Understanding how pre-existing mantle wedge depletion shapes arc magma chemistry, distinct from subduction inputs, remains a key challenge in petrogenesis studies in subduction zone volcanoes. Here we present new high-precision Zn isotopic data (expressed as delta Zn-66, relative to JMC(Lyon) standard) for 62 arc lavas from western Pacific arc-front (Mariana, Izu, and Kurile) and rear-arc (Mariana) volcanoes. After correcting delta Zn-66 for early fractional crystallization, our results show that near-primitive arc-front lavas exhibit delta Zn-66 values systematically similar to 0.06 parts per thousand lower than rear-arc lavas, similar to 0.11 parts per thousand lower than back-arc basin and mid-ocean ridge basalts. Crucially, delta Zn-66 correlates positively with source depletion indicators (e.g., Ta/Zn, Nb/Yb), but shows no correlation with slab input proxies (e.g., Ba/La, Ce/Mo, delta Mo-98), demonstrating that subduction fluids/melts minimally affect Zn isotopes in arc magmas. Notably, progressive mantle depletion (e.g., from lherzolite to harzburgite) drives a compositional transition and corresponding delta Zn-66 variation in residual mantle spinel (e.g., from Al-rich to Cr-rich). This source lithology transition triggers inter-mineral isotopic re-equilibrium, which in turn decreases the Zn isotope fractionation factor (alpha Zn-66(Melt-Residue)) in more refractory sources. By incorporating this effect, our newly-developed partial melting models reveal that Mariana arc-front magmas formed from 5-10 % melting of mantle previously depleted by 9-12 % melt extraction, whereas rear-arc magmas derived from 5 % melting of less depleted mantle (3-9 % pre-depletion). This pre-depletion at a mature oceanic arc is most likely driven by prior melt extractions during back-arc spreading. Our Zn isotopic data therefore provides direct evidence that ambient mantle wedge infertility profoundly controls the geochemical diversity of arc-front magmas.

Abstract The slab thermal state model predicts that only cold slabs should retain some of their intra‐slab water beyond subduction zones, while warmer slabs should be nearly dry past the volcanic arc front. Such predictions are yet to be fully tested, as they mostly rely on numerical modeling. To further test the slab thermal model, here we have examined slab‐sensitive elemental and isotopic tracers in recently erupted basalts (<5 Myr) from along and across an arc transect at end‐member types of cold (NE Japan) and hot subduction zones (SW Japan and Ryukyu) and beyond (eastern China intraplate volcanism). We show that the oceanic crust and the incoming hydrated mantle from the cold subducted Pacific plate are the main water carriers beyond subduction zones. Only cold slabs may thus recycle part of their intra‐slab H2O into the lower mantle. Warmer slabs are too dry past the back‐arc or too short‐lived to exert a first order control on deep water recycling.

The source characteristics and melting mechanisms for forearc basalts formed during subduction initiation in the western Pacific plate remain unclear. Here we present new Hf-Nd isotopes and trace element data for the latest erupted forearc basalts collected during oceanic drilling in the Bonin forearc. These basalts have εNd values comparable to mid-ocean ridge basalts but much higher εHf, plotting above the terrestrial array, and elevated Lu/Hf and Sm/Nd ratios that indicate an unusually depleted mantle source. Systematic forward modeling indicates that these basalts cannot have been generated solely from the melting of the asthenosphere; both 160 Ma and Proterozoic refractory mantle are required to explain the observed Hf-Nd isotope decoupling. This study proposes that the Bonin forearc lithospheric mantle may have formed during ancient events. Rollback of the earliest subducting slab caused substantial thinning of the forearc lithosphere and triggered extensive melting in both the asthenospheric and lithospheric mantle. Re-melting of both ancient and younger refractory lithospheric mantle contributed to the generation of Izu–Bonin forearc basalts during the earliest stages of subduction initiation, according to hafnium–neodymium isotope analyses.

