ALBERT

Description: The effect of magmatic sulfide precipitation on the potential of magmatic systems to produce porphyry-type ore deposits is still a matter of debate. In particular, we need to know whether magmatic sulfide precipitation has an impact on the Cu and Au content of the exsolving magmatic volatile phases and, by this way, on the Cu/Au ratio of porphyry deposits. The Javorie volcano is a perfect place to explore these questions. First, it hosts several Au-only porphyry-type mineralized occurrences which have among the lowest Cu/Au ratios reported in the literature. Secondly, the geology of the Javorie volcano and the timing of porphyry Au mineralization are well established. The evolution of the Javorie magmatic system was reconstructed by detailed petrographic studies and laser ablation inductively coupled plasma mass spectrometry analysis of minerals, melt inclusions and sulfide inclusions. The Javorie volcano was formed during the post-subduction magmatic activity affecting the Western Carpathians. It is a typical stratovolcano, composed dominantly of basaltic andesites and andesites which were intruded by several small stocks of dacitic to dioritic composition. According to our thermobarometric data, the volcano was fed by a transcrustal magmatic system in which two levels of magma chambers could be identified. Part of the magma evolved in the lower crust as suggested by the occurrence of magmatic garnet antecrysts in some of the studied rocks. The occurrence of magmatic sulfide inclusions in garnet indicates that sulfide saturation was reached in this lower crustal magma chamber. Most of the rocks crystallized in an upper crustal magma chamber (∼2 ± 1 kbar) that was fed by a basaltic to basaltic andesite magmas. A large variation in temperatures, ranging between 820°C and 1025°C, recorded by the extrusive and intrusive rocks suggest either that the upper crustal magma chamber was thermally zoned, or that the temperature of the whole magma chamber varied dramatically during its lifetime. Magmatic sulfide inclusions are present in all minerals and rocks of the upper crustal magma chamber, independent of their timing relative to porphyry Au mineralization (pre-, syn-, post-ore). These observations suggest that the magmatic system was sulfide saturated during its entire evolution. With very few exceptions, the precipitating sulfides were composed of monosulfide solid solution containing 0·2–9·2 wt % Cu and 0·05–11 ppm Au. The presence of these magmatic sulfides, together with results of a numerical model, suggest that the primitive magma feeding the upper crustal magma chamber contained less than 2·75 wt % H2O and that only a minor part of the magmatic sulfides was fractionated out of the system. Finally, the Cu/Au ratios measured in the magmatic sulfide inclusions and the ones predicted for the exsolved aqueous fluids are 10 to 100 times higher than the Cu/Au ratios of the porphyry deposits. Therefore, the extremely low Cu/Au ratios of the porphyry deposits must have been acquired during the hydrothermal stage.

Description: Arc magmas are thought to be generated by partial melting of the mantle wedge above the subduction slab, which is triggered by the fluids from the dehydration of subducting oceanic crust. Among the dehydration reactions, those occurring at the depths of the blueschist-to-eclogite transition are considered to be very important. The sodium amphibole glaucophane (□Na2Mg3Al2Si8O22(OH)2, where □ represents a vacancy) is characteristic of blueschists, so that determining the higher-temperature stability of end-member glaucophane helps constrain the maximum temperature of the transition between blueschists and eclogites. A reversed determination of the dehydration reaction 2 glaucophane = 4 jadeite + 3 enstatite + 2 quartz + 2 H2O was done in the system Na2O–MgO–Al2O3–SiO2–H2O over the pressure-temperature (P–T) range of 2·5–3·3 GPa and 760–900 °C for durations of 24–96 hours, using synthetic phases as starting materials. The reaction was bracketed at 830–850 °C at 2·5 GPa and at 810–830 °C at 2·9 GPa in the presence of water. In addition to pure water, 5 molality H2O–NaCl (mole fraction XNaCl = 0·08) and 5 molality H2O–CO2 (XCO2 = 0·08) fluids were used to check the effects of NaCl and CO2, respectively, on the dehydration reaction. The H2O–NaCl fluid shifts the reaction boundary at 2·5 GPa from 840 °C to a lower T (800 °C), while the CO2–H2O fluid shifts the boundary to a higher T (860 °C). At these high P–T conditions, the fluid, even without any added NaCl or CO2, is a silicate-bearing aqueous fluid. The different effects of NaCl and CO2 are attributed to differences in their capabilities to inhibit the solubility of silicates, such as quartz in water, and in changing the activity of water at such P–T conditions. The difference can shift the depth of dehydration by 6 km for an average low dT/dP geothermal gradient of 325 °C/GPa. The shift in the boundary could be considerably larger for shallower dT/dP slab-top geothermal gradients. The experimental results show a good fit with those from thermodynamic modelling and aqueous geochemical calculation.

