Geochronology, Petrogenesis and Oxidation State of the Northparkes Igneous Suite, New South Wales, Australia: Implications for Magma Fertility

New geochronological and geochemical data for the barren and ore-associated suites from the Northparkes porphyry Cu-Au deposits, Australia, have implications for magma fertility. The Goonumbla and Wombin Volcanics and intrusions are barren in the Northparkes area. A sample from Wombin suite yielded a zircon U-Pb age of 433.8 ± 3.1 Ma, whereas the ore-associated porphyries yielded ages between 441.8 ± 3.7 and 436.3 ± 4.5 Ma. The bulk of the mineralization at Northparkes is associated with a K-feldspar-phyric quartz monzonite porphyry (K-QMP), which gave U-Pb zircon ages of 441.8 ± 3.7 and 441.1 ± 2.5 Ma. Whole-rock Sr-Nd isotope compositions of the Goonumbla, Wombin, and ore-associated suites are similar, with (87Sr/86Sr)i = 0.704112 to 0.704424 and εNd = 5.6 to 6.9, which is typical of primitive intraoceanic island arcs, and their Pb isotope values lie within the MORB array. Most of the zircons from the Wombin and ore-associated suites have arc mantle-like O-Hf isotope compositions, with δ18O values that vary from 6.13 to 4.95, and εHf(t) from 11.5 to 6. These results suggest that the Goonumbla, Wombin, and ore-associated suites originated from typical arc mantle. The magmas that produced the Goonumbla and Wombin suites were dominated by plagioclase-pyroxene fractionation, and the Wombin suite has a low oxidation state with ΔFMQ between ~0 and 1.5. They were relatively reduced and dry. This combination resulted in early sulfide saturation, probably without reaching fluid saturation. Trace element modeling shows that plagioclase-amphibole dominated the later stages of fractionation of the ore-associated suite, implying that it had a higher water content than the barren suites. It was also more oxidized (ΔFMQ from ~0 to 4). The result was late sulfide saturation, which was followed shortly thereafter by voluminous fluid release. As a consequence, the ore-forming fluid effectively transferred Cu and Au from the magma to the site of hydrothermal ore deposition. We suggest that the higher water content and oxidation state of the ore-associated suite was due to the deep underlying magma chamber, which was recharged by many more pulses of magma than the chamber that underlay the barren suites. This is more effective in raising the concentration of incompatible water and ferric iron in the residual melt than straight fractional crystallization. High oxygen fugacities and water contents played a significant role in determining the timing of sulfide and fluid saturation, respectively, and as a result, they had a critical influence on magma fertility.

Rapid, paced metamorphism of blueschists from laser-based Lu-Hf garnet-domain geochronology and LA-ICMPS trace element mapping

<p>Unravelling the timing and rate of subduction-zone metamorphism requires linking the composition of petrogenetic indicator minerals in blueschists and eclogites to time. Garnet is a key mineral in this regard, not in the least because it best records P-T conditions and changes therein and can be dated, using either Lu-Hf or Sm-Nd chronology. Bulk-grain garnet ages are the norm and can provide important and precise time constraints on reactions across both facies. Domain dating, i.e., dating of individual growth zones, moves beyond that. Domain dating by combining mechanical micro-milling and Sm-Nd chronology yielded important constraints on garnet-growth and fluid-release rates for blueschists (e.g., Dragovic et al., 2015). Developing this method for Lu-Hf chronology and, importantly, for "common-sized" garnet (≤1 cm) provides an important opportunity to further explore the potential of this approach.</p><p>We combined a low-loss micro-sampling technique in laser cutting with a refined Lu-Hf routine to precisely date multiple growth zones of a sub-cm-sized garnet in a blueschist. The targeted grain from a glaucophane-bearing micaschist from Syros Island, Greece, was chemically characterized by major- and trace-element mapping (EPMA, LA-ICPMS) and five zones were extracted using a laser mill. The three core and inner mantle zones are chemically comparable and identical in age within a 0.1 Myr precision (2σ). The outer two zones are chemically distinct and are resolvably younger (0.2-0.8 Myr). The timing of these two major garnet-growth episodes, together with the variations in trace-element chemistry, constrain important fluid-release reactions, such as chloritoid-breakdown. The data show that the integral history of garnet growth in subduction zones may be extremely short (<1 Myr), but may, even in that short timeframe, consist of multiple short pulses. Garnet-forming reactions clearly are localized and, thus, associated with focussed high-flux fluid flow. Beyond subduction-zone processes, our new protocol for zoned garnet Lu-Hf geochronology of "common-sized" garnet opens possibilities for constraining the causes and rates of garnet growth and in turn, the pace of tectonic processes in general.</p><p> </p><p><sub><em>Dragovic, B., Baxter, E.F. and Caddick, M.J., 2015. Pulsed dehydration and garnet growth during subduction revealed by zoned garnet geochronology and thermodynamic modeling, Sifnos, Greece. Earth and Planetary Science Letters, 413, pp.111-122.</em></sub></p>

The corundum conundrum: Constraining the compositions of fluids involved in metasomatic corundum formation.

