ALBERT

Description: Highlights • Statistically different gas geochemistry was observed in two adjacent springs. • About 74% of helium was contributed by the mantle. • Excess N2 relative to Ar was attributed to subducted materials and seawater mixing. • Magmatic CO2 has been largely removed by calcite precipitation in the reaction zone. • The residual CO2 may also be supplied by microbial oxidation of alkanes. Gas emissions from hydrothermal systems can serve as indicators of subsurface activity. In addition to gas sources, hydrothermal gas geochemistry is strongly influenced by secondary processes that occur during/after hydrothermal circulation. Here, we observed statistically significant differences in the geochemical characteristics (except for helium isotopes) of bubbling gases discharged from two adjacent vents in the Northern Luzon Arc. Helium (3He/4He = 4.25–7.09 Ra) in both vents was controlled by mixing between mantle and crustal components, where about 74% of helium was contributed by the mantle. Differences in N2/Ar ratios (∼ 300–330) of the two neighboring springs are attributed to subducted materials and seawater mixing (contributing ∼2.5% N2 and Ar), rather than phase separation in the reaction zone. Specifically, Ar was mainly supplied by atmospheric components that dissolved in the percolated seawater with only 8%–9% contributed by the excess radiogenic 40Ar. Excess N2 relative to Ar was mainly supplied by the decomposition of subducted materials (83%–92%) of the South China Sea plate beneath the Philippine Sea Plate. The Lutao gases showed low CO2 concentrations (0.07–22.2 mmol/mol), despite the high 3He/4He ratios indicating a significant contribution of magmatic components. Magmatic CO2 may have been largely consumed by the high Ca Lutao vent fluids via carbonate precipitation in the reaction zone. Alternatively, stable carbon isotope compositions (δ13C) indicate that Lutao CO2 may be supplied by microbial oxidation of alkanes (e.g., CH4 with concentrations of 14.6–173 mmol/mol in the samples), with fractionation factor ΔCO2–CH4 ranging from −15‰ to −25‰ and conversion rates of 〈10%. Up to 65% of the CO2 in the 2016 samples experienced secondary calcite precipitation in the discharge zone. Our results indicate that recycled subducted materials could potentially affect the geochemical characteristics of gases discharged from arc-volcanic systems. In addition, the influence of secondary processes needs to be considered before tracing the sources of hydrothermal fluids and/or gases, especially in shallow-water hydrothermal systems.

Description: Highlights • New geophysical data and samples redefine submarine volcanism in Sicilian Channel. • Three dominant bands of volcanism are distinguished. • Ancient, eroded structures aligned at 120° are tied to faulted banks in the north. • Younger band of similarly aligned volcanism in the south is linked to grabens. • Youngest structures comprise small, dispersed volcanoes with distinct orientation. Abstract The origin and role of volcanism in continental rifts remains poorly understood in comparison to other volcano-tectonic settings. The Sicilian Channel (central Mediterranean Sea) is largely floored by continental crust and represents an area affected by pronounced crustal extension and strike-slip tectonism. It hosts a variety of volcanic landforms closely associated with faults, which can be used to better understand the nature and distribution of rift-related volcanism. A paucity of appropriate seafloor data in the Sicilian Channel has led to uncertainties regarding the location, volume, sources and timing of submarine volcanism. To improve on this situation, we use newly acquired geophysical data (multibeam echosounder and magnetic data, sub-bottom profiles) and dredged seafloor samples to: (i) re-assess the evidence for submarine volcanism in the Sicilian Channel and define its spatial pattern, (ii) infer the relative age and style of magmatism, and (iii) relate this to the dominant tectonic structures in the region. Quaternary rift-related volcanism has been focused at Pantelleria and Linosa, at the northwest boundaries of their respective NW-SE trending grabens. Subsidiary and older volcanic sites potentially occur at the Linosa III and Pantelleria SE seamounts, collectively representing the only sites of recent volcanism that can be directly related to the main rift process. These long-lived polygenetic volcanic landforms have been shaped by magmatism that is directly correlated with extensional faulting and buried igneous bodies. Older volcanic landforms, sharing a similar scale and alignment, occur to the north at Nameless Bank and Adventure Bank. These deeply eroded volcanoes have likely been inactive since the Pliocene and are probably related to earlier stages of crustal thinning and underlying feeder structures in the northern region of the Sicilian Channel. Along a similar alignment, Pinne Bank, SE Pinne Bank and Cimotoe in the northern Sicilian Channel lack a surface volcanic signature but are associated with intrusive bodies or deeply buried volcanic rock masses. Terrible Bank, in the same region, also shows evidence of ancient, polygenetic magmatism, but was subject to significant erosion and lacks a prominent alignment. The much younger volcanism at Graham Volcanic Field and along the northern Capo-Granitola-Sciacca Fault Zone differs markedly from that observed in the other study areas. Here, the low-volume and scattered volcanic activity is driven by shallow-water mafic magma eruptions, which gave rise to small individual cones. These sites are associated with large fault structures away from the main rift axis and may have a distinct magmatic origin. Dispersed active fluid venting occurs across both ancient and young volcanic sites in the region and is directly associated with shallow magmatic bodies within tectonically-controlled basins. Our study provides the foundation for an updated tectonic and magmatic framework for the Sicilian Channel, and for future detailed chronological and geochemical assessment of the sources and evolution of magmatic processes in the region.

