A comparison of the Nickel and the conventional geothermometers with respect to the Jagersfontein and the Matsoku kimberlite peridotite xenolits

Bibliography: leaves 116-125.The accuracy of the experimental (Canil, 1994; T-Canil) and the empirical (Ryan et al., 1996; T-Ryan) calibrations of the Ni geothermometer has been evaluated on two suites of geochemically and geothermobarometrically well characterised mantle xenoliths from Matsoku and Jagersfontein by comparison to the more commonly used conventional geothermometers. The two published calibrations of the Ni geothermometer are in agreement to within ±500C in the temperature range of ~900°C to 1200°C. Outside this temperature range, the two calibrations differ by between 75 and 150°C. The importance of the Ni geothermometer in diamond exploration and the studies of the mantle makes the resolution of this discrepancy very important. In addition to issues of calibration, errors in the determination of trace levels of Ni abundances in garnets may affect the accuracy of the Ni geothermometer. A 'reliable' Ni in garnet dataset was, therefore, required to minimise errors associated with Ni compositions used in temperature determination by Ni geothermometry. Thus, Ni compositions of garnets determined by PIXE, LA-ICP-MS and SIMS were compared to select the most 'reliable' dataset. Four matrix-matched secondary garnet standards were developed for the cross-checking and testing of the accuracy of data. The standards were developed by multi-method analyses, which included PIXE (using both the GeoPIXE and GUPIX software programmes for data reduction), LA-ICP-MS and solution ICP-MS. PIXE and LA-ICP-MS data were found to agree to within their 2σ errors of, respectively, 2 to 10% and 4 to 12% for a concentration range of ~15 to 112 ppm Ni. However, PIXE analyses were found to be superior in terms of smaller beam width, allowing several repeat analyses, and analysis of small and altered garnets thereby producing a relatively larger dataset

Geothermobarometry of Cordierite-Bearing Metapelites of the Hudson Highlands, Southeastern NY

Various P-T determinations have been reported on metasedimentary rock suites in the New Jersey and Hudson Highlands using different geothermobarometers, but all have yielded relatively large P-T ranges and none have used cordierite in a quantitative way. The presence of variable cordierite and its narrow stability field in metapelitic gneisses provides an excellent opportunity to better constrain the peak P-T conditions. Calcium depleted, clay-rich, arkosic sedimentary protolith was deposited during the Mesoproterozoic in a continentaloceanic arc basin during the Elzevirian orogeny (~1300-1200 Ma). The Ottawan metamorphic event (~1090-1020 Ma) transformed this protolith into the metasedimentary unit exposed in the Hudson Highlands, southeastern NY (Gates et al., 2001). Twenty samples from outcrops with inlerlayered metapelites and metapsammitic gneisses were sampled from four localities within Harriman State Park, NY in the western Hudson Highlands. The determination of the metamorphic conditions based on the prograde phases of the biotite-cordierite, biotite-cordieritegarnet, and biotite-garnet metapelitic gneisses is the focal point of interest in the current study. Whole-rock major elements analysis of the composite suite of samples defines the differentiation of the metapsammitic and metapelitic gneisses as: metapelites = weight % K2O/ CaO \u3e 1; metapsammitic gneiss = weight % K2O/ CaO \u3c1. Chemical composition and weight % oxide data was collected from five samples that displayed co-existing garnet, biotite, and locally occurring plagioclase and cordierite, to perform geothermobarometry. Garnet-cordierite and garnet-biotite thermometers and grt-sil-crd-qtz, garnet-aluminasilicate-plagioclase (GASP), and garnet-biotite-plagioclase-quartz (GBPQ) barometers were used. The P-T determinations for five samples with appropriate mineralogy are as follows: QB-7: 588 °C (±38) at 4.4 kilobars (± 2) SB-5: 593 °C (± 19) at 5.3 kilobars (± 0.4) QB-1: 654 °C (±18) at 5.1 kilobars (± 0.1) TB-3: 745 °C (± 18) at 6.0 kilobars (± 0.2) SB-7: 730 °C (± 19) at 6.1 kilobars (± 0.1) The P-T reported is indicative of a transition from middle-amphibolite to lower-granulite facies metamorphism. The appearance and disappearance of cordierite with increasing metamorphic grade is expressed as biotite-cordierite gneiss-\u3ebiotite-cordierite-garnet gneiss-\u3egarnet-biotite gneiss. The presence of cordierite in the metapelites of Hudson Highlands is not as imperative as previously thought in regards to determining peak metamorphism conditions. However, its appearance and disappearance provides a unique opportunity to determine the rates of metamorphic processes for the metasedimentary unit exposed in the Hudson Highlands