A quantitative literacy course titled "Computational Geology" has been taught at the University of South Florida for over 20 years at the undergraduate level, largely by the same professor. Due to the extended course tenure, a unique opportunity presented itself to qualitatively study what the perceived broader impacts of the course were on selected alumni from three or more years post-graduation. Viewing data from a 2016 course alumni interview project through a transcendental phenomenological lens, we looked for evidence to describe the lived experiences of nine course alumni. The alumni were baccalaureate geology graduates of USF, at least three years removed from graduation at the time of interview, who had taken and passed the Computational Geology course, and been working in private, public, or academic sectors. We discovered that the course positively impacted students' self-efficacy toward mathematics, nurtured a growing appreciation for using math as a tool to solve problems relevant to the students' personal life and career, and motivated students to learn and appreciate quantitatively literate behaviors and how quantitative literacy applies to their personal and professional life. We assert that instructors who consider the affective domain (i.e., feelings, emotions, and attitudes) while teaching mathematics leverage a powerful approach to impact student learning and incite a long-lasting impact on students' appreciation for quantitative literacy.

The geochemistry of Mariana forearc serpentinites sheds light on how the subducting slab starts to devolatilize. International Ocean Discovery Program Expedition 366 targeted three serpentinite seamounts: Yinazao, Fantangisña and Asùt Tesoru, with corresponding slab depths of 13, 14 and 18 km, respectively. The fluid-mobile elements (FMEs) B, Rb, Cs, and Li become more enriched in the eruptive serpentinites with increasing slab depth. Yinazao serpentinites show strongly elevated Sr and Ba contents, which complements depletions in these elements seen in mafic clasts entrained in the serpentinites. Correlations between the FMEs and 87Sr/86Sr of the serpentinites reflect interactions among slab fluid, mantle and the subducting slab. With the enrichments of Sr-Ba in the Yinazao and B-Rb-Cs-Li in the Asùt Tesoru serpentinites, their 87Sr/86Sr converge toward values similar as those of the mafic clasts from values between sediments and the mantle. These observations indicate that sediment is the source of the earliest slab-derived fluids, which experienced extensive interaction first with mantle wedge peridotites to produce high pH fluids, and later with subducted seamount basalts. The decomposition of plagioclase in subducting basalts leads to precipitation of CaCO3 from the high pH serpentinizing fluids and elevated Ca-Sr-Ba contents under low carbonate alkalinity conditions, as seen in the Yinazao serpentinites. The breakdown of clay minerals after lawsonite formation and the lizardite-antigorite transition release geochemically distinct fluids, with high B-Rb-Cs-Li and low Sr. This episode of fluid release results in the dissolution of CaCO3 precipitated in the shallow subduction channel, and the generation of rodingite assemblages and epidote. Finally, these fluids evolve to have high carbonate alkalinity and low Ca-Sr-Ba contents, as seen in Asùt Tesoru serpentinites as well as those in South Chamorro and Conical Seamounts. This study reveals early plagioclase breakdown via high pH fluids in the subduction channel is important both for the generation of hydrous minerals and the precipitation of CaCO3 in the subducting slab, subduction channel, and shallow mantle wedge.

Understanding the chemical composition of materials within the subducting slab is crucial for studying slab dehydration and melting mechanisms. Previous research has primarily focused on examining sediments and altered basalts collected near the seafloor, which has overlooked the chemical changes occurring within the forearc subduction channel. The Mariana forearc serpentinite mud volcanoes offer a unique opportunity to investigate metabasite xenoliths, providing valuable insights into the compositional changes of subducting slab materials at forearc depths. In this study, we present Pb-Nd-Hf isotopic data for blueschist-facies mafic clasts obtained from the As & ugrave;t Tesoru serpentinite seamount in the Mariana forearc. These data are combined with existing Sr isotope and trace element data to address the aforementioned issues. The mafic clasts display Nd-Hf isotopic signatures reminiscent of the HIMU- and EM1-type basalts discovered in the Magellan seamounts. However, their Sr-Pb isotopes display mixed signatures, indicating the influence of sediments and altered midocean ridge basalts. These findings underscore the importance of serpentinizing fluids resulting from the interaction between sediment pore water and forearc mantle peridotite in altering the Sr-Pb elemental and isotopic compositions of subducted slab materials. This process is achieved through interactions between slab materials and serpentinizing fluids within the subduction channel. It is important to emphasize that the blueschist-facies mafic clasts themselves do not directly serve as source materials for arc volcanism. However, their Sr-Pb isotopes can undergo further modifications through exchanges with unaltered seamount basalts facilitated by serpentinizing fluids within the deep subduction channel, which extends from depths of 30 to 100 km. In this scenario, the mafic clasts with modified isotopic signatures could potentially contribute as source materials for arc volcanism.