Description: Monazite is a common mineral in metapelitic rocks including those which underwent ultra-high pressure (UHP) metamorphism. During metamorphic evolution monazite adapts its composition to the changing mineral assemblage, especially in its heavy rare earth element contents. We studied this process in diamond-bearing gneiss containing monazite, from Saxnäs in the Seve Nappe Complex of the Scandinavian Caledonides. Although the rock has been re-equilibrated under granulite facies and partial melting conditions, it still preserves minerals from the UHP stage: garnet, kyanite, rutile, and especially diamond. Microdiamonds occur in situ as inclusions in garnet, kyanite and zircon, either as single-crystals or polyphase inclusions with Fe-Mg carbonates, rutile and CO2. Both monazite and diamond occur in the rims of garnet showing the highest pyrope content and a secondary peak of yttrium. Such a position indicates thermally activated diffusion under high temperature at the end of prograde metamorphism. Monazite compositions show negative Eu anomalies, which we interpret to be inherited from the source rock, not reflecting the coexistence with plagioclase and/or K-feldspar which are unstable at UHP conditions. Our results suggest that the effect of whole-rock composition may be more important than that of coexisting phases. The UHP monazite was most likely formed from allanite during subduction and prograde metamorphism. The monazites included in garnet and kyanite are mostly unaltered, whereas those in the matrix show breakdown coronas consisting of apatite, REE-epidote/allanite and REE carbonate, likely formed due to pressure decrease and cooling. U-Th-Pb chemical age dating of monazites yields an isochron centroid age of 472 ±3 Ma. We interpret this age as monazite growth under UHP conditions related to subduction of the Baltica continental margin in Early Ordovician time.

Description: Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, Re), major and trace element abundances, and 187Re-187Os systematics are reported for xenoliths and lavas from Aitutaki (Cook Islands), to investigate the composition of Pacific lithosphere. The xenolith suite comprises spinel-bearing lherzolites, dunite, and harzburgite, along with olivine websterite and pyroxenite. The xenoliths are hosted within nephelinite and alkali basalt volcanic rocks (187Os/188Os = ∼0.1363 ±13; 2 SD; ΣHSE = 3-4 ppb). The volcanic host rocks are low-degree (2-5%) partial melts from the garnet stability field and an enriched mantle (EM) source. Pyroxenites have similar HSE abundances and Os isotope compositions (Al2O3 = 5.7-8.3 wt %; ΣHSE = 2-4 ppb; 187Os/187Os = 0.1263 to 0.1469) to the lavas. The pyroxenite and olivine websterite xenoliths directly formed from - or experienced extensive melt-rock interaction with - melts similar in composition to the volcanic rocks that host the xenoliths. Conversely, the Aitutaki lherzolites, harzburgites and dunites are similar in composition to abyssal peridotites with respect to their 187Os/188Os ratios (0.1264 ±82), total HSE abundances (ΣHSE = 8-28 ppb) and major element abundances, forsterite contents (Fo89.9±1.2), and estimated extents of melt depletion ( 15%). These peridotites are interpreted to sample relatively shallow Pacific mantle lithosphere that experienced limited melt-rock reaction and melting during ridge processes at ∼90 Ma. A survey of maximum ages of Pacific mantle lithosphere from the Cook (Aitutaki = ∼1.5 Ga), Austral (Tubuai’i = ∼1.8 Ga), Samoan (Savai’i = ∼1.5 Ga) and Hawaiian (Oa’hu = ∼2 Ga) island groups shows Mesoproterozoic to Neoproterozoic depletion ages are preserved in the xenolith suites. The variable timing and extent of mantle depletion preserved by the peridotites is, in some instances, superimposed by extensive and recent melt depletion as well as melt refertilization. Collectively, Pacific Ocean island mantle xenolith suites have similar distributions and variations of 187Os/188Os and highly siderophile element abundances to global abyssal peridotites. These observations indicate that Pacific mantle lithosphere is typical of oceanic lithosphere in general, and that this lithosphere is composed of peridotites that have experienced both recent melt depletion at ridges and prior and sometimes extensive melt depletion across several Wilson cycles spanning periods in excess of two billion years.

