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Platinum-group element (PGE) + Re abundances and Os isotopic compositions were determined for metasomatized peridotite and cumulate pyroxenite xenoliths from Kharchinsky Volcano, Kamchatka. Two peridotites have moderately high PGE concentrations with 2.9–3.3 ppb Os and smoothly variable PGE patterns similar to abyssal peridotites and encompassing primitive and depleted mantle compositions. One of these has unradiogenic Os (187Os/188Os = 0.1148) at the lower limit of compositions observed in abyssal peridotite which results in a time of Re depletion (TRD) model age of ∼1.8 Ga. These contrasts Kharchinsky peridotites that are Pt-Ir enriched with sharply fractionated PGE patterns with Pt/Pd = 132–168 and Ir/Ru ∼ 4.0. This pattern of enrichment in the heavier and more highly siderophile Pt and Ir over the lighter and more siderophile-chalcophile Ru and Pd ( Fleet and Stone, 1991 , Fleet et al., 1991 ) is consistent with sulfur depletion under oxidizing metasomatic conditions that leave a residue with PGE host minerals that are dominantly alloys. A single Ru-enriched peridotite with low PGE abundances is interpreted to reflect advanced stages of paragenesis driven by metasomatism and resulting in the loss of all PGE and Re relative to Ru. All Kharchinsky peridotites included in this study have relatively unradiogenic Os (187Os/188Os = 0.1148–0.1314) but some are coupled to PGE and lithophile element characteristics that require oxidizing metasomatic conditions (Ce/Ce*≪1.0) and desulfurization leading to a general loss of PGE and data patterns that are consistent overall with low abundances of relatively radiogenic Os in arc peridotites globally ( Brandon et al., 1996 , Widom et al., 2003 , Saha et al., 2005 ). Kharchinsky pyroxenites, which are relatively undeformed and only weakly metasomatized, have similarly fractionated PGE patterns with enrichments in Pt (5.7–46 ppb) and Pd (0.5–2.7 ppb) but with Os, Ir, and Ru abundances 20 that will produce rapid ingrowth of 186Os relative to 187Os. Deep recycling and storage of such subduction-modified rocks might preclude the need for core-mantle exchange to explain 186Os-enrichments in komatiites and Hawaiian picrites originating in the deep mantle ( Brandon et al., 1998 , Brandon and Walker, 2005 ).

The remnant rear-arc segment of the early Izu-Bonin arc, known as the Kyushu-Palau Ridge (KPR), is a key location where magmatic outputs can be constrained during the lifetime of an island arc. We present new geochemical data for coarse-grained basaltic to andesitic volcaniclastic sandstones derived from the KPR and deposited in the Amami Sankaku Basin (IODP Site U1438, Unit III rocks) in the time period 40–30 Ma. Bulk disaggregated and cleaned volcaniclastic sandstones of Unit III at Site U1438 retain primary magmatic signatures and can be used to infer the evolution of magmatic sources of the juvenile Izu-Bonin island arc through time. A sharp increase of slab-derived components to the source of KPR magmatism developed at about 35 Ma, indicated by increasing Th/La and decreasing Sm/La, Yb/Hf and Nb/Nd. Systematic variations in trace element ratios and increasing trace element abundances in younger samples through the 40–30 Ma time window are decoupled from Hf-Nd isotope ratios, which are measurably more depleted (eHf = 16.5–15, eNd = 9.6–8.2) than boninites produced during the preceding magmatic phase and sampled in the modern Izu-Bonin forearc. Hafnium isotopic compositions in the Unit III sandstones remain little-changed and similar to the subducting Pacific Plate after 40 Ma and do not revert to highly radiogenic compositions of the Indian-type MORB mantle wedge which is reflected in highly-depleted basalts produced at Site U1438 and in the forearc (commonly eHf ≥ 18.0). The overall pattern recorded in Unit III sandstones indicates that the Pacific-type MORB slab-melt component, which was present in the preceding boninite phase of magmatism, persisted after 40 Ma, while the subducted sediment component in the boninite source was lost or significantly reduced. Variations in trace element ratios (at constant eNd and near-constant and radiogenic eHf) and in high field strength element abundances of the early Izu Bonin arc are controlled by the addition of a subducted Pacific MORB melt or supercritical fluid to the mantle wedge. A subducted MORB (slab melt) component is thus sampled throughout the early life of the Izu-Bonin arc and in the currently active Izu-Bonin arc-backarc system.

