First magmatism in the New England Batholith, Australia: Forearc and arc-back-arc components in the Bakers Creek Suite gabbros

The New England Orogen, eastern Australia, was established as an outboard extension of the Lachlan Orogen through the migration of magmatism into forearc basin and accretionary prism sediments. Widespread S-type granitic rocks of the Hillgrove and Bundarra supersuites represent the first pulse of magmatism, followed by I-A nd A-types typical of circum-Pacific extensional accretionary orogens. Associated with the former are a number of small tholeiite-gabbroic to intermediate bodies of the Bakers Creek Suite, which sample the heat source for production of granitic magmas and are potential tectonic markers indicating why magmatism moved into the forearc and accretionary complexes rather than rifting the old Lachlan Orogen arc. The Bakers Creek Suite gabbros capture an early (∼305Ma) forearc basalt-like component with low Th/Nb and with high Y/Zr and Ba/La, recording melting in the mantle wedge with little involvement of a slab flux and indicating forearc rifting. Subsequently, arc-back-arc like gabbroic magmas (305-304Ma) were emplaced, followed by compositionally diverse magmatism leading up to the main S-type granitic intrusion (∼290Ma). This trend in magmatic evolution implicates forearc and other mantle wedge melts in the heating and melting of fertile accretion complex sediments and relatively long (∼10Myr) timescales for such meltingThis research was partially supported by an Australian National University PhD Research Scholarship to Seann J. McKibbin, who is currently a postdoctoral fellow of the Research Foundation – Flanders (Fonds Wetenschapplijke Onderszoek; FWO

Triple oxygen isotope variations in magnetite from iron-oxide deposits, central Iran, record magmatic fluid interaction with evaporite and carbonate host rocks

Oxygen isotope ratios in magnetite can be used to study the origin of iron-oxide ore deposits. In previous studies, only 18O/16O ratios of magnetite were determined. Here, we report triple O isotope data (17O/16O and 18O/16O ratios) of magnetite from the iron-oxide-apatite (IOA) deposits of the Yazd and Sirjan areas in central Iran. In contrast to previous interpretations of magnetite from similar deposits, the triple O isotope data show that only a few of the magnetite samples potentially record isotopic equilibrium with magma or with pristine magmatic water (H2O). Instead, the data can be explained if magnetite had exchanged O isotopes with fluids that had a mass-independently fractionated O isotope composition (i.e., MIF-O), and with fluids that had exchanged O isotopes with marine sedimentary carbonate rocks. The MIF-O signature of the fluids was likely obtained by isotope exchange with evaporite rocks of early Cambrian age that are associated with the IOA deposits in central Iran. In order to explain the triple O isotope composition of the magnetite samples in conjunction with available iron isotope data for magnetite from the deposits, we propose that magnetite formed from magmatic fluids that had interacted with evaporite and carbonate rocks at high temperatures and at variable water/rock ratios; e.g., magmatic fluids that had been released into the country rocks of a magma reservoir. Additionally, the magnetite could have formed from magmatic fluids that had exchanged O isotopes with SO2 and CO2 that, in turn, had been derived by the magmatic assimilation and/or metamorphic breakdown of evaporite and carbonate rocks

Cosmic spherules from Widerøefjellet, Sør Rondane Mountains (East Antarctica)