<p>Corundum, including the variety ruby, is found in numerous locations in the Archaean North Atlantic Craton of southern Greenland. Corundum owes its occurrence to fluid-induced interaction among high-grade metamorphic lithologies of contrasting chemistry. Here, we present constraints on the conditions of corundum formation and the compositions of the fluids involved for the Storø and Maniitsoq ruby localities. We use thermodynamic modelling of mineral and mineral-fluid equilibria, and complement these with experimentally obtained data on mineral solubility to show that metasomatism took place at 650-725˚C and 7 kbar, involving a boron-rich, acidic fluid of low <em>f</em>O<sub>2</sub> and low X(CO<sub>2</sub>). Aqueous concentrations of aluminium are low and indicate that corundum saturation is the result of residual aluminium enrichment rather than aluminium mobilisation. Intrusion of the <em>ca.</em> 2.55 Ga Qôrqut granite and associated fluid release is the likely source of boron, and U-Pb dating of rutile inclusions is consistent with a temporal link between ruby formation and granite emplacement. Interaction with meta-dunite and Fe-sulfides modified the oxidized magmatic fluid, introduced SO<sub>4</sub>, and produced the reduced, high X<sub>Mg</sub> and K-rich fluid recorded by the corundum-bearing samples. These results highlight a complex interplay among lithologies involved in corundum-formation, but also demonstrate that corundum formation is a predictable part of the geological history where a magmatic intrusion expels a pulse of fluid through its lithologically heterogeneous carapace.</p>

Localized megathrust slip controlled by metasomatic reactions in subduction mélanges

<p>Aseismic megathrust slip down-dip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. Here, we show that subduction mélanges deformed at ~370–500 °C in warm-slab environments record fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions induced albitization of metapelite, resulting in depth-dependent rheological behavior. In a mélange deformed at ~370 °C, near the down-dip limit of the seismogenic zone, very fine-grained metasomatic albite facilitated grain boundary diffusion creep at stresses less than those in the surrounding metapelite and metabasalt, contributing to an overall decreased megathrust strength. In a mélange deformed at ~500 °C, near the mantle wedge corner, metasomatic reactions led to brittle fracturing, albite grain growth, and incorporation of strengthened albitized metapelite blocks into a chlorite-actinolite matrix deforming at locally elevated strain rate of ~10<sup>-10</sup> s<sup>-1</sup>. We suggest that metasomatic reactions facilitate localized changes in megathrust slip mode with depth, potentially providing a mechanism for change from viscous creep to SSEs with tremor.</p>

Tracking the recent dynamics of Mt. Vesuvius from joint investigations of ground deformation, seismicity and geofluid circulation

We reconstruct the composite dynamics of Mt. Vesuvius volcano in the period 2012–2019 from the study of ground deformation, seismicity, and geofluid (groundwater and fumarolic fluids) circulation and recognize complex spatio-temporal variations in these observables at medium (years) and short (months) time-scales. We interpret the observed patterns as the combined effect of structural changes affecting the volcanic edifice and variations of the dynamics of the hydrothermal system. In particular, we identify a change in the activity state of Mt. Vesuvius. After the activity reached minimum levels in 2014, the centroid of the surface manifestations migrated towards the SE. Episodic variations of co-seismic and aseismic deformation and fluid release, if analysed separately, would likely have been interpreted as pseudo-random oscillations of the background geophysical and geochemical signals. When organised in a comprehensive, multiparametric fashion, they shed light on the evolution of the volcano in 4D (x,y,z, time) space. These inferences play a crucial role in the formulation of civil protection scenarios for Mt. Vesuvius, a high risk, densely urbanized volcanic area which has never experienced unrest episodes in the modern era of instrumental volcanology.

Pyrite and pyrrhotite in a prograde metamorphic sequence, Hyland River region, SE Yukon: implications for orogenic gold

The distribution of pyrite and pyrrhotite is documented within an andalusite-sillimanite type (high-temperature, low-pressure) metasedimentary succession exposed in the Hyland River region of southeastern Yukon, Canada. The following metamorphic zones are recognized: chlorite, biotite, cordierite/staurolite (porphyroblast-in), andalusite, sillimanite, and K-feldspar + sillimanite. Pyrite occurs in the chlorite zone through the biotite zone, while pyrrhotite occurs from the chlorite zone to K-feldspar + sillimanite zone. The pyrite-pyrrhotite transition, therefore, occupies an interval in the chlorite and lower biotite zones that is terminated upgrade by a pyrite-out isograd in the upper part of the biotite zone or lowest grade part of the cordierite/staurolite zone. Pressure and temperature conditions of the rocks were estimated from phase equilibrium modelling and from Raman spectroscopy of carbonaceous material (RSCM) thermometry. Modelling indicates pressures of 3.7-4.1 kbar with temperatures of ~425 °C at the biotite isograd, 560-570 °C for chlorite-out/porphyroblast-in, ~575 °C for andalusite-in, 575-600 °C for the sillimanite isograd, and 645-660 °C at the K-feldspar + sillimanite isograd. RSCM temperatures are greater than or equal to 420 °C in the Chl zone, 500 °C at the Bt isograd, 525-550 °C for porphyroblast-in isograd, ~550 °C at the And isograd, and 580 °C at the Sil isograd. These results suggest the pyrite-pyrrhotite transition occurs from less than or equal to 420°C to ~560 °C. Thermodynamic modelling shows 0.6 wt. % H2O is released during metamorphism over the ~140 °C interval of the pyrite-pyrrhotite transition. The gradual release of fluid in the biotite zone is interpreted to have broadened the pyrite-pyrrhotite transition compared to other studies that predict a small interval of vigorous fluid release associated with volumetric chlorite consumption. Samples from the pyrite-pyrrhotite transition zone contain lower whole rock and pyrite Au values than samples from unmetamorphosed/lower rocks, suggesting that Au was removed from the rock at conditions below the pyrite-pyrrhotite transition (<420 °C). The chlorite zone and higher-grade metamorphic rocks of the Hyland River area do not appear to be a plausible source region for orogenic gold.