Description: Since the initial discovery of the non-exponential mass fractionation (non-EMF) of Nd isotopes analysis in 2002, similar deviations from an EMF pattern have been reported for measurements of a number of isotope systems (e.g., Si, Ge, Sr, Sn, Ba, Yb, W, Os, Hg and Pb) with MC-ICP-MS. However, the previous controversial reports on the magnitude of the deviations from EMF suggest that instrumental mass bias behaviour of MC-ICP-MS is neither fully understood nor well-characterised. Consequently, the standard approach of using a mass dependent fractionation (MDF) correction model (e.g., exponential law) may lead to both inaccurate and imprecise results. In this study, we systematically characterise the instrumental mass fractionation of MC-ICP-MS using Nd isotope measurements carried out under different plasma conditions, quantified using the normalised argon index (NAI) as an estimate of plasma temperature. Our results indicate that the mass bias of MC-ICP-MS is not always a simple exponential function of mass but shows systematic deviations from an EMF behaviour, which are closely associated with decreased NAIs. As a result, the conventional exponential correction yields a 143Nd/144Nd value of 0.512257 for the reference material BHVO-2 when the NAI is low, which is 722 ppm lower than the reported value of 0.512979. By tuning the plasma to higher NAIs (higher plasma temperatures), the deviations from the EMF array are systematically attenuated and the exponential correction is able to correct for the instrumental mass bias under high NAIs. In contrast, a regression correction model for Nd isotopes is developed to account for the observed mass fractionation behaviour that does not follow EMF under low NAIs, given that the regression correction relies on the observed loglinear fractionation of different isotope pairs and does not require both isotope ratios to undergo EMF. We expect that the analytical protocol and fundamental insights gained in this study are applicable to a wide range of other isotope measurements with MC-ICP-MS.

Description: This chapter provides an overview of near-surface geochemical processes operating on Earth, with special emphasis placed on (i) marine weathering such as alteration and dissolution of silicates, carbonates and terrigenous riverine particles in the ocean, complemented by (ii) reverse weathering reactions leading to marine authigenic clay formation, and the impact of these phenomena on ocean alkalinity budget and the chemical and isotope composition of seawater. Model simulations of the above processes provide estimates of the global marine fluxes of major cations (Na+, K+, Mg2+, Ca2+) and alkalinity in the ocean induced by silicate weathering and dissolution of terrigenous material in seawater. Additional constraints on silicate vs. carbonate weathering, oceanic/coastal CaCO3 cycling, and paleo-seawater reconstructions are provided via the stable and radiogenic isotope systems of alkali and alkaline earth metals (Li, K, Mg, Ca, and Sr isotopes) that are discussed within the context of marine and reverse weathering in the present and past ocean. Key points • Impact of weathering processes on marine elemental cycles and the ocean alkalinity budget. • Alteration and dissolution of silicate minerals and riverine particles in the ocean quantified via thermodynamic equilibrium (PHREEQC) calculations, in seawater and top sediment settings. • Estimates of global ocean fluxes of dissolved cations (Na+ , K+ , Mg 2+ , Ca2+ ) and alkalinity induced by alteration and dissolution of terrigenous material in seawater and marine sediments. • Principles and mechanisms of isotope variability in nature (mass-dependent and radiogenic isotope effects) observed for alkali and alkaline earth metals. • Silicate vs. carbonate weathering and coastal carbon/carbonate cycling constrained via stable and radiogenic Ca and Sr, and Li isotopes. • Oceanic processes, marine carbonate chemistry (alkalinization vs. acidification), and paleo-seawater reconstructions constrained via d44 Ca, d88 Sr, d26 Mg proxies and numerical (MATLAB) modeling. • Emerging metal isotope proxies (d41 K) for silicate and reverse weathering in the ocean.