A newly discovered sedimentary accumulation of micrometeorites in the Sør Rondane Mountains of East Antarctica, close to the Widerøefjellet summit at ~2750 meter above sea level, is characterized in this work. The focus here lies on 2099 melted cosmic spherules larger than 200 μm, extracted from 3.2 kg of sampled sediment. Although the Widerøefjellet deposit shares similarities to the micrometeorite traps encountered in the Transantarctic Mountains, both subtle and more distinct differences in the physicochemical properties of the retrieved extraterrestrial particles and sedimentary host deposits are discernable (e.g., types of bedrock, degree of wind exposure, abundance of metal-rich particles). Unlike the Frontier Mountain and Miller Butte sedimentary traps, the size fraction below 240 μm indicates some degree of sorting at Widerøefjellet, potentially through the redistribution by wind, preferential alteration of smaller particles, or processing biases. However, the cosmic spherules larger than 300 μm appear largely unbiased following their size distribution, frequency by textural type, and bulk chemical compositions. Based on the available bedrock exposure ages for the Sør Rondane Mountains, extraterrestrial dust is estimated to have accumulated over a time span of ~1 to 3 Ma at Widerøefjellet. Consequently, the Widerøefjellet collection reflects a substantial reservoir to sample the micrometeorite influx over this time interval. Petrographic observations and 3D microscopic CT imaging are combined with chemical and triple-oxygen isotopic analyses of silicate-rich cosmic spherules larger than 325 μm. The major element composition of 49 cosmic spherules confirms their principally chondritic parentage. For 18 glassy, 15 barred olivine, and 11 cryptocrystalline cosmic spherules, trace element concentrations are also reported on. Based on comparison with evaporation experiments reported in literature and accounting for siderophile and chalcophile element losses during high-density phase segregation and ejection, the observed compositional sequence largely reflects progressive heating and evaporation during atmospheric passage accompanied by significant redox shifts, although the influence of (refractory) chondrite mineral constituents and terrestrial alteration cannot be excluded in all cases. Twenty-eight cosmic spherules larger than 325 μm analyzed for triple-oxygen isotope ratios confirm inheritance from mostly carbonaceous chondritic precursor materials (~55% of the particles). Yet, ~30% of the measured cosmic spherules and ~50% of all glassy cosmic spherules are characterized by oxygen isotope ratios above the terrestrial fractionation line, implying genetic links to ordinary chondrites and parent bodies currently unsampled by meteorites. The structural, textural, chemical, and isotopic characteristics of the cosmic spherules from the Sør Rondane Mountains, and particularly the high proportion of Mg-rich glass particles contained therein, imply a well-preserved and representative new sedimentary micrometeorite collection from a previously unstudied region in East Antarctica characterized by distinct geological and exposure histories

Mn-Cr chronology and trace element systematics of olivine from angrite and pallasite meteorites

{u2075}{u00B3}Mn-{u2075}{u00B3}Cr short-lived nuclide dating of certain meteoritic materials can be achieved in situ using Secondary Ion Mass Spectrometry (SIMS) as an alternative to bulk methods. This thesis reports on the Mn-Cr systematics of olivine from two differentiated meteorite classes: angrites and pallasites. Much of the previously published SIMS Mn-Cr data relies on reference materials that are not compositionally matched to unknowns, particularly with respect to olivine (e.g. Mg-rich San Carlos olivine is used alongside Fe-rich meteoritic olivine). Previous studies may suffer from systematic biases due to the differing sensitivities of SIMS to Mn and Cr. To assess olivine Mn-Cr relative sensitivity factors (RSFs), synthetic olivines with varying Mg-Fe-Ca components were produced using gas-mixing furnaces and piston-press apparatus, and analysed by SIMS and LaserAblation Inductively-Coupled-Plasma Mass-Spectrometry (LA-ICP-MS). RSF is correlated with Fe-content of olivine; San Carlos olivine is therefore an inappropriate standard for most SIMS Mn-Cr work. For angrites, a mixing model was used to account for RSF, using three of the synthetic olivines. Two quenched angrites (D'Orbigny and Sahara 99555) and two plutonic angrites (NWA 4590 and 4801) were analysed with Sensitive High-mass Resolution Ion Micro-Probe (SHRIMP) Reverse Geometry (-RG) and SHRIMP-II. The initial {u2075}{u00B3}Mn/{u2075}{u2075}Mn for D'Orbigny and Sahara 99555 was found to be 3.52 ({u00B1}0.16) x 10-6 and 3.57 ({u00B1}0.22) x 10-6 , which is in agreement with some previous work but not with the SIMS study of Sugiura et al. (2005). NWA 4590 and 4801 were found to have initial {u2075}{u00B3}Mn/{u2075}{u2075}Mn of 1.00 ({u00B1}0.27) x 10-6 and 0.52 ({u00B1}1.2) x 10-6 respectively; less precise than but in agreement with previous work. To check if the difference between this study and Sugiura et al. (2005) represents inter-laboratory bias or meteorite heterogeneity, the same piece of D'Orbigny as analysed in that study was reanalysed using SHRIMP-RG; initial {u2075}{u00B3}Mn/{u2075}{u2075}Mn of 3.61 ({u00B1}0.39) x 10-6 was found, still in disagreement with Sugiura et al. (2005). The source of bias remains elusive; it is not due to differing RSF since both studies found similar values for this variable. For pallasite olivine, trace-elemental maps were made using LA-ICPMS to provide context for Mn-Cr measurements. Two contrasting pallasites were selected: Brahin, which has fragmental olivine, and Brenham, which has rounded olivine. Both olivine types are normally zoned for divalent elements Ni and Co, but non-divalent elements AI, Cr, Ti are heterogeneously distributed, often with no relationship to grain morphology. In Brenham, these complex elemental structures are superimposed on normal zoning. Preservation of these structures probably hinges on slow diffusion of AI, which other non-divalent elements associate with to maintain charge-balance. SIMS measurements indicate initial {u2075}{u00B3}Mn/{u2075}{u2075}Mn of -2.25 ({u00B1}4.9) x 10-6 and 2.55 ({u00B1}0.62) x 10-5 for Brahin and Brenham respectively; the latter is an unreasonably high value and is unlikely to reflect Mn-Cr chronology. This apparent value could reflect loss of {u2075}{u2075}Mn after decay of {u2075}{u00B3}Mn, although the geochemical behaviour of Mn is unfavourable for such a model. Instead, excess {u2075}{u00B3}Cr* derived from Mn-rich phosphates originally present in the metallic groundmass could have been introduced into olivine rims during the grain growth and annealing process which formed rounded olivine

