ALBERT

Description: Recent data suggest an active role for chloride-bearing alkali carbonatitic melts in the formation and evolution of kimberlites, whereas experiments indicate that chlorides could be responsible for liquid immiscibility during melting of carbonated mantle rocks. This study considers melting trends in kimberlite-related chloride–carbonate–silicate systems at a pressure of 5·5 GPa. The direction of these trends largely depends on the chlorine concentration in the system. All trends start with a Cl-rich carbonatitic liquid coexisting with crystalline phases. Melting of a peridotite–carbonate–chloride system containing 4·4 wt % Cl results in a gradual transition from a carbonatitic melt (5–7 wt % SiO 2 , MgO/CaO = 0·5–0·6, ~2 wt % Cl) at 1000–1100°C through a Cl-rich carbonate–silicate melt (12–15 wt % SiO 2 , MgO/CaO = 0·6, and up to 14 wt % Cl) at 1360–1400°C towards a Cl-bearing ultrabasic carbonate–silicate melt (UCSM), i.e. kimberlite-like (26–29 wt % SiO 2 , MgO/CaO = 1·5–2·8, and 6–4 wt % Cl), at 1500–1600°C. This trend results from the specific behavior of chlorides, which are found to be stable crystalline phases up to about 300°C above the solidus. In addition, the trend touches a miscibility gap at about 1450°C, although immiscibility does not significantly influence the melt evolution. In contrast, the melting trend of a peridotite system with about 1·3 wt % Cl does not intersect the miscibility gap and proceeds from a carbonatitic melt (~5 wt % SiO 2 , MgO/CaO = 0·4–0·5, ~2 wt % Cl) at 1100–1150°C toward the UCSM (~25 wt % SiO 2 , MgO/CaO = 1·7–1·8, and 0·6–1·0 wt % Cl) at 1500–1600°C. Chloride–carbonate–silicate systems containing 17 wt % Cl show an abrupt transition from the chloride–carbonate liquid toward the UCSM because of the immiscibility gap between carbonate–silicate and chloride–carbonate melts. Melting relations in all studied chloride–carbonate–silicate systems are exclusively regulated by peritectic reactions between the silicate phases and carbonate constituents (mostly CaCO 3 ) of the melts. Chlorides decrease the solidus temperatures of carbonated peridotites, but do not reduce the temperature interval for the transition from carbonate- to silicate-dominated melts. The experimental results suggest that Cl-rich carbonatite liquids preserved in diamonds and the Cl-rich kimberlites of the Udachnaya-East kimberlite pipe (Yakutia) could represent a linked system of chloride-rich liquids that evolved over a wide temperature interval during incipient melting and evolution of kimberlite magma from a carbonated mantle source containing 3–4 wt % Cl.

Description: A tonalitic vein from a representative traverse section from the Paleoarchean Sand River biotite-amphibole gneiss consists of two domains – orthopyroxene-bearing and orthopyroxene-absent domains – which grade from center to the margin, along its length. The orthopyroxene-bearing domain is heavily altered with respect to the surrounding orthopyroxene-absent domain, which preserves a prominent magmatic texture typical of crystallized melt. Contrasting textures were observed in terms of monazite occurrence in fluorapatite in the two domains. While monazite occurs as inclusions in fluorapatite in the orthopyroxene-bearing domain, it prominently occurs as irregular rims along fluorapatite margins in the orthopyroxene-absent domain. The textural relation of monazite, occurring as both inclusions and irregular rims in the same fluorapatite grains, and different mineral chemical characteristics of both types of monazite, supports the operation of melting accompanying fluid-induced dehydration in the Sand River orthogneiss. T- a H 2 O pseudosection modeling reproduced the mineral assemblages of the domain representing melt portion formed during dehydration. The Paleoproterozoic timing of the events is characterized by a 40 Ar/ 39 Ar amphibole age of 2037 ± 10 Ma from the dehydrated gneiss.

Description: To constrain effects of chloride-bearing H 2 O–CO 2 fluids on complex natural assemblages during high-grade metamorphism and anatexis, we report the results of experiments on the interaction of biotite–hornblende tonalitic gneiss from the Sand River Formation (Limpopo Complex, South Africa) with H 2 O–CO 2 , H 2 O–CO 2 –KCl, H 2 O–CO 2 –NaCl, and H 2 O–CO 2 –(K, Na)Cl fluids at 550 MPa, 750 and 800°C, and varying chloride/(H 2 O + CO 2 ) ratios with molar CO 2 /(CO 2 + H 2 O) = 0·5. Heating of solid cylinders of gneiss at both temperatures in the absence of a free fluid phase produced no changes in the gneiss phase assemblage. The equimolar H 2 O–CO 2 fluid at 750°C also did not significantly influence the phase assemblage. Addition of KCl to the fluid at 750°C resulted in formation of the clinopyroxene + K-feldspar (+ ilmenite/titanite) assemblage after biotite, hornblende and plagioclase. Orthopyroxene accompanied by amphibole appeared only at 800°C as a result of biotite breakdown in the presence of H 2 O–CO 2 and low-salinity H 2 O–CO 2 –KCl fluids. Increase in the KCl content in the fluid at 800°C resulted in the production of a clinopyroxene-bearing assemblage. Increase of the NaCl content stabilized amphibole in an assemblage with either orthopyroxene (at low NaCl concentrations) or clinopyroxene. Nevertheless, clinopyroxene (+ albite) is stable only at high salt concentrations. Comparison of the experimental results with the results of thermodynamic modeling using the Gibbs free energy minimization method (PERPLE_X software) showed that mineral reactions and assemblages in the run products were governed by the activities of alkali components imposed by KCl and NaCl in the H 2 O–CO 2 fluids, and decrease of the water activity served as an additional factor stabilizing anhydrous assemblages. No melts formed at 750°C in the presence of the H 2 O–CO 2 –KCl fluids. These fluids provoked melting only at 800°C with formation of rhyolitic melts. With increasing KCl content of the fluid, the melt composition changed to potassic rhyolitic with Al 2 O 3 〈 13·5 wt %, CaO 〈 2 wt %, K 2 O + Na 2 O 〉 7 wt %, FeO/(FeO + MgO) 〉 0·8, K 2 O/Na 2 O 〉 1, and moderate enrichment in Cl (0·2–0·6 wt %). Increasing NaCl content caused melting at 750°C and shifted the melt composition towards trachytic and trachyandesitic compositions at both 750 and 800°C. The experiments support a model for the formation of ferroan A-type granite–syenite complexes via crustal melting in the presence of H 2 O–CO 2 –salt fluids in extensional tectonic settings. They demonstrate a possible link between A-type granitoids and mid-crustal dehydration zones in amphibolite- to granulite-facies terrains and allow a new interpretation of mineral assemblages within these zones in terms of variations in fluid salinity.