Deciphering magmatic processes in response to growth and destruction at Taranaki Volcano, New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Earth Sciences at Massey University (Manawatū campus), Palmerston North, New Zealand

Taranaki Volcano is an atypical back-arc andesitic stratovolcano located on the Taranaki Peninsula of the North Island in New Zealand. Volcanism started c. 200 kyr ago and the edifice went through at least 14 sector collapse events with the same number of corresponding growth cycles, expanding the surrounding volcanic apron over its lifetime, which presently is populated and farmed. Previous studies focussed on the modern edifice (<14 ka), the tephra deposits (<29 ka), and the volcaniclastic stratigraphy over the 200 kyr volcanic history. However, there is a significant knowledge gap in relation to the evolution of the Taranaki volcanic system during successive edifice growth cycles (i.e. inter-collapse states) and the response of the magmatic system to unloading of the edifice. In order to unravel the subaerial and subvolcanic aspects of these growth cycles, the sedimentary textures, lithologies and stratigraphy of the volcaniclastic mass-flow deposits were investigated in the southwestern sector of the Taranaki ring plain, which provides a nearly continuous stratigraphic record of the time period of c. 65-34 ka that comprises three edifice regrowth phases. Volcaniclastic mass-flow deposits were the focus of this study, providing an opportunity to explore sedimentological and geochemical characteristics of eruptive periods of Taranaki Volcano, as there are no proximal sites available close to the modern edifice. Due to the well-exposed volcaniclastic successions along the coastline of Taranaki Volcano, a classification framework was established for the globally applicable categorization of volcaniclastic mass-flow deposits in ring plain settings. Additionally, the development of the stratigraphic model of the time period c. 65-34 ka highlighted the high frequency of widely distributed volcanic mass-flow events, approximately occurring 4-5 times in every 4-10 kyr. As these deposits encompass the characteristics of eruptive periods, vesicular pyroclasts were analysed in order to investigate the time related aspects of the Taranaki magmatic system during edifice growth cycles. Based on the analysis of 220 lapilli-sized pyroclasts, whole-rock compositions were reproduced by a mixing model, indicating that the volcanic rocks originate from melt-mush mixing processes. The mixing ratios varied within the individual growth cycles and further revealed that the melt-mush ratios define the produced whole-rock compositions, where the assimilant endmember is a primitive mush and the melt endmember is a trachyandesitic ascending melt. The temporal variation of the pyroclast geochemistry showed that within the inter-collapse states (i.e. growth cycles), the range of bulk rock compositions display a broadening pattern over time towards pre-collapse states. This chemostratigraphic pattern was attributed to edifice loading affecting crustal magmatic processes over time. Whole-rock geochemical results demanded a detailed investigation of the crystal mush, from which Taranaki Volcano is fed, producing the basaltic to trachyandesitic magmas. The textural and chemical analyses of the Taranaki crystal cargo revealed reoccurring and specific crystal patterns and proved the antecrystic origin of the majority of the clinopyroxene, plagioclase and amphibole crystals. The observations highlighted that the Taranaki magmatic plumbing system involved repeated magma recharge, melt-mush mixing and crystal convection processes affecting the produced magmas within the time period of c. 65-34 ka. Crystallisation conditions (i.e. P-T-H₂O) were estimated applying thermobarometric modelling on clinopyroxene and amphibole phenocrysts. Results of the clinopyroxene rim equilibration modelling suggested source depths of approximately 26-12 km (±7.5), which outline the mid- to lower-crustal regions and further indicate polybaric rim crystallization processes. In addition, hygrometry approximations indicated that within the individual growth cycles, melts with various properties (2.9-3.7 to 3.9-4.8 wt% H₂O) arrived at different regions of the crystal mush at mid-crustal depths. Altogether, textural, chemical and hygrothermobarometric analyses outlined the spatiotemporal complexity of the Taranaki magmatic plumbing system and the connected magmatic processes of andesitic volcanism. The interconnected sedimentological and geochemical studies of this research provided an understanding of mid-crustal melt-mush mixing processes producing the Taranaki magmas within a complex, interconnected vertical mush domain affected by the temporal influence of edifice loading and unloading during consecutive edifice growth cycles

Table Cape vent xenolith suite, northwest Tasmania: Mineralogy and implications for crust-mantle lithology and Miocene geotherms in Tasmania.