The sparsity of a direct record for the moment of subduction zone initiation has led to various models describing the infancy and evolution of modern oceanic subduction systems. Recently, with increases in available samples and geochemical data for subduction zone initiation-to-mature-arc lavas, better constraints on subduction evolution are possible. Here, by systemically modeling the time-space pattern and geochemical characters of forearc magmas with forward numerical modeling, we attempt to search for a best-fit geodynamic scenario where Izu-Bonin-Mariana-type subduction tends to develop. Our modeling and geochemical constraints have identified a necessary and possibly transitory pre-subduction zone initiation trenchward contraction consistent with observed Izu-Bonin-Mariana forearc magma geochemistry. Our results also reveal a typical maturation process for Izu-Bonin-Mariana-type oceanic subductions, controlled by the pace of the upper plate’s rifting and solidification.

Arc lavas display significant chemical and isotopic heterogeneity mainly due to recycled materials from subducting slabs. However, the extent to which different types of subducted sediments and oceanic crust contribute to the petrogenesis of arc magmas, as well as the roles of the mantle wedge and overlying crust, remain debated. Potassium (K) isotopes have the potential to provide new insights into the processes and sources of arc magmatism because sediments and altered oceanic crust are highly enriched in K and have distinct delta K-41 values compared with the mid-ocean ridge basalts and upper mantle (-0.42 +/- 0.08 parts per thousand, 2SD). Here we report K isotopic compositions of 32 well-characterized arc lavas from the circum-Pacific margins. We find low delta K-41 values (-0.86 parts per thousand similar to -0.38 parts per thousand) in the Setouchi arc samples, which we interpret as the result of incorporation of isotopically light sediments into the subarc mantle. The Kurile and Panama arc lavas have high delta K-41 values (-0.36 parts per thousand similar to 0.02 parts per thousand) and their delta K-41 values correlate positively with the Ba/Th ratios, indicating similar to 0.5-2% fluid additions from dehydrated altered crust. Adakites have variable but overall heavy K isotope compositions (-0.44 parts per thousand similar to -0.01 parts per thousand) , which correlate with the diagnostic trace-element indicators of Sr/Y, La/Yb and K/U. These results indicate limited K isotope fractionation during metamorphic dehydration of hot slab subduction, and/or fluid metasomatism before the slab melting. Our study suggests that fluids and melts originating from subducting sediments and slabs exerted distinct influences on the origins of different types of arc magmas, demonstrating the significance of K isotopes in studying the petrogenesis of arc magmas.

While chemical variability in volcanic arc lavas erupted perpendicular to the strike of subduction has been observed and studied for many years, how these variations may reflect slab dehydration or melting processes is still actively debated. Here we report new data for cross-arc geochemical variations in Quaternary volcanic rocks from the Kurile arc. Correlations among multiple isotopic tracers (B–Sr–Nd–Hf) and key elemental ratios (B/Nb, Ba/Nb, Th/Nb and Hf/Nd) show that these arc lavas reflect the influence of three discernible subduction components. Shallow slab-sourced low-temperature hydrous fluids (high B/Nb, Ba/Nb, and δ11B) and deeper high-temperature hydrous melts (moderate B/Nb, Ba/Nb and δ11B, and low Hf/Nd) show characteristics similar to those seen in Izu-Bonin-Mariana arc lavas and likely reflect similar slab-derived origins. In addition to these components, Kurile volcanic front samples document a component with high Th/Nb and depleted mantle-like Hf/Nd. This component may reflect Kurile forearc mantle that had been metasomatized by shallow slab-derived melts associated with the subduction of the Izanagi-Pacific ridge in the Eocene. We propose a new model to interpret the across-arc geochemical variations in the Kurile arc lavas. Kurile forearc mantle, with high Th/Nb due to Eocene ridge subduction, was added to the subduction channel via subduction erosion by the downgoing Pacific plate. This subduction interface reservoir was subsequently metasomatized by deep-sourced hydrous fluids and melts from the slab beneath the volcanic front. The gradual prograde metamorphic modification of these multiply-metasomatized, subducted materials provides the flux that triggered Kurile volcanic front magmatism.