Description: The knowledge of the fractionation behaviour between phases in isotopic equilibrium and its evolution with temperature is fundamental to assist the petrological interpretation of measured oxygen isotope compositions. We report a comprehensive and updated internally consistent database for oxygen isotope fractionation. Internal consistency is of particular importance for applications of oxygen isotope fractionation that consider mineral assemblages rather than individual mineral couples. The database DBOxygen is constructed from a large dataset of published experimental, semi-empirical and natural data, which were weighted according to type. It includes fractionation factors for 153 major and accessory mineral phases and a pure H2O fluid phase in the temperature range of 0–900°C, with application recommended for temperatures of 200–900°C. Multiple primary data for each mineral couple were discretized and fitted to a model fractionation function. Consistency between the models for each mineral couple was achieved by simultaneous least square regression. Minimum absolute uncertainties based on the spread of the available data were calculated for each fractionation factor using a Monte Carlo sampling technique. The accuracy of the derived database is assessed by comparisons with previous oxygen isotope fractionation calculations based on selected mineral/mineral couples. This database provides an updated internally consistent tool for geochemical modelling based on a large set of primary data and including uncertainties. For an effective use of the database for thermometry and uncertainty calculation we provide a MATLAB©-based software ThermoOx. The new database supports isotopic modelling in a thermodynamic framework to predict the evolution of δ18O in minerals during metamorphism.

Description: The Orestes Melt Zone (OMZ) is a massive contact melt zone (∼20 m thick by several kilometers long), located in the McMurdo Dry Valleys of Antarctica. The OMZ formed at shallow crustal depths by melting of the A-type Orestes Granite owing to intrusion of the underlying, doleritic Basement Sill. The OMZ can be divided broadly into two melting facies. The upper melting facies is distal from the contact and formed by melting at low temperature and water-saturated, or near water-saturated, conditions. The lower melting facies is proximal to the contact and formed by melting at high temperature and water-undersaturated conditions. Separate melting reactions occurred in both of the melting facies, resulting in distinct textures and melt compositions. Melting in the distal facies generated melts with compositions that plot near a predicted eutectic composition. Melting in the proximal facies was accomplished in part by replacement reactions in restitic feldspars. These reactions resulted in the development of plagioclase mantles on both restitic plagioclase and K-feldspar, and melt compositions that diverged from predicted minimum melt along an unexpected path, towards enrichment in orthoclase component. Thermal modeling indicates that this melt zone was active for a minimum of ∼150 years, with a contact temperature of ∼900 °C. Upon cooling, recrystallization generated ocellar textures around restitic quartz, as well as faceted albite as a late-stage product. Observations of the OMZ, combined with thermal modeling, provide new insights into the origin of rapakivi and albite granites. This study has implications for the origin of these two associated granite types in other geological settings.

Description: The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.