Description: Highlights • a high-fidelity RANS CFD method is used to simulate the flow through netting panels. • The influence of netting solidity, twine diameter, mesh opening angle and incident angle is examined. • Mesh opening angle, solidity and angle of incidence greatly influence the hydrodynamic force coefficients and efficiency. To ensure the economic and environmental sustainability of the fisheries and aquaculture industries, it is necessary to address issues related to fuel consumption, environmental degradation, and fish welfare. Hence, we need a thorough understanding of the filtration efficiency and the hydrodynamic forces acting on towed fishing gears and netting structures. Here we apply a Reynolds-averaged Navier-Stokes (RANS) CFD method to model the flow through netting panels, where we vary the operational and design parameters of flow speed, netting solidity, twine diameter, mesh opening angle and the incidence angle of the flow to the panel. Thus, we create a simulated data set which we analyze to provide a fundamental understanding of the functional relationships for the pressure drop and tangential drag coefficients, and the flow deflection in terms of these parameters. We pay particular attention to the effect of mesh opening angle, a parameter that has not received much attention in the literature. We demonstrate that it has a large influence on the drag and lift coefficients and consequently on the hydrodynamic efficiency of netting panels. These results will be particularly useful for reducing the hydrodynamic forces on netting structures and improving the fuel efficiency of towed fishing gear operations.

Description: Gypsum makes up about one fifth of giant salt deposits formed by evaporation of seawater throughout Earth’s history. Although thermodynamic calculations and precipitation experiments predict that gypsum precipitates when the salinity of evaporating seawater attains about 110 g kg-1, gypsum deposits of the Mediterranean Salt Giant often bear the geochemical signature of precipitation from less saline water masses. Addressing this geochemical riddle is important because marine gypsum deposition and continental gypsum erosion affect the global carbon cycle. We investigated gypsum deposits formed in the marginal basins of the Mediterranean Sea during the Messinian Salinity Crisis (about 6 million years ago). These often bear low-salinity fluid inclusions and isotopically light crystallization water, confirming previous published reports that the Mediterranean Salt Giant harbors low-salinity gypsum deposits. A geochemical model constrained by fluid inclusion salinity and isotope (87Sr/86Sr, δ34SSO4, δ18OH2O, δDH2O) measurements excludes that Ca2+- and SO42--enriched continental runoff alone provides the trigger for gypsum precipitation at low salinity. We propose that, concurrent with the prevalent evaporative conditions and with Ca2+- and SO42--bearing runoff, the biogeochemical sulfur cycle is capable of producing a spatially-restricted and temporally-transient increase of Ca2+ and SO42- within benthic microbial mats, creating local chemical conditions conductive to gypsum precipitation. This hypothesis is supported by the presence of dense packages of fossils of colorless sulfur bacteria within gypsum in several Mediterranean marginal basins, together with independent geochemical and petrographic evidence for an active biogeochemical sulfur cycle in the same basins. Should this scenario be confirmed, it would expand the range of environments that promote marine gypsum deposition; it would also imply that an additional, biological coupling between the calcium, sulfur and carbon cycles exists.