Typically, the change in lithospheric thickness associated with fracture zones relates directly to the vigor of secondary convection or mantle flow patterns. Therefore, one might expect that mantle flow considerably boosted by the presence of a mantle plume would easily overcome the lithospheric steps created at fracture zone locations. However, to date, there are no studies to verify this assumption. Numerical models based on an example from the SW Indian Ridge suggest that the axial flow driven by a plume (the Marion plume) is indeed likely to be curtailed by the long-offset fracture zones1.We have investigated the interactions between the Jan Mayen fracture zone and Iceland mantle plume in the NE Atlantic by considering (a) the lithospheric and asthenospheric regional configuration and (b) the geochemistry of rocks produced by submarine volcanism.Several global lithospheric models indicate a thinning of the lithosphere on both sides of the Jan Mayen Fracture transform, despite the difference in age of the two adjacent oceanic basins. However, the tomographic models indicate a gap in the asthenospheric flow at the lithosphere-asthenosphere depth under Jan Mayen transform fault, and only a narrow northward channel of this flow is visible under the westernmost part of the fracture zone.Vesteris seamount is an alkaline seamount placed in the central part of the Greenland Basin, located ca. 480 km west from slow-spreading Mohn's ridge and ca. 250 km north from the Jan Mayen Fracture Zone. Vesteris is a solitary volcanic center far away from an active ridge regime with an eruptive age ranging from 650 – 10 ka 2. Here we report new results from geochemical analysis of several samples dredged during the East Greenland Sampling campaign EGS-2012 from the flanks of Vesteris. Whole-rock major and trace elements, together with isotopes and olivine phenocryst mineral data, are used to decipher the source of volcanism at Vesteris Seamount.The Sr-Nd-Pb isotopic signatures indicate that Vesteris volcanism is unrelated to the Iceland mantle plume. Low NiO concentrations in highly forsteritic olivines from Vesteris alkali basalt suggest that the source was dominantly peridotitic. Rare Earth Elements profiles indicate very low degrees of partial melting of a deep mantle source in the presence of residual garnet.Vesteris seamount was formed in a location of a relatively steep gradient of the lithospheric-asthenospheric boundary and close to the northward mantle flow that is carving the Greenland thick lithosphere. The results suggest that the Iceland mantle flow may not have crossed the Jan Mayen Transform Fault; instead, the seamount tapped into a mantle reservoir in the Greenland Basin that preserved the complex history of the Greenland craton and adjacent terranes. REFS. (1) Georgen and Lin, 2003, Plume-transform interactions at ultra-slow spreading ridges: Implications for the SW Indian Ridge, G-cubed, doi:10.1029/2003GC000542; (2) Mertz & Renne, 1995, Quaternary multi-stage alkaline volcanism at Vesteris Seamount (Norwegian—Greenland Sea): evidence from laser step heating 40Ar/39Ar experiments, Journal of Geodynamics, doi:10.1016/0264-3707(94)E0001-B.

The Azores Plateau in the North Atlantic is a classic example of near-ridge oceanic plateau (600 km) associated with the upwelling of the Azores mantle plume. The radiogenic isotope signatures of Azores lavas show systematic inter-island variations, which are often interpreted in terms of sampling several distinct, chemically enriched reservoirs from the Azores plume [1].Here we discuss new radiogenic cerium isotope data on Azores lavas in the context of recent isotope data on olivine-hosted melt inclusions [2]. Olivine-hosted melt inclusions have very high neodymium isotope ratios (up to εNd = 18.1), suggesting that variably depleted mantle is the dominant component of the Azores mantle source [2]. Radiogenic Ce isotopes reflect the time-integrated La/Ce ratio of the mantle source. La/Ce approaches zero values in incompatible element depleted mantle, while the Sm/Nd and Lu/Hf ratios retain higher, more variable values. Melts from variably depleted mantle therefore develop distinct signatures in Ce–Nd–Hf space [3].The new Ce isotope values for 36 whole-rock lava samples covering the whole Azores Plateau reveal a number of parallel, vertically stacked trends in Ce–Nd and Ce–Hf isotope space, pointing to variably incompatible depleted end-members, that are not discernible in Sr–Nd–Pb–Hf isotope space. The observed isotope trends in Ce–Nd–Hf space are readily explained by variable contribution of melts from volumetrically dominant, but variably depleted mantle and similar, but inherently heterogeneous enriched local plume components. Hence, although not directly reflected in the erupted basalts on a whole-rock scale [1, 2], variable contribution of melts from a variably, in part highly depleted mantle control the isotope composition of Azores lavas.These results indicate the North Atlantic mantle below the Azores is variably depleted and contains highly depleted domains. The lavas closest to the proposed plume center [4] do not correspond to either extreme in terms of mantle depletion, suggesting mantle depletion in Azores is inherently complex and not a simple mixing product between plume and ridge mantle. [1] Béguelin et al. (2017) Geochimica et Cosmochimica Acta, 218, 132-152.[2] Stracke et al. (2019) Nature Geoscience, 12(10), 851-855.[3] Willig et al. (2020) Geochimica et Cosmochimica Acta, 272, 36-53.[4] Bourdon et al. (2005) Earth and Planetary Science Letters, 239, 42-56.