Description: Amphibolite- and granulite-facies metamorphic rocks are common in the eastern Himalayan syntaxis of southeastern Tibet. These rocks are composed mainly of gneiss, amphibolite and schist that underwent various degrees of migmatization to produce leucogranites, pegmatites and felsic veins. Zircon U–Pb dating of biotite gneiss, leucocratic vein and vein granite from the syntaxis yields consistent ages of ∼49 Ma, indicating crustal anatexis during continental collision between India and Asia. Garnets in these rocks are categorized into peritecitc and anatectic varieties based on their mode of occurrence, mineral inclusions and major- and trace-element zoning. The peritectic garnets mainly occur in the biotite gneiss (mesosome layer) and leucocratic veins. They are anhedral and contain abundant mineral inclusions such as high-Ti biotites and quartz, and show almost homogeneous major-element compositions (except Ca) and decreasing HREE contents from core to rim, indicating growth during the P- and T-increasing anatexis. Peak anatectic conditions at 760–800°C and 9–10·5 kbar are well constrained by phase equilibrium calculations, mineral assemblages, and garnet isopleths. In contrast, anatectic garnets only occur in the vein granite. They are round or subhedral, contain quartz inclusions, and exhibit increasing spessartine and trace-element contents from core to rim. The garnet–biotite geothermometry and the garnet–biotite–plagioclase–quartz geobarometry suggest that the anatectic garnets crystallized at ∼620–650°C and 4–5 kbar. Some garnet grains show two-stage zoning in major and trace elements, with the core similar to the peritectic garnet but the rim similar to the anatectic garnet. Mineralogy, whole-rock major- and trace-element compositions and zircon O isotopes indicate that the two types of leucosomes were produced by hydration (water-present) melting and dehydration (water-absent) melting, respectively. The leucocratic veins contain peritectic garnet but no K-feldspar, have lower whole-rock K2O contents and Rb/Sr ratios, higher whole-rock CaO contents and Sr/Ba ratios, and show homogeneous δ18O values that are lower than those of relict zircons, indicating that such veins were produced by the hydration melting. In contrast, the vein granite contains peritectic garnet and K-feldspar, has higher whole-rock K2O contents and Rb/Sr ratios, lower whole-rock CaO contents and Sr/Ba ratios, and shows comparable δ18O values with those of relict zircons, suggesting that this granite were generated by the dehydration melting. Accordingly, both hydration and dehydration melting mechanisms have occurred in the eastern Himalayan syntaxis.

Description: Cumulate processes in granitic magma systems are thought by some to be negligible and by others to be common and widespread. Because most granitic rocks lack obvious evidence of accumulation, such as modal layering, other means of identifying cumulate rocks and estimating proportions of melt lost must be developed. The approach presented here utilizes major and trace element compositions of hornblende to estimate melt compositions necessary for zircon saturation. It then compares these estimates with bulk-rock compositions to estimate proportions of extracted melt. Data from three arc-related magmatic systems were used (English Peak pluton, Wooley Creek batholith, and Tuolumne Intrusive Complex). In all three systems, magmatic hornblende displays core-to-rim decreases in Zr, Hf, and Zr/Hf. This zoning indicates that zircon must have fractionated during crystallization of hornblende, at temperatures greater than 800 °C. This T estimate is in agreement with Ti-in-zircon thermometry, which yields a maximum T estimate of 855 °C. On the basis of this evidence, concentrations of Zr in melts from which hornblende and zircon crystallized were calculated by (1) applying saturation equations to bulk-rock compositions, (2) applying saturation equations to calculated melt compositions, and (3) using hornblende/melt partition coefficients for Zr. The results indicate that melt was lost during crystallization of the granitic magmas, conservatively at least as much as 40 %. These results are in agreement with published estimates of melt loss from other plutonic systems and suggest that bulk-rock compositions of many granitic rocks reflect crystal accumulation and are therefore inappropriate for use in thermodynamic calculations and in direct comparison of potentially consanguineous volcanic and plutonic suites.