculated δ 18 O H 2 O values of the metamorphic fluids in the schist-rich zones decrease from about 15‰ in the lower grades to an average of about 8.5‰ in the migmatite. The δD values of OH-minerals (muscovite, biotite, chlorite, and glaucophane) in the schist-rich zones also decrease with increase in grade. The calculated δD H 2 O values for the metamorphic fluid decrease from −5‰ in the glaucophane zone to an average of about −70‰ in the migmatite. The δD values of water in fluid inclusions in quartz segregations in the higher grade rocks are consistent with this trend. Theδ 18 O values of silicate minerals and quartz segregations in marble-rich zones are usually very large and were controlled by exchange with the adjacent marbles. The δD values of the OH minerals in some marble-rich zones may reflect the value of water contained in the rocks prior to metamorphism. Detailed data on 20 marble units show systematic variations of δ 18 O values which depend upon metamorphic grade. Below the 540°C isograd very steep δ 18 O gradients at the margins and large δ 18 O values in the interior of the marbles indicate that oxygen isotope exchange with the adjacent schist units was usually limited to the margins of the marbles with more exchange occurring in the stratigraphic bottom than in the top margins. Above the 540°C isograd lower δ 18 O values occur in the interior of the marble units reflecting a greater degree of recrystallization and the occurrence of Ca-Mg-silicates. Almost all the δ 13 C values of the marbles are in the range of unaltered marine limestones. Nevertheless, the δ 13 C values of most marble units show a general correlation with δ 18 O values. The CO 2 H 2 O mole ratio of fluid inclusions in quartz segregations range from 0.01 to 2. Theδ 13 C values of the CO 2 range from −8.0 to 3.6‰ and indicate that at some localities CO 2 in the metamorphic fluid was not in carbon isotopic equilibrium with the marbles.

A mechanism for preferential H2O leakage from fluid inclusions in quartz, based on TEM observations

Preferential leakage of H2O from fluid inclusions containing multiple gas components has been suspected in natural metamorphic rocks and has been demonstrated experimentally for synthetic H2O-CO2-rich inclusions in natural quartz. Knowledge of the physical and chemical characteristics of the leakage mechanism, which may be very complex, increases the value of natural fluid inclusions to metamorphic geology. It is proposed that crystal defects play a major role in nondecrepitative preferential H2O leakage through quartz, and remain effective during metamorphism. Inclusions with either an internal overpressure or underpressure produce strain in the adjacent quartz crystal via the nucleation of many dislocations and planar defects (like Dauphiné twin boundaries). These defects allow preferential loss of H2O from H2O-CO2-rich inclusions at supercritical conditions. The transport capacity of this leakage mechanism is enhanced by nucleation of small bubbles on defect structures. The nucleation of these bubbles seems to be a recovery process in strained crystals. Solubility gradients of quartz in water in a crystal with internally underpressurized inclusions may result in optical visible implosion halos in a three dimensional spatial arrangement, caused by the growth of small bubbles at the expense of the larger original fluid inclusion. Natural fluid inclusions from Naxos (Greece) are always associated with numerous interlinked dislocations. These dislocations may have been produced by plastic derormation or by crystal growth related processes (e.g. crack healing). The presence of small bubbles on these dislocations indicates that a similar leakage mechanism for H2O must have occurred in these rocks.

Experimental post-entrapment water loss from synthetic CO2-H2O inclusions in natural quartz