To deconvolve contributions from the four overlapping hotspots that form the “hotspot highway” on the Pacific plate—Samoa, Rarotonga, Arago-Rurutu, and Macdonald—we geochemically characterize and/or date (by the 40Ar/39Ar method) a suite of lavas sampled from the eastern region of the Samoan hotspot and the region “downstream” of the Samoan hotspot track. We find that Papatua seamount, located ~60 km south of the axis of the Samoan hotspot track, has lavas with both a HIMU (high μ = 238U/204Pb) composition (206Pb/204Pb = 20.0), previously linked to one of the Cook-Austral hotspots, and an enriched mantle I (EM1) composition, which we interpret to be rejuvenated and Samoan in origin. We show that these EM1 rejuvenated lavas at Papatua are geochemically similar to rejuvenated volcanism on Samoan volcanoes and suggest that flexural uplift, caused by tectonic forces associated with the nearby Tonga trench, triggered a new episode of melting of Samoan mantle material that had previously flattened and spread laterally along the base of the Pacific plate under Papatua, resulting in volcanism that capped the previous HIMU edifice. We argue that this process generated Samoan rejuvenated volcanism on the older Cook-Austral volcano of Papatua. We also study Waterwitch seamount, located ~820 km WNW of the Samoan hotspot, and provide an age (10.49 ± 0.09 Ma) that places it on the Samoan hotspot trend, showing that it is genetically Samoan and not related to the Cook-Austral hotspots as previously suggested. Consequently, with the possible exception of the HIMU stage of Papatua seamount, there are currently no known Arago-Rurutu plume-derived lava flows sampled along the swath of Pacific seafloor that stretches between Rose seamount (~25 Ma) and East Niulakita seamount (~45 Ma), located 1400 km to the west. The “missing” ~20-million-year segment of the Arago-Rurutu hotspot track may have been subducted into the northern Tonga trench, or perhaps was covered by subsequent volcanism from the overlapping Samoan hotspot, and has thus eluded sampling. Finally, we explore tectonic reactivation as a cause for anomalously young volcanism present within the western end of the Samoan hotspot track.

The distribution of water in the upper mantle plays a crucial role in the Earth's deep water cycle, magmatism, and plate tectonics. To better constrain how these large-scale geochemical systems operate, peridotite and pyroxenite mantle xenoliths from Kilbourne Hole (KH) and Rio Puerco (RP) along the Rio Grande Rift (NM, USA) were analyzed for water, and major and trace element contents. These xenoliths sample a lithosphere whose composition was influenced by subduction and rifting, and can be used to examine the effects of melting, metasomatism, and sub-solidus equilibration on the behavior of water. The first result is that in KH xenoliths, olivines underwent negligible H loss during xenolith ascent, i.e. preserved their mantle water contents. These olivine water contents are used to calculate mantle viscosities of 0.5–184 · 1021 Pa·s. These viscosity values are more than 40 times higher than those of the asthenosphere and show that KH peridotites represent samples from the lithosphere. The preservation of olivine water contents is exceptional for off-cratonic xenoliths, and the KH peridotites provide the first estimate of the average concentration of water in Phanerozoic continental mantle lithosphere at 81 ± 30 ppm H2O. The mantle lithosphere beneath the Rio Grande rift is nevertheless heterogeneous with water contents ranging from

Intraplate magmatic provinces found away from plate boundaