A Tale of Two Arcs: Petrogenesis of Ultramafic Xenoliths Sampling the Upper Mantle Wedge Beneath the West Bismarck Island Arc

Peridotite xenoliths transported to the surface in basaltic magma from the upper mantle wedge beneath the West Bismarck island arc, Papua New Guinea, present a rare opportunity to assess the nature of the mantle wedge in an active intra-oceanic island arc. This thesis reports comprehensive new geochemical and isotopic data for harzburgites, pyroxenites and dunites, from the island of Ritter, in order to understand how partial melting and hydrous metasomatism generate chemically, isotopically and physically distinct mantle. The highly depleted major and moderately incompatible trace element composition of residual phases and the radiogenic strontium isotope composition of texturally well-equilibrated harzburgites are best explained by hydrous partial melting and metasomatism associated with a previous period of subduction. Harzburgites that record textural evidence for melt-rock reaction, meanwhile, have elevated equilibration temperatures, oxidised spinel compositions, elevated olivine water contents and strontium isotope compositions identical to regionally erupted basalts. These features reflect interaction between ambient mantle and primary hydrous, oxidised basaltic melts in the upper mantle wedge. Modelling of trace element diffusion profiles in olivine constrains this event to approximately one year before exhumation. The low water contents of both coarse-grained olivine and orthopyroxene are consistent with equilibrium in chemically depleted upper mantle. The absence of hydrated silicon vacancies in olivine despite overall increases in water content during melt-rock reaction indicates that the mantle wedge may not change significantly in mechanical strength during hydrous melting and metasomatism. Chemical and radiogenic signatures of subduction are thus more likely to survive convective homogenisation. The whole-rock budget of highly siderophile elements (HSE) is contained within heterogeneously distributed trace sulphides and inferred alloy phases, and is controlled by both partial melting and metasomatism. An absence of any correlation of Sr isotopes with either HSE or Os isotopes indicates these elements may be immobile in slab-derived fluids. Elevated concentrations of Pt and Pd in pyroxenite are mirrored by depletions in dunite, demonstrating that melt-rock reaction is instead responsible for enrichments in these elements in arc mantle. A correlation between whole-rock 187Os/188Os and phosphorous in olivine offers clues to ancient processes unrelated to active subduction, not recorded by any other chemical or isotopic system

The responses of the four main substitution mechanisms of H in olivine to H2O activity at 1050 °C and 3 GPa

The water solubility in olivine (CH2O) has been investigated at 1050 °C and 3 GPa as a function of water activity (aH2O) at subsolidus conditions in the piston-cylinder apparatus, with aH2O varied using H2O–NaCl fluids. Four sets of experiments were conducted to constrain the effect of aH2O on the four main substitution mechanisms. The experiments were designed to grow olivine in situ and thus achieve global equilibrium (G-type), as opposed to hydroxylating olivine with a pre-existing point-defect structure and impurity content (M-type). Olivine grains from the experiments were analysed with polarised and unpolarised FTIR spectroscopy, and where necessary, the spectra have been deconvoluted to quantify the contribution of each substitution mechanism. Olivine buffered with magnesiowüstite produced absorbance bands at high wavenumbers ranging from 3566 to 3612 cm−1. About 50% of the total absorbance was found parallel to the a-axis, 30% parallel to the b-axis and 20% parallel to the c-axis. The total absorbance and hence water concentration in olivine follows the relationship of CH2O∝aH2O2, indicating that the investigated defect must involve four H atoms substituting for one Si atom (labelled as [Si]). Forsterite buffered with enstatite produced an absorbance band exclusively aligned parallel the c-axis at 3160 cm−1. The band position, polarisation and observed CH2O∝aH2O are consistent with two H substituting for one Mg (labelled as [Mg]). Ti-doped, enstatite-buffered olivine displays absorption bands, and polarisation typical of Ti-clinohumite point defects where two H on the Si-site are charge-balanced by one Ti on a Mg-site (labelled as [Ti]). This is further supported by CH2O∝aH2O and a 1:1 relationship of molar H2O and TiO2 in these experiments. Sc-doped, enstatite-buffered experiments display a main absorption band at 3355 cm−1 with CH2O∝aH2O0.5 and a positive correlation of Sc and H, indicating the coupled substitution of a trivalent cation plus a H for two Mg (labelled as [triv]). Our data demonstrate that extreme care has to be taken when inferences from experiments conducted at aH2O=1 are applied to the mantle, where in most cases, a low aH2O persists. In particular, the higher exponent of the [Si] substitution mechanism means that the contribution of this hydrous defect to total water content will decrease more rapidly with decreasing aH2O than the contributions of the other substitution mechanisms. The experiments confirm previous results that the [Mg] mechanism holds an almost negligible amount of water under nearly all T-P-fO2-fH2O conditions that may be anticipated in nature. However, the small amounts of H2O we find in substituting by this mechanism are similar in the experiments on forsterite doped with either Sc or Ti to those in the undoped forsterite at equivalent aH2O (all buffered by enstatite), confirming the assumption that, thermodynamically, CH2O substituting by each mechanism does not depend on the water concentration that substitutes by other mechanisms.We gratefully acknowledge the Australian Research Council (ARC) support through DP110103134 to JH and HON, and FL130100066 to HON, which partly supported PT during the final stages of this project. RS acknowledges an Australian Postgraduate Award

Gemtelligence: Accelerating Gemstone classification with Deep Learning

The value of luxury goods, particularly investment-grade gemstones, is greatly influenced by their origin and authenticity, sometimes resulting in differences worth millions of dollars. Traditionally, human experts have determined the origin and detected treatments on gemstones through visual inspections and a range of analytical methods. However, the interpretation of the data can be subjective and time-consuming, resulting in inconsistencies. In this study, we propose Gemtelligence, a novel approach based on deep learning that enables accurate and consistent origin determination and treatment detection. Gemtelligence comprises convolutional and attention-based neural networks that process heterogeneous data types collected by multiple instruments. Notably, the algorithm demonstrated comparable predictive performance to expensive laser-ablation inductively-coupled-plasma mass-spectrometry (ICP-MS) analysis and visual examination by human experts, despite using input data from relatively inexpensive analytical methods. Our innovative methodology represents a major breakthrough in the field of gemstone analysis by significantly improving the automation and robustness of the entire analytical process pipeline

A shock recovery experiment and its implications for Mercury's surface : The effect of high pressure on porous olivine powder as a regolith analog

We conducted classic dynamic high - pressure experiments on porous San Carlos (SC) olivine powder to examine if and how different shock stages modify corresponding reflectance mid – infrared (MIR) spectra. Microscopic investigation of the thin sections produced of our shocked samples indicates local peak pressures of >60 GPa along with all lower grade shock stages. Spectral analyses of optically identified shock areas were documented and compared in terms of Christiansen Feature (CF) and the position of olivine – diagnostic Reststrahlenbands (RBs). We found that one RB (fundamental vibrations of the orthosilicate - ion) of olivine occurring at 980 cm−1 (corresponding to ≈ 10.2 μm) shows the least energetic shift in the investigated MIR spectra and could therefore serve as a proxy for the presence of olivine in remote sensing application. Furthermore, a peak located at ≈ 1060 cm−1 (≈ 9.4 μm) shows a significant intensity change probably related to the degree of shock exposure or grain orientation effects, as we observe a decline in intensity of this band from our averaged reference olivine spectra of our IRIS database (diffuse reflectance measurement) down to spectra of grains showing mosaicism and recrystallized areas. We also report the presence of a weak band in some of the olivine spectra located at ≈ 1100 cm−1 (9.1 μm) that has an influence on the position of the CF when spectral data of olivine are averaged.Peer reviewe

A connection between magma chamber processes and eruptive styles revealed at Nisyros-Yali volcano (Greece)

Arc volcanoes generally emit water-rich, high-viscosity silicic magmas, which are prone to erupt explosively. However, effusive behavior is a common occurrence despite the high-H2O, high viscosity conditions. The contrasting shift from effusive to explosive behavior (and vice-versa) at any individual volcano raises the question on what controls eruptive style. Permeability development in conduits allows magma to outgas and is clearly a key factor. However, an important question is whether magma reservoir processes can also have an influence on eruptive styles. The answer could have direct impact on predicting eruptive behavior. Hence, we explore this potential connection by analyzing nine alternating effusive and explosive silicic deposits that were emplaced during distinct eruptions at the active Nisyros-Yali volcanic center. The lavas and pyroclastic deposits are compositionally similar. This indicates a negligible influence of the bulk rock composition on different eruptive styles. The crystal contents vary between units, without any clear correlation with eruptive style (from nearly aphyric to ~45 vol% crystals). Mineral textures and chemistry do show variations between effusive and explosive eruptions, with a larger proportion of resorbed plagioclase and, in most cases, more evolved amphiboles present in the lava flows. Mineral thermo-barometry and hygrometry show that the storage zones of magmas generating effusive eruptions evolved towards colder and more water-rich conditions (710–790 °C; 5.6–6.5 wt% H2O) than their explosive counterparts (815–850 °C; 4.2–4.6 wt% H2O). At storage pressures of 1.5–2 kbar, relevant for Nisyros-Yali, the volatile saturation level is reached at >5 wt% H2O. Therefore, it is likely that the magmas reached water-saturation before generating effusive eruptions, and were undersaturated before explosive events. We hypothesize that the presence of exsolved volatiles in the subvolcanic reservoir can enhance the outgassing potential of the magma during conduit ascent. Hence, the rhyolitic effusive-explosive transitions can be influenced by the pre-eruptive exsolved versus dissolved state of the volatiles in the magma chamber. This can lead to the less explosive eruptions for the most water-rich reservoir conditions