Artificial fluid inclusions were hydrothermally synthesized by crack healing in natural Brazilian quartz. Two original experiments E421 and E679 with a H2O-CO2 fluid were carried out at 835 K, 200 MPa, over 38 days, and at 856 K, 211 MPa, over 35 days, respectively. In both experiments homogeneous three-phase (a vapor and two liquids) fluid inclusions were synthesized with 22 and 20 mol% CO2, respectively. The CO2 phases homogenize to the vapor phase at 302.2 ± 0. l K (E679, cores 1 and 2), and to the liquid phase at 303.6 ± 0.3 K (E421, core 3), 303.9 ± 0.2 K (E421, core 4). The CO2-H2O phases homogenize to the vapor phase at 573.6 ± 0.4 K (E679, core 1 and 2), 575.6 ± 1.5 K (E421, core 3), and 576.1 ± 0.8 K (E421, core 4). Micro cracks and the new hydrothermally precipitated quartz, which directly surrounds the inclusions, were studied with TEM and SEM. The healed cracks have numerous growth imperfections that provide many possible routes for fluid transport. Dislocation arrays and small channels were observed and are often connected to the inclusions. Quartz cores 3 and 4 were subsequently re-equilibrated for 21 and 27 days respectively under hydrothermal conditions with pure H2O, at both a lower pressure of 100 MPa (E463) and a higher pressure of 365 MPa (E490) than that of the original experiment E421. The temperatures of the re-equilibration experiments were equal to the original (835 K). In E463, the internally overpressured re-equilibration, only traces of solution and precipitation of quartz were evident with minor transformation of the angular walls to more rounded forms. Volume increase for some inclusions resulted from decrepitation. The homogenization of the CO2-H2O phases to the gas phase occurred at higher temperatures, up to 604 K. In E490, the internally underpressured re-equilibration, major solution and precipitation resulted in the transformation of irregular shaped inclusion walls and formation of secondary inclusions halos. The homogenization of the CO2-H2O phases to the gas phase occurred at lower temperatures, down to 565 K. The fluids in inclusions from both re-equilibration experiments were found to have lower densities than the original fluids synthesized. This is quantified by the increased volumetric proportions of CO2 vapor. The CO2 fraction in inclusions was found to have increased, by up to 54 mol%. The change in homogenization temperatures and the decrease in the proportions of H2O in the original inclusions favours a model in which preferential transport of H2O occurs along mobile dislocation lines, small intercrystalline nanocracks, and/or channels. Results from experiment V1 and V4, using cores 1 and 2, indicate that the changes observed in the re-equilibration experiments are not artifacts of the experimental method.

Metamorphic Zoning in the High-Grade Proterozoic of Rogaland-Vest Agder, SW Norway

The high-grade low-pressure metamorphism in Rogaland increases towards the southwest to reach a maximum near the border of the Egersund intrusive complex. The first isograd is the “hypersthene line”, marking the appearance of hypersthene in rocks of leucogranitic composition. A few kilometres westward hornblende + quartz are replaced by pyroxenes in mafic rocks, and the amphibole colour grades from green to brown in Ti-bearing rocks. Somewhat west of the hypersthene line garnet + silli-manite give way to cordierite + spinel/magnetite. A second isograd heralds the partial decomposition of garnet to orthopyroxene + cordierite + spinel/magnetite. In rocks richer in Fe, garnet + sillima-nite react to spinel/magnetite + quartz. In potash-rich compositions garnet + biotite + quartz ± sillimanite react to osumilite + orthopyro¬xene + spinel/magnetite. Near the intrusive complex a third isograd marks the incoming of metamorphic pigeonite in leucogranitic rocks. The temperature range of the whole zoning from the hypersthene line to the complex is estimated at 700-1050°C at a pressure of 4–6 kb, although remnants of a higher-pressure older metamorphism may be present. Somewhat younger retrograde metamorphism is widespread, and includes decomposition of osumilite and the growth of garnet rims.

CHARACTERIZATION OF SYNTHETIC Na-BEIDELLITE

Na-beidellite, a member of the smectite group, was grown hydrothermally from a gel of composition 0.35Na2O·2.35Al2O3·7.3SiO2 in NaOH solutions at a pH between 7.5 and 13.5, a pressure of 1 kbar, and a temperature of 350°C. The synthetic Na-beidellite was characterized by means of scanning electron microscopy, X-ray powder diffraction, infrared spectroscopy, electron microprobe, inductively coupled plasma-atomic emission spectroscopy, and thermogravimetric analysis. The unit-cell parameters of the orthorhombic cell are: a = 5.18, b = 8.96, and c = 12.54 Å. The cation-exchange capacity was determined to be 70 meq/100 g. A maximum of 40 wt. % water was present and reversibly lost by heating to about 55°C. The loss of water caused a decrease of the basal spacing to 9.98 Å. At temperatures ≥600°C, the Na-beidellite started to dehydroxylate, reaching its maximum in the range 600°–630°C. At 1100°C the remaining solid recrystallized to Al6Si2O13 (mullite) and SiO2 (cristobalite).