The sources and origins of the mobile element phases that enrich convergent plate margin mantle sources and trigger arc melting remain controversial. New high-precision Pb isotope data in combination with Sr isotopes of Kurile arc volcanic rocks reveal that the decomposition of serpentinite below the subducting slab Moho played an important role in generating slab fluid/melt to fertilize the subarc mantle. The Kurile arc volcanic rocks have compositional features indicative of a role for subducted slab inputs. However, the classic model of decompo-sition of hydrous minerals in subducting marine sediments and altered basalts to produce slab-derived fluid and/ or melt phases do not explain Kurile lava compositions. The correlation between Pb and Sr isotopes in Kurile lavas requires that the slab fluids or melts must involve substantial Pb and Sr contributions from comparatively unaltered basalt/dolerite/gabbro with an isotopic signature similar to Indian Ocean mantle. We suggest that the decomposition of serpentinites from below the subducting slab Moho led to substantial fluid-driven extraction of Pb and Sr from relatively unaltered oceanic crust as these deep fluids transited upward through the subduction interface into the mantle wedge. The presence of Indian-type oceanic crust in the northwestern Pacific basin is consistent with recent plate reconstruction results showing that the Cretaceous-age Izanagi-Pacific ridge, which likely created the oceanic crust beneath the present Kurile arc, was adjacent to the mantle of the Neo-Tethys Ocean. A switch from Pacific-type to Indian-type subducting oceanic crust occurred between 50 and 42 Ma beneath the Izu-Bonin arc based on the Pb isotope systematics of erupted lavas from this period. This observation may indicate that subduction of the Izanagi-Pacific ridge along the Asian continental margin between 50 and 40 Ma reduced the velocity of the Pacific plate relative to that of the Philippine Sea Plate, and led to dextral strike -slip of the Izu-Bonin-Mariana subduction zone.

EDITORIAL article Front. Earth Sci., 21 November 2022Sec. Petrology Volume 10 - 2022 | https://doi.org/10.3389/feart.2022.1092337

Understanding relationships between different components involved in subduction-related recycling of sediment at arc volcanic systems has long been a focus of study. It is generally understood that volcanic arc lavas incorporate materials from the down-going slab including fluids, fluid mobile elements (FME), and fluid modified large ion lithophile elements (LILE), largely derived from subducted sediments and serpentinized ultramafic rocks. Past studies have presumed bulk sediment contributions rather than addressing localized variations in sediment composition. We examine trace element variability along the Lesser Antilles Island Arc (LAIA) and show that sediment inputs via fluid transfer to the mantle wedge are necessary to explain high B (up to 50 ppm), high B/La ratios, and higher As, Cs, Rb in the central LAIA. In contrast, the northern portion of LAIA is enriched in Ba, Sr and U/Th, with low FME. Grenada has low FME abundances, low Ba/La, but higher Sm/La. Our results suggest that modification of LAIA mantle source by fluid addition is important for the central islands but find that sediment enrichment is more likely in the northern section of the LAIA. Li/Yb ratios suggest the Grenada signature is chemically more comparable to sediment from DSDP Site 144 (Demerara Rise), while Martinique magmas in the central LAIA are affiliated to sediments from Site 543 (offshore Dominica). Unexpectedly, the northern portion of LAIA is not directly relatable to Site 543 sediments but instead trends back toward a sediment source similar in composition to Site 144 but with lower B, lower La/Sm and higher Ba. Binary mixing models suggest the northern islands are likely influenced by small amounts of sediment melt similar to average Site 144 sediments, but with Ba concentrations ranging from 1,250 to 1,500 ppm. Trace element ratios then likely result from different levels of contribution from sediment packages both spatially and temporally along the arc, with some variation evident among samples from the same island. We suggest that the process of recycling subducted sediment along the arc is variable along the length of the LAIA, wherein the trace element relationships are likely strongly influenced by the types of sediment being subducted along strike.