The role of the antigorite + brucite to olivine reaction in subducted serpentinites (Zermatt, Switzerland)

Metamorphic olivine formed by the reaction of antigorite + brucite is widespread in serpentinites that crop out in glacier-polished outcrops at the Unterer Theodulglacier, Zermatt. Olivine overgrows a relic magnetite mesh texture formed during ocean floor serpentinization. Serpentinization is associated with rodingitisation of mafic dykes. Metamorphic olivine coexists with magnetite, shows high Mg# of 94–97 and low trace element contents. A notable exception is 4 µg/g Boron (> 10 times primitive mantle), introduced during seafloor alteration and retained in metamorphic olivine. Olivine incorporated 100–140 µg/g H2O in Si-vacancies, providing evidence for low SiO2-activity imposed by brucite during olivine growth. No signs for hydrogen loss or major and minor element diffusional equilibration are observed. The occurrence of olivine in patches within the serpentinite mimics the former heterogeneous distribution of brucite, whereas the network of olivine-bearing veins and shear zones document the pathways of the escaping fluid produced by the olivine forming reaction. Relic Cr-spinels have a high Cr# of 0.5 and the serpentinites display little or no clinopyroxene, indicating that they derive from hydrated harzburgitic mantle that underwent significant melt depletion. The enrichment of Mg and depletion of Si results in the formation of brucite during seafloor alteration, a pre-requisite for later subduction-related olivine formation and fluid liberation. The comparison of calculated bulk rock brucite contents in the Zermatt-Saas with average IODP serpentinites suggests a large variation in fluid release during olivine formation. Between 3.4 and 7.2 wt% H2O is released depending on the magnetite content in fully serpentinized harzburgites (average oceanic serpentinites). Thermodynamic modelling indicates that the fluid release in Zermatt occurred between 480 °C and 550 °C at 2–2.5 GPa with the Mg# of olivine varying from 68 to 95. However, the majority of the fluid released from this reaction was produced within a narrow temperature field of < 30 °C, at higher pressures 2.5 GPa and temperatures 550–600 °C than commonly thought. Fluids derived from the antigorite + brucite reaction might thus trigger eclogite facies equilibration in associated metabasalts, meta-gabbros, meta-rodingites and meta-sediments in the area. This focused fluid release has the potential to trigger intermediate depths earthquakes at 60–80 km in subducted oceanic lithosphere.

Decarbonation Reactions Involving Ankerite and Dolomite under upper Mantle P,T-Parameters: Experimental Modeling

An experimental study aimed at the modeling of dolomite- and ankerite-involving decarbonation reactions, resulting in the CO2 fluid release and crystallization of Ca, Mg, Fe garnets, was carried out at a wide range of pressures and temperatures of the upper mantle. Experiments were performed using a multi-anvil high-pressure apparatus of a “split-sphere” type, in CaMg(CO3)2-Al2O3-SiO2 and Ca(Mg,Fe)(CO3)2-Al2O3-SiO2 systems (pressures of 3.0, 6.3 and 7.5 GPa, temperature range of 950–1550 °C, hematite buffered high-pressure cell). It was experimentally shown that decarbonation in the dolomite-bearing system occurred at 1100 ± 20 °C (3.0 GPa), 1320 ± 20 °C (6.3 GPa), and 1450 ± 20 °C (7.5 GPa). As demonstrated by mass spectrometry, the fluid composition was pure CO2. Composition of synthesized garnet was Prp83Grs17, with main Raman spectroscopic modes at 368–369, 559–562, and 912–920 cm−1. Decarbonation reactions in the ankerite-bearing system were realized at 1000 ± 20 °C (3.0 GPa), 1250 ± 20 °C (6.3 GPa), and 1400 ± 20 °C (7.5 GPa). As a result, the garnet of Grs25Alm40Prp35 composition with main Raman peaks at 349–350, 552, and 906–907 cm−1 was crystallized. It has been experimentally shown that, in the Earth’s mantle, dolomite and ankerite enter decarbonation reactions to form Ca, Mg, Fe garnet + CO2 assemblage at temperatures ~175–500 °C lower than CaCO3 does at constant pressures.