Geothermometry and Geobarometry in Rogaland and Preliminary Results from the Bamble Area, S Norway

Mineral textures in Rogaland and Bamble areas give evidence for polymetamorphism, beside the usual low-grade retrogradations in or below the greenschist facies. Due to the fact that geothermobarometry is based on the assumption of chemical equilibrium among minerals, the application has to be treated with an enormous amount of caution in such areas. Especially the presence (or absence) of index minerals must be used as a control upon the derived results. For example, in both areas involved the main Al-silicate is sill. The few occurrences of kya in SE Norway indicate high pressure retrogradation, while the few occurrences of anda in SW Norway presumably point at low pressure retrogradation. In fact most rocks in Rogaland are reset during the retrograde M3 metamorphism. Part of the chemical rock system, particularly biot, cord, oxide mineral, is reequilibrated in a temperature range of about 550-700 °C at about 3–5 kb total pressure. The conditions are regionally varying. These results are achieved by the use of the garn-cord, garn-biot, garn-opx-plag-qtz, cpx-opx and ilm-ulvo thermo-barometers. In order to establish the conditions of earlier metamorphisms a search for relic equilibrium domains is inevitable, which are not necessarily one restricted part of a thin section with contacting minerals. For the M2 metamorphism the calibrated conditions yielded 700-1050 °C grading to the lower temperatures towards ENE, at pressures of about 4 kb. These estimates are derived by the use of thermo- barometry on cpx-opx, ilm-ulvo, fay-qtz-fesil and some garn-opx assemblages. Equilibrium domains of mineral assemblages useful for geothermo-geobarometry upon the Ml metamorphism are not unambigous enough to establish PT-conditions. Based on experimentally derived mineral stability fields some estimates in the temperature range from 5–7 kb can be made. The regional gradients are not well understood yet. The main metamorphism in Bamble near Tvedestrand took place at temperatures of about 750 °C and about 7 kb. These results were achieved with garn-cord, garn-biot and garn-opx-plag-qtz thermometry. Only some local chemical resetting is established. The retrogradation was more or less isobaric down to about 600 °C passing into the kyanite stability field.

Nitrogen storage in biotite: An experimental study of the ammonium and potassium partitioning between 1M-phlogopite and vapour at 2 kb

Abstract The NH+4 and K+ distribution between phlogopite and a chloride vapour phase was investigated at 550°C and 2000 bars. Some preliminary data are reported for 650°C and 2000 bars. The apparent distribution coefficient, K ∗ D = (NH + 4 K + ) phlogopite p,t (NH + 4 K + ) vapour 1 bar, 25°C , is 1.29 ± 0.30 at 550°C and 1.44 ± 0.20 for the preliminary data at 650°C. At 550°C a series of runs was performed to study the equilibration time of the exchange reaction. Bulk compositions requiring a large change in solid composition did not equilibrate within 28 days, indicating a rather sluggish reaction mechanism. The extrapolation to low NH+4 concentrations in natural rock systems may provide estimates of NH+4 concentrations for metamorphic fluids in equilibrium with NH+4 bearing minerals. The assumption that Rb+ behaves similarly to NH+4 allows a rough estimate of the distribution coefficients for NH+4 in other minerals. Semiquantitative modelling based on the derived distribution coefficients of NH+4 and Rb+ may elucidate fluid rock equilibria during closed system behaviour. The release of nitrogen by breakdown of NH4-bearing biotite during prograde or retrograde processes in metamorphic rocks may explain the presence of N2-gas in fluid inclusions.

