Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of K2O-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000μg/g), whereas globules and the interface are Cl poor (ca. 300μg/g). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty (87Sr/86Sr of 0.70580±10), indicating that they likely formed from the same source. However, globules show high δ11B (ca. + 7‰), whereas their host andesites are isotopically lighter (ca. – 1 ‰), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low δ11B forearc basalt parental magmas (average 87Sr/86Sr of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered

An experimental study of the partitioning of trace elements between rutile and silicate melt as a function of oxygen fugacity

Subduction zone or arc magmas are known to display a characteristic depletion of High Field Strength Elements (HFSE) relative to other similarly incompatible elements, which can be attributed to the presence of the accessory mineral rutile (TiO2) in the residual slab. Here we show that the partitioning behavior of vanadium between rutile and silicate melt varies from incompatible (∼0.1) to compatible (∼18) as a function of oxygen fugacity. We also confirm that the HFSE are compatible in rutile, with D(Ta)> D(Nb)>> (D(Hf)>/∼ D(Zr), but that the level of compatibility is strongly dependent on melt composition, with partition coefficients increasing about one order of magnitude with increasing melt polymerization (or decreasing basicity). Our partitioning results also indicate that residual rutile may fractionate U from Th due to the contrasting (over 2 orders of magnitude) partitioning between these two elements. We confirm that, in addition to the HFSE, Cr, Cu, Zn and W are compatible in rutile at all oxygen fugacity conditions

Partition behavior of platinum-group elements during the segregation of arsenide melts from sulfide magma

Evidence of immiscibility between arsenide and sulfide melts has been observed both in experimental studies and natural samples from several localities worldwide (e.g., Ronda, Spain; Beni Bousera, Morocco; Dundonald Beach South, Canada). Platinum-group elements (PGE) have shown to have a strong affinity for arsenide melts, but little is known about their partitioning behavior between arsenide and sulfide melts. In this study, we experimentally determine the partition coefficients of PGE (Pt, Pd, Ir, Ru, and Os) between both types of melt in As-saturated sulfide systems doped with trace amounts of PGE. Results show that all PGE display a strong preference to the arsenide melt with DAs/sulf meltsPGE ranging from 20 to 2700, with Ir and Pt showing a marked preference for arsenide melts. These partition coefficients values are similar to estimates made from natural samples and demonstrate that the separation of arsenide melts from sulfide magmas can be an efficient mechanism to scavenge PGE from magmas and to fractionate Os, Ru, and Pd from Pt and Ir

Understanding Re–Os systematics and model ages in metamorphosed Archean ultramafic rocks: A single mineral to whole-rock investigation

International audienc

Partitioning of highly siderophile elements between monosulfide solid solution and sulfide melt at high pressures

Base metal sulfides (Fe–Ni–Cu–S) are ubiquitous phases in mantle and subduction-related lithologies. Sulfides in the mantle often melt incongruently, which leads to the production of a Cu–Ni-rich sulfide melt and a solid residue called monosulfide solid solution (mss). Even though peridotite-hosted sulfides, which tend to be more Ni-rich, are likely completely molten at mantle potential temperatures, the same is not true for eclogite-hosted Ni-poor, Fe-rich sulfides. Because of this, solid crystalline mss may persist at higher pressures and equilibrate with co-existing sulfide melt along colder geotherms, like those associated with subduction zones. Because highly siderophile elements (HSE—Pt, Pd, Rh, Ru, Os, Ir, and Re) are known to fractionate as a result of mss/sulfide-melt equilibrium, the persistence of an mss/sulfide-melt assemblage to higher pressures may lead to the fractionation of these elements during the subduction process. In this contribution, we carried out an experimental investigation of the partitioning behavior of the HSE, as well as Cu and Ni, between mss and sulfide melt over a pressure and temperature range relevant to equilibration between Earth's surface and transition zone depths (0.1 MPa to 14 GPa; 930–1530 °C), and variable Ni contents in sulfide. Results show that at higher pressures, the HSE are considerably less fractionated as a result of mss and sulfide melt equilibrium compared to lower pressure conditions. This is exemplified by a lowering of the $D^{mss/melt}_{i}$ for the more compatible HSE (Ru, Os, Ir, Rh and Re) from around 10 at 0.1 MPa to just above or below unity at 14 GPa. Moreover, the higher the Ni content of the bulk sulfide assemblage, the larger the degree of change in the magnitude of HSE fractionation seen over the pressure range studied. The exchange coefficient ($K^{Ru-Pt}_{D}$) between highly compatible HSE (Ru) and less compatible Pt illustrates a notable contrast. In the Ni-poor composition (E1), $K^{Ru-Pt}_{D}$ changes from 27 at 0.1 MPa to 6 at 14 GPa. In contrast, the Ni-rich composition exhibits a broader range, with $K^{Ru-Pt}_{D}$ ranging from 150 to 17 across the same pressure interval. Our results highlight key differences between experimental data obtained at lower and higher pressure, and how composition, namely the Ni content of sulfide, affects HSE partitioning behavior