A general assumption about the geochemical behavior of phosphorus (P) is that it exists exclusively in the +5 oxidation as phosphate. However, in extremely reducing environments, other oxidation states of phosphorus such as +3 may also be stable. Such environments—if prevalent globally—may determine planetary habitability, which is in part governed by nutrient availability, including the availability of the element phosphorus. Here we show a route to P liberation from water-rock reactions that are thought to be common throughout the Solar System. We report the speciation of phosphorus in several serpentinite rocks and muds to include the ion phosphite (HPO32− with P3+) and show that reduction of phosphate to phosphite may be predicted from thermodynamic models of serpentinization. Furthermore, the amount of phosphite exceeds the amounts predicted from thermodynamic models in three of nine samples analyzed. As a result, as olivine and other silicates in ultramafic rocks alter to serpentine minerals, phosphorus as the significantly more soluble and reactive phosphite ion should be released under low redox conditions, liberating this key nutrient for life. Thus, this element may be accessible to developing life where water is in direct contact with ultramafic rock, providing a source of this nutrient to potentially habitable worlds.

Subduction zones have played a central role in exchanging volatiles (H2O, CO2, S, halogens) between the different Earth's reservoirs throughout its history. Fluids that are released as the subducted plates dehydrate are major agents that transfer these volatiles inside the Earth; but the origins and the compositional evolution of the slab fluids as plates begin to sink are yet to be understood. To explore processes that tool place during subduction infancy, here we examine the compositions of proto-arc magmas from the Izu-Bonin-Mariana (IBM) convergent margin that formed during subduction inception; and we compare these to a modern example of near-trench spreading in the southeast Mariana fore-arc rift (SEMFR). There is a temporal and spatial evolution in the slab fluid composition that is accompanied with a change in the fluid reservoirs, as subduction progresses. During the early stages of the subduction zone, dehydration of the serpentinized subducting mantle likely triggered dehydration and melting of the altered oceanic crust in the amphibolite facies to produce boninites. As the subduction zone matured, the volcanic arc front was displaced away from the trench. The arc magmas captured deeper slab fluids released from the subducted oceanic crust, the sediments and the underlying serpentinized mantle. Dehydration and melting of the subducted sediment became more prevalent with time and increasing slab depth (>= 100 km) to produce arc magmas. This compositional evolution was associated with a deepening of magma generation, which is likely accompanied with the progressive serpentinization of fore-arc mantle and a slab rollback. Hence, fore-arc mantle serpentinization might have facilitated arc maturation and subduction stabilization throughout the IBM history.

How subduction-related magmatism starts at convergent plate margins is still poorly understood. Here we show that boron isotope variations in early-formed boninites from the Izu-Bonin arc, combined with radiogenic isotopes and elemental ratios document rapid (~0.5 to 1 Myr) changes in the sources and makeup of slab inputs as subduction begins. Heterogeneous hornblende-granulite facies melts from ocean crust gabbros ± basalts fluxed early melting to generate low silica boninites. Hydrous fluids from slab sediments and basalts later fluxed the low silica boninites mantle source to produce high silica boninites. Our results suggest that initially the uppermost parts of the slab were accreted near the nascent trench, perhaps related to early low-angle subduction. The rapid changes in slab inputs recorded in the boninites entail a steepening subduction angle and cooling of the plate interface, allowing for subduction of slab sediment and basalt, and generating hydrous fluids at lower slab temperatures.

Abstract International Ocean Discovery Program Expedition 352 to the Izu‐Bonin forearc cored over 800 m of basement comprising boninite and boninite‐series lavas. This is the most extensive, well‐constrained suite of boninite series lavas ever obtained from in situ oceanic crust. The boninites are characterized as high‐silica boninite (HSB), low‐silica boninite (LSB), or basaltic boninite based on their SiO2‐MgO‐TiO2 relations. The principal fractionation products of all three series are high‐Mg andesites (HMA). Lavas recovered >250 meters below seafloor (mbsf) erupted at a forearc spreading axis and are dominated by LSB and HMA. Lavas recovered from <250 mbsf erupted off‐axis and are dominated by HSB. The axial and off‐axis lavas are characterized by distinct chemostratigraphic trends in their major, trace, and isotopic compositions. The off‐axis lavas are chemically similar to boninite from the type locality at Chichijima, with concave‐upward rare earth elements patterns. In contrast, the more abundant axial lavas have distinctly light rare earth element‐depleted patterns and represent a new, previously unsampled precursor to the Chichijima‐type boninite lavas. Petrogenetic modeling suggests that the axial lavas formed by fluxing of refractory mantle (likely the residue from forearc basalt extraction), with amphibolite‐facies melt derived from subducting altered oceanic crust. The upper, off‐axis lavas require an additional component of sediment‐derived melt in addition. Both models are consistent with previously published isotopic data.