Description: The dynamic processes associated with subducting tectonic plates and rising plumes of hot material are typically treated separately in dynamical models and seismological studies. However, various types of observations and related models indicate these processes overlap spatially. Here we use precursors to PP and SS reflecting off mantle transition zone discontinuities to map deflections of these discontinuities near three subduction zones surrounding the Caribbean Plate: 1) Lesser Antilles, 2) Middle America and 3) northern South American subduction zones. In all three regions slow seismic anomalies are present behind the sinking slab within the transition zone in tomographic images. Using array methods, we identify precursors and verify their in-plane propagation for MW ≥ 5.8 events occurring between the years 2000 and 2020 by generating a large number of source receiver combinations with reflection points in the area, including crossing ray paths. The measured time lag between PP/SS arrivals and their corresponding precursors on robust stacks are used to measure the depth of the mantle transition zone discontinuities. In all three areas we find evidence for upward deflection of the 660 discontinuity behind the sinking slab, consistent with the presence of hot plume material (average temperature anomalies of 180 to 620 K), while there is not a corresponding downward deflection of the 410 km discontinuity. One interpretation of these disparate observations is suggested based on comparison to existing models of mantle convection and subduction: plume material rising across 660 km discontinuity could be entrained by lateral flow in the transition zone induced by the nearby sinking slab, and thus delaying the rise of hot material across the 410 km discontinuity.

Description: Highlights • Ankaramites are Ca-rich and Ni-poor porphyritic basalts that are common in oceanic arcs. • Melt inclusions from Kibblewhite Volcano show similar compositions to ankaramites. • Ankaramite is a primary magma component in oceanic arcs. • Interaction between melt and mantle can produce ankaramitic melts. • Harzburgite formed by melt-mantle interactions is the source of high-Mg andesites. Abstract Ankaramites, which are clinopyroxene-rich basalts with primitive whole-rock compositions (Mg# 〉65), are common in oceanic arcs and are characterized by high whole-rock CaO/Al2O3 (〉1.0) ratios and olivine crystals with anomalously low nickel contents (〈0.2 wt% NiO). These geochemical characteristics cannot be explained by the melting of ordinary mantle peridotite. However, their origin is critical for understanding the formation of primary magmas in oceanic arcs. Here, we investigated olivine-hosted melt inclusions (MIs) from ankaramites and magnesian andesites of the Kibblewhite Volcano in the Kermadec arc. The MIs from the ankaramites have similar major and trace element characteristics to the host rocks, indicating that the ankaramites did not result from an accumulation of mafic minerals but rather represent the primary magma in the Kibblewhite Volcano. The MIs from the magnesian andesites were hosted in forsteritic olivine xenocrysts with a wide range of NiO contents (Fo90–92; 0.13–0.39 wt% NiO) and have similar major element compositions to the ankaramites but exhibit a wide range of CaO/Al2O3 (0.85–1.54). The trace element characteristics of the MIs from the magnesian andesites do not match those of the host rocks, indicating that they are not primary melts of the magnesian andesites but primitive basaltic melts generated before the magnesian andesites formed. Interestingly, the CaO/Al2O3 ratio of MIs from the magnesian andesites was negatively correlated with the NiO content of their host olivines. This correlation suggests that the composition of the primary basaltic magmas of the Kibblewhite Volcano changed continuously from peridotite-derived to ankaramitic. This correlation could not be explained by grain-scale process, crustal anatexis, or contribution of slab-derived carbonate-rich fluids. Instead, we propose that this correlation can be explained by the interaction of the ascending primary basaltic melts with the lithospheric mantle. During melt-mantle interaction, the assimilation of clinopyroxene and fractionation of olivine and orthopyroxene caused the CaO/Al2O3 ratio to increase in the melt and the Ni content to decrease. Furthermore, because the magnesian andesites have low CaO/Al2O3 ratios and could be derived from a clinopyroxene-poor mantle lithology, the interaction between the melt and mantle may also be closely related to the origin of the magnesian andesites at Kibblewhite Volcano. This interpretation provides a new perspective on the origin of the oceanic arc ankaramites and why primary andesitic and basaltic magmas coexist in the Kibblewhite Volcano.