Formation mechanisms of macroscopic globules in andesitic glasses from the Izu–Bonin–Mariana forearc (IODP Expedition 352)

The Izu–Bonin–Mariana volcanic arc is situated at a convergent plate margin where subduction initiation triggered the formation of MORB-like forearc basalts as a result of decompression melting and near-trench spreading. International Ocean Discovery Program (IODP) Expedition 352 recovered samples within the forearc basalt stratigraphy that contained unusual macroscopic globular textures hosted in andesitic glass (Unit 6, Hole 1440B). It is unclear how these andesites, which are unique in a stratigraphic sequence dominated by forearc basalts, and the globular textures therein may have formed. Here, we present detailed textural evidence, major and trace element analysis, as well as B and Sr isotope compositions, to investigate the genesis of these globular andesites. Samples consist of $K_{2}O$-rich basaltic globules set in a glassy groundmass of andesitic composition. Between these two textural domains a likely hydrated interface of devitrified glass occurs, which, based on textural evidence, seems to be genetically linked to the formation of the globules. The andesitic groundmass is Cl rich (ca. 3000$\mu$g/g), whereas globules and the interface are Cl poor (ca. 300$\mu$g/g). Concentrations of fluid-mobile trace elements also appear to be fractionated in that globules and show enrichments in B, K, Rb, Cs, and Tl, but not in Ba and W relative to the andesitic groundmass, whereas the interface shows depletions in the latter, but is enriched in the former. Interestingly, globules and andesitic groundmass have identical Sr isotopic composition within analytical uncertainty ($^{87}$Sr/$^{86}$Sr of 0.70580 $\pm$ 10), indicating that they likely formed from the same source. However, globules show high $\delta^{11}$B (ca. + 7‰), whereas their host andesites are isotopically lighter (ca. – 1 ‰), potentially indicating that whatever process led to their formation either introduced heavier B isotopes to the globules, or induced stable isotope fractionation of B between globules and their groundmass. Based on the bulk of the textural information and geochemical data obtained from these samples, we conclude that these andesites likely formed as a result of the assimilation of shallowly altered oceanic crust (AOC) during forearc basaltic magmatism. Assimilation likely introduced radiogenic Sr, as well as heavier B isotopes to comparatively unradiogenic and low $\delta^{11}$B forearc basalt parental magmas (average $^{87}$Sr/$^{86}$Sr of 0.703284). Moreover, the globular textures are consistent with their formation being the result of fluid-melt immiscibility that was potentially induced by the rapid release of water from assimilated AOC whose escape likely formed the interface. If the globular textures present in these samples are indeed the result of fluid-melt immiscibility, then this process led to significant trace element and stable isotope fractionation. The textures and chemical compositions of the globules highlight the need for future experimental studies aimed at investigating the exsolution process with respect to potential trace element and isotopic fractionation in arc magmas that have perhaps not been previously considered

The mechanism of borosilicate glass corrosion revisited

Currently accepted mechanistic models describing aqueous corrosion of borosilicate glasses are based on diffusion-controlled hydrolysis, hydration, ion exchange reactions, and subsequent re-condensation of the hydrolyzed glass network, leaving behind a residual hydrated glass or gel layer. Here, we report results of novel oxygen and silicon isotope tracer experiments with ternary Na borosilicate glasses that can be better explained by a process that involves the congruent dissolution of the glass, which is spatially and temporally coupled to the precipitation and growth of an amorphous silica layer at an inwardly moving reaction interface. Such a process is thermodynamically driven by the solubility difference between the glass and amorphous silica, and kinetically controlled by glass dissolution reactions at the reaction front, which, in turn, are controlled by the transport of water and solute elements through the growing corrosion zone. Understanding the coupling of these reactions is the key to understand the formation of laminar or more complex structural and chemical patterns observed in natural corrosion zones of ancient glasses. We suggest that these coupled processes also have to be considered to realistically model the long-term performance of silicate glasses in aqueous environments