The Miocene Table Cape vent erupted a diverse mantle-crust xenolith suite within its fractionated nephelinitic matrix. Assemblages include mantle metaperidotites, garnet-metawebsterites and rarer garnet-metadinopyroxenitcs, garnct-mctawehrlites, metawebsterites and crusta] two-pyroxene granulites. Most metapyroxenires and granulites represent the Ti-Al-bearing augite suite and their bulk geochemistry indicates transitional olivine basalt magmatic affinities. Metasomatised, hydrous lithologies are only rarely present. Co-existing pyroxenes in the xenoliths provide re-equilibration temperature estimates from 860-1 0750C (for the whole suite) and temperature-pressure estimates for the garnet metawebsterires from 1055-1 070°C and 1.2-1.4 CPa. This gives a Miocene mantle geotherm gradient at least 80--130°C higher than the Southeast Australian (SEA) western Victorian geotherms. However, considerations of Moho from new seismic surveys below Table Cape (~.32 km) suggest that the indicated georherm is more strongly perturbed in its lower levels than at the mantle-crust transition. This localised perturbation is attributed to magma chamber in the mantle (Boat Harbour just prior to Table Cape vent activity. Tasmanian Miocene geotherms (Table Cape, Bow Hill) achieve relatively high gradients and reinforce suggestions of local variation in East Australian geothermal gradients, They illustrate the potential complexities in com paring xenolith- derived geotherms from different areas in general, both from thermometer/barometer selection and from associated magmatic heat inputs

PRESSURE-TEMPERATURE EVOLUTION OF METAPELITES WITHIN THE ANACONDA METAMORPHIC CORE COMPLEX, SOUTHWESTERN MONTANA

This study shows the complete metamorphic evolution of metapelitic footwall rocks in the Anaconda metamorphic core complex, located in southwestern Montana. Pseudosections were constructed using the computer program THERMOCALC (v. 3.26) in conjunction with the internally consistent dataset of Holland and Powell (1998, version 5.5, updated Nov. 2003) in the model system NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3- SiO2-H2O). Microprobe data reveals that garnet is very almandine rich and contains very little manganese, thus not requiring MnO as a component in the model system. X-ray mapping of garnet within metapelites indicate only one growth phase. Petrographic analysis in conjunction with pseudosections reveal peak metamorphism occurred at 3.3-5.3 kbars and 590-665°C with an assemblage of biotite + garnet + sillimanite + quartz + plagioclase + muscovite. Chemical composition of garnet, biotite, and plagioclase all verify peak metamorphic conditions on contoured pseudosections. Evolution of footwall rocks occurred along a prograde, clockwise path that was largely stable within sillimanite stability as evidenced by its abundance. These amphibolite facies metapelites equilibrated at or near the solidus

Thermodynamic modelling of metamorphic processes: state of the art in pseudosection approach

Understanding global-scale orogenic processes related to supercontinents, and their relationship to the secular evolution of the Earth’s lithosphere, represent important challenges for Earth scientists today. The record of these processes is preserved in the microstructures, mineral assemblages and mineral compositions of lithospheric rocks exhumed to the Earth’s surface. Given a well-characterized microstructural evolution, thermodynamic modelling is the key to quantifying changes in pressure and temperature, with the rate of these changes being provided by rock-forming and accessory mineral-based geochronology. Thus, metamorphic rocks provide Pressure– Temperature–time–deformation (P–T–t–d) data that help to parameterize orogenic processes

Earthquakes as Precursors of Ductile Shear Zones in the Dry and Strong Lower Crust

The rheology and the conditions for viscous flow of the dry granulite facies lower crust are still poorly understood. Viscous shearing in the dry and strong lower crust commonly localizes in pseudotachylyte veins, but the deformation mechanisms responsible for the weakening and viscous shear localization in pseudotachylytes are yet to be explored. We investigated examples of pristine and mylonitized pseudotachylytes in anorthosites from Nusfjord (Lofoten, Norway). Mutual overprinting relationships indicate that pristine and mylonitized pseudotachylytes are coeval and resulted from the cyclical interplay between brittle and viscous deformation. The stable mineral assemblage in the mylonitized pseudotachylytes consists of plagioclase, amphibole, clinopyroxene, quartz, biotite,6garnet6K-feldspar. Amphibole-plagioclase geothermobarometry and thermodynamic modeling indicate that pristine and mylonitized pseudotachylytes formed at 650\u20137508C and 0.7\u20130.8 GPa. Thermodynamic modeling indicates that a limited amount of H2O infiltration (0.20\u20130.40 wt. %) was necessary to stabilize the mineral assemblage in the mylonite. Diffusion creep is identified as the main deformation mechanisms in the mylonitized pseudotachylytes based on the lack of crystallographic preferred orientation in plagioclase, the high degree of phase mixing, and the synkinematic nucleation of amphiboles in dilatant sites. Extrapolation of flow laws to natural conditions indicates that mylonitized pseudotachylytes are up to 3 orders of magnitude weaker than anorthosites deforming by dislocation creep, thus highlighting the fundamental role of lower crustal earthquakes as agents of weakening in strong granulites