Arc magmas oxidized by water dissociation and hydrogen incorporation in orthopyroxene

Elevated H2O concentrations and oxygen fugacities are two fundamental properties that distinguish magmas formed in subduction zones from new crust generated at mid-ocean ridges. However, the mechanism of magma oxidation and how it relates to the increase in H2O remain unclear. In this study, we used infrared spectroscopy of mantle wedge orthopyroxene to trace the temporal and spatial evolution of oxygen fugacity during the transport of hydrous arc melts towards the crust. A positive correlation between equilibrium oxygen fugacity and orthopyroxene H2O concentrations for the peridotite samples studied allowed the assignment of specific, commonly observed absorption bands to redox-sensitive crystallographic defects. H2O content associated with these redox-sensitive defects increases in concentration across individual crystals, uniquely preserving the time-dependent transition from reduced to oxidized conditions during the migration of hydrous melts through the mantle wedge. A separate but related process of reaction with H2 occurring primarily during the earliest stages of melt–mantle reaction may be fundamental in generating the oxidized nature of hydrous melts. Our study proposes that the oxidized nature of arc magmas may not be a primary feature, but is instead acquired progressively as hydrous primary melts react with the surrounding mantle

Microplastics in agricultural drainage water: A link between terrestrial and aquatic microplastic pollution

Microplastic (MP) contamination has been reported to be higher in terrestrial compared to aquatic environments. This is probably due to the fact that plastic items are mostly produced and used in terrestrial environments and have a longer residence time. However, there are several links between the terrestrial and aquatic environments. We analyzed drainage water samples from agricultural soils in the Seeland, a heavily drained agriculturally intensive area in Switzerland for its MP (>100 μm) concentration and composition. We foundMP in relevant numbers (mean 10.5 ± 9.5 N L−1). The polymers were mainly PA and PE, and the size distribution showed an exponential increase with decreasing particle size. The results show considerableMP concentrations in drainage water and imply a transport of MP in soils down to the drainage pipes. Given the large areas drained both in Switzerland and globally, it is proposed that MP leaching from soil can be a significant source of MP to aquatic ecosystems. Such a contribution should be considered when dealing with MP cycling on a local to global scale

FTIR spectra and absorbances of mantle orthopyroxene from Ritter, West Bismarck island arc

Data are average infrared absorbances, H2O contents and spectra for arc mantle orthopyroxene from Ritter peridotite xenoliths, West Bismarck island arc. Also included are spectra extracted from a profile across a zoned orthopyroxene from an individual sample

Elucidating the processes affecting highly primitive lavas of the Borgarhraun flow (northern Iceland) using trace elements in olivine

Olivine is typically the first phase to crystallise from basaltic melts and its chemistry can therefore inform on the earliest stages of magmatic evolution, not recorded by later crystallising phases. Despite the potential of olivine for understanding primitive differentiation, limited analytical capabilities have previously restricted the range of elements that can be routinely measured. Consequently, important processes controlling early magma evolution may have been overlooked or misidentified. This study reports a wide range of minor and trace elements in forsteritic (up to Fo92.2) olivine macrocrysts from the primitive Borgarhraun lava flow in northern Iceland. We define two distinct populations of olivine based on their forsterite (Fo) content and then apply minor and trace element data to discern mixing and crystallisation of subtly different high-MgO parental melts. High-Fo (90.9–92.2 mol%) olivines show approximately linear trends between Cr and other incompatible trace elements (Li, Na, Ca, Ti, Al and Y), implying mixing and concurrent crystallisation of two highly primitive melts. Low-Fo (87.4–90.0 mol%) olivines show trends that indicate mixing and crystallisation of multiple, genetically distinct and less primitive melts. The outermost 50 μm of the olivine microcrysts record diffusive re-equilibration of the olivine macrocrysts to a single, significantly more evolved carrier liquid over an ascent timescale of 70–250 days. Compared to the rest of Iceland, the Borgarhraun olivine macrocrysts are distinguished by their Cr contents, which extend from 97 to 1150 ppm. The uniquely steep trend in Fo vs. Cr can be explained by early crystallisation of Cr-spinel and Cr-rich clinopyroxene, stabilised by high pressures of differentiation (>0.8 GPa). Chromium-forsterite systematics may therefore be a powerful tool for qualitatively assessing relative pressures of crystallisation for different magmatic systems. Collectively, our new dataset clearly demonstrates the importance of measuring trace elements in olivine for identifying the formative stages and conditions of basaltic magmatic systems.ISSN:0016-7037ISSN:1872-953