Description: Available experimental data on chemical composition and crystal structure of K-bearing clinopyroxenes are compiled together with the results of atomistic simulations and thermodynamic calculations of mineral equilibria. It is shown that the limited solubility of K2O in clinopyroxene from crustal rocks cannot be ascribed to the strong non-ideality of mixing between diopside (CaMgSi2O6) and K-jadeite (KAlSi2O6) components. The more likely reason is the instability of the potassic endmember with respect to other K-bearing phases. As the volume effects of typical K-jadeite-forming reactions are negative, the incorporation of K in the clinopyroxene structure becomes less difficult at higher pressure. Atomistic simulations predict that the thermodynamic mixing properties of diopside-K-jadeite solid-solutions at high temperature approach those of a regular mixture with a relatively small positive excess enthalpy. The standard enthalpy of formation ({Delta}fH0 = -2932.7 kJ/mol), the standard volume (V0 = 6.479 J mol-1 bar-1) and the isothermal bulk modulus (K0 = 145 GPa) of K-jadeite were calculated from first principles, and the standard entropy (S0 = 141.24 J mol-1 K-1) and thermal-expansion coefficient ( = 3.3 x 10-5 K-1) of the K-jadeite endmember were estimated using quasiharmonic lattice-dynamic calculations based on a force-field model. The estimated thermodynamic data are used to compute compositions of K-bearing clinopyroxenes in diverse mineral assemblages within a wide P-T interval. The review substantiates the conclusion that clinopyroxene can serve as an effective container for K at upper-mantle conditions.

Description: A clear case study of local-scale, fluid-induced dehydration of the Paleoarchean Sand River biotite–hornblende gneiss from the Central Zone of the Limpopo Complex is presented here. Field and petrographic examination of three adjacent zones—darker Sand River orthogneiss with local occurrence of orthopyroxene and clinopyroxene, a lighter intermediate gneissic zone with more orthopyroxene than the Sand River orthogneiss, and tonalitic veins containing large orthopyroxene-bearing patches—indicates the local transformation of a light grey, fine- to medium-grained, hornblende–biotite gneiss into a greenish brown, medium- to coarse-grained orthopyroxene-bearing dehydration zone. Field evidence indicates that the tonalitic veins were emplaced in discrete ductile shear zones, with development of large orthopyroxene-bearing patches in a sigmoidally transposed foliation bounded by shear planes. Orthopyroxene-forming reaction textures after biotite and amphibole together with the occurrence of microveins of K-feldspar along quartz–plagioclase grain boundaries in the three adjacent zones, and the higher modal abundance of orthopyroxene and K-feldspar with lesser biotite and amphibole from the Sand River orthogneiss to the intermediate gneissic zone to the orthopyroxene-bearing patches, indicate that the three adjacent zones represent progressive stages of the dehydration process. Such K-feldspar microveins along quartz–plagioclase grain boundaries have been proposed as evidence for the presence and passage of a low H 2 O activity fluid. Further, the occurrence of monazite inclusions in fluorapatite in orthopyroxene-bearing zones suggests dissolution and reprecipitation involving a free fluid phase. Fluid inclusion studies indicate the presence of a fluid with CO 2 , NaCl and H 2 O components, with higher salinity of the fluid (up to 29% NaCl) in the orthopyroxene-bearing patches relative to the intermediate gneissic zone. The increase in Cl content in amphibole, biotite and fluorapatite from the Sand River orthogneiss to the orthopyroxene-bearing patches supports the presence of a Cl-rich brine fraction in the fluid responsible for the dehydration process. Further, the increase in An content of plagioclase at the contact with the K-feldspar rims on quartz reflects an increase in potassium activity in the fluid. The whole-rock major, trace and rare-earth element enrichment or depletion patterns of the orthopyroxene-bearing zones relative to the precursor support the dehydration process. The diffuse contact relationship of a granite pegmatite occurring in the vicinity of the dehydration zones, together with fluid inclusion and whole-rock major, trace and rare element characteristics of samples collected along a traverse from the granite pegmatite to the Sand River orthogneiss, suggests a scenario in which the dehydrating fluids derived from an external source utilized lithological contrasts, such as the gneiss–pegmatite boundaries, as fluid conduits. Dehydration of the gneissic wall-rock occurred where permeability was sufficient for fluid penetration. The occurrence of orthopyroxene-bearing tonalitic veins along deformation-transposed foliation planes further attests to a structural control to the channeling of the dehydrating fluids.