DEVELOPMENT OF AMMONIUM-SAPONITES FROM GELS WITH VARIABLE AMMONIUM CONCENTRATION AND WATER CONTENT AT LOW TEMPERATURES*

Ammonium-saponite is hydrothermally grown at temperatures below 300°C from a gel with an overall composition corresponding to (NH4)0.6Mg3Si34Al0.6O10(OH)2. The synthetic saponite and coexisting fluid have been characterized by means of X-ray powder diffraction, X-ray fluorescence, Induced Coupled Plasma-Atomic Emission Spectroscopy, thermogravimetric analysis, transmission electron microscopy, CEC determination using an ammonia selective electrode, and pH measurement. In the crystallization model developed, crystallization started with the growth of individual tetrahedral layers with an aluminum substitution not controlled by the A1IV/A1VI ratio in the gel and hydrothermal fluid, on which the octahedral Mg layers can grow. During the synthesis, individual sheets stacked to form thicker flakes while lateral growth also took place. The remaining A1VI partly replaced ammonium as the interlayer cation.

Halogen geochemistry of fluid during amphibolite-granulite metamorphism as indicated by apatite and hydrous silicates in basic rocks from the Bamble Sector, South Norway

Abstract The halogen chemistry of apatites, biotites, amphiboles and titanites from metabasites in the Proterozoic Bamble Sector amphibolite-granulite facies transition zone (Norway) has been studied to gather data on the behaviour of halogens during original magmatism and subsequent metamorphism. Three groups of apatites could be recognized: (1) FOH-apatites with more than 50 mole % F and virtually no Cl. (2) FOH-apatites with up to 15 mole % of Cl. (3) Cl-rich apatites with up to 6.73 wt.% Cl. The latter are the most Cl-rich metamorphic apatites ever reported. The F-contents of coexisting biotite do not follow this division, but their Cl-contents do. Log( z.sfnc;HF z.sfnc;H 2 O ) values for the coexisting fluid phase calculated from biotite are slightly lower than those calculated from apatite. Log( z.sfnc;HCl z.sfnc;H 2 O ) values in the coexisting fluid phase calculated from biotite are substantially higher than those calculated from apatite. The log( z.sfnc;HF z.sfnc;H 2 O ) reflects the composition of the peak metamorphic fluid phase. The apatites show three types of patterns of F-contents: (1) W-shaped, (2) Rimward increasing and (3) Rimward decreasing. Pattern (2) reflects progressive metamorphism. Pattern (3) is thought to reflect growth starting near peak metamorphism and going on during retrograde dilution of the peak metamorphic fluid by a newly introduced aqueous one. Some apatites show local resorption effects along cracks. No regional trend in calculated log( z.sfnc;HF z.sfnc;H 2 O ) values is present. As different from other granulite terrains, the Bamble biotites and amphiboles from granulite facies samples do not have higher halogen contents as those from the amphibolite facies area. The samples show that halogen bearing fluids were controlled at a very restricted scale during peak as well as retrograde metamorphism.

The metapelitic rocks of the polymetamorphic Precambrian of Rogaland, SW Norway

The relations between cordiente, hercynite and magnetite in specimens of the garnetiferous migmatites within the osumilite-in isograd from the high-grade polymetamorphic Precambrian of Rogaland, SW Norway, have been studied, using an electron microprobe. In most specimens the primary spinel of the M 2 stage of metamorphism is exsolved into hercynite and magnetite. The exsolution continued during subsequent cooling to moderate temperature conditions of the M 2 stage or even to retrogressive conditions of metamorphism of the M 4 stage. Fe-Mg distributions give evidence that cordierite and hercynite are in equilibrium at these mentioned conditions, while their textural relations belong to the high-grade metamorphic stage M 2. This implies that neither cordierite nor spinel are suitable for use in geothermometry of high-grade metamorphic stage M 2 in Rogaland. It is tentatively concluded that re-equilibration of Mg and Fe continues to lower retrograde temperatures in the Fe-rich cordierite-hercynite pairs than in Mg-rich pairs.