Petrogenesis of D'Orbigny-like angrite meteorites and the role of spinel in the angrite source

Angrite meteorites are samples of early planetesimal magmatic rocks, distinguished from more typical basaltic eucrites by compositions that are silica undersaturated, relatively oxidized, and with high CaO/Al2O3. The latter is not expected from nebular, chondritic materials that might form a primitive mantle, such as a source enriched in refractory inclusions with fixed CaO/Al2O3 (e.g., CV chondrite). Here we present results of reversal crystallization experiments for two possible parental angrite compositions (approximating the D'Orbigny meteorite) to investigate the role of spinel as a sink for Al2O3. This mineral has previously been produced with angritic melts during forward melting of CV chondrite and may be abundant in the angrite source. At oxidizing conditions, we confirm that spinel is a liquidus phase and that angritic magmas form near the olivine-anorthite-spinel-liquid peritectic. A stability gap separates Al-rich liquidus spinels and lower temperature spinels, the latter of which are similar to those in basaltic eucrites. Al-rich spinel is likely more abundant in the angritic source than other Fe-rich core-forming components such as metal or sulfide, and a CV chondrite-like composition generates most features of angrite magmas by fractionation of observed olivine and liquidus spinel. Direct CaO excess, via carbonate addition, is therefore limited. In this model, discrepancies remain for Li, Sc, Cr(-Al), and Ba, which may record local accretion conditions or early processing. The possible role of spinel as a sink for Al-26 may have strong influence on the thermal evolution of the angrite parent body.This research was supported by an Australian National University PhD scholarship to SMcK, who is currently a postdoctoral fellow of the Research Foundation—Flanders

Mn–Cr dating of Fe- and Ca-rich olivine from 'quenched' and 'plutonic' angrite meteorites using Secondary Ion Mass Spectrometr

Angrite meteorites are suitable for Mn–Cr relative dating (⁵ᶟMn decays to ⁵ᶟCr with a half life of 3.7 Myr) using Secondary Ion Mass Spectrometry (SIMS) because they contain olivine and kirschsteinite with very high ⁵⁵Mn/⁵²Cr ratios arising from very low Cr concentrations. Discrepant Mn–Cr and U–Pb time intervals between the extrusive or ‘quenched’ angrite D’Orbigny and some slowly cooled or ‘plutonic’ angrites suggests that some have been affected by secondary disturbances, but this seems to have occurred in quenched rather than in slow-cooled plutonic angrites, where such disturbance or delay of isotopic closure might be expected. Using SIMS, we investigate the Mn–Cr systematics of quenched angrites to higher precision than previously achieved by this method and extend our investigation to non-quenched (plutonic or sub-volcanic) angrites. High values of 3.54 (±0.18) × 10⁻⁶ and 3.40 (±0.19) × 10⁻⁶ (2-sigma) are found for the initial ⁵ᶟMn/⁵⁵Mn of the quenched angrites D’Orbigny and Sahara 99555, which are preserved by Cr-poor olivine and kirschsteinite. The previously reported initial ⁵ᶟMn/⁵⁵Mn value of D’Orbigny obtained from bulk-rock and mineral separates is slightly lower and was probably controlled by Cr-rich olivine. Results can be interpreted in terms of the diffusivity of Cr in this mineral. Very low Cr concentrations in Ca-rich olivine and kirschsteinite are probably charge balanced by Al; this substitutes for Si and likely diffuses at a very slow rate because Si is the slowest-diffusing cation in olivine. Diffusion in Cr-rich Mg–Fe olivine is probably controlled by cation vacancies because of deficiency in charge-balancing Al and is therefore more prone to disturbance. The higher initial ⁵ᶟMn/⁵⁵Mn found by SIMS for extrusive angrites is more likely to reflect closure of Cr in kirschsteinite at the time of crystallisation, simultaneous with closure of U–Pb and Hf–W isotope systematics for these meteorites obtained from pyroxenes. For the younger angrites Northwest Africa (NWA) 4590 and 4801 we have found initial ⁵ᶟMn/⁵⁵Mn values which are consistent with more precise work, at 0.90 (±0.4) × 10⁻⁶ and 0.13 (±1.1) × 10⁻⁶ respectively. Our work shows that SIMS can usefully constrain and distinguish the ages of angrites of different petrologic groups. In reviewing the petrology of angrites, we suggest that NWA 2999, 4590, and 4801 underwent a secondary partial melting and Cr (+/−Pb) disturbance event that the sub-volcanic Lewis Cliff 86010, and perhaps the plutonic Angra dos Reis, did not. With our higher initial ⁵ᶟMn/⁵⁵Mn for D’Orbigny and Sahara 99555 as well as previous data, a combined quenched angrite initial 53Mn/55Mn of 3.47 (±0.12) × 10⁻⁶ (2-sigma, MSWD 1.00) yields consistent Mn–Cr and U–Pb intervals between these angrites and Lewis Cliff 86010. Discrepant Mn–Cr timescales for other plutonic and sub-volcanic angrites represents resetting during the secondary partial melting event at ∼4557.2 Ma and indicates a relative order of disturbance of isotope systems: Mn–Cr in olivine before U–Pb in pyroxene, with Hf–W in pyroxene being the most resistant.This research was supported by grants DP034277 and DP0666751 from the Australian Research Council to TRI and YA, and an Australian National University PhD Research Scholarship

A re-evaluation of the Mn-Cr systematics of olivine from the angrite meteorite D'Orbigny using secondary ion mass spectrometry

'Quenched' angrite meteorites are among the best time markers of igneous activity in early formed planetesimals of the Solar System. They can be precisely dated by the Mn-Cr extinct nuclide decay system because they contain olivine with high Mn/Cr. Never

Mn-Cr relative sensitivity Factors for Secondary Ion Mass Spectrometry analysis of Mg-Fe-Ca olivine and implications for the Mn-Cr chronology of meteorites

Secondary Ion Mass Spectrometry (SIMS) is used to date meteoritic olivine using the 53Mn-53Cr short-lived decay scheme. This involves simultaneously measuring the Cr-isotope composition and Mn/Cr ratio of a sample, but the relative sensitivity of SIMS t

New constraints on the formation of main group pallasites derived from in situ trace element analysis and 2D mapping of olivine and phosphate

Pallasites are stony-iron meteorites consisting mainly of olivine and Fe-Ni metal with a formation history that remains widely debated. Despite their simple mineralogy, relatively limited data are available on their geochemistry and the lateral elemental distribution in individual pallasite olivine crystals. In this work, laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) was used for the elemental analysis of both pallasite olivine and phosphate phases, including 2D trace element mapping of olivine crystals. While the results obtained are in good agreement with literature values, important differences are observed for Al and Ni concentrations compared with bulk analytical methods. The element distribution maps reveal complex zoning in pallasite olivine, which can be explained based on i) diffusion gradients formed during olivine cooling, ii) the crystal chemistry of element substitution due to charge-balancing, and iii) inherited features of olivine before the metal-olivine mixing. Oscillatory zoning in an olivine crystal from Imilac provides strong evidence for the olivine not being a restite of partial melting, but rather having crystallized from a melt. Concentrations of Cr and Al are correlated both within single olivine crystals and between olivine crystals of different pallasites, forming a 1:1 linear trend. This likely results from a spinel-type charge-balancing substitution mechanism in pallasite olivine. Additionally, principle component analysis of laterally resolved multi-element concentration data of pallasite olivine provides an independent measure of the genetic relationships between the different main group pallasites within their parent body (-ies). Rare earth element (REE) contents of pallasite phosphate grains range from concentrations typically measured for chondritic or primitive achondrite-like phosphates to more (light) RE-depleted signatures, revealing the overall primitive nature of the melts from which the phosphate minerals crystallized. Phosphate in pallasites may thus have formed through the consumption of apatites during pro-grade metamorphism, melting, and melt extraction. Combined, the trace element signatures of olivine and phosphate in pallasites of the main group (PMG) suggest that these meteorites are common products of planetary processes on chondritic or primitive achondrite-like precursor bodies, with the different subgroups highlighting distinct formation and evolution histories

Methodological Aspects of Criteria for Assessing the Development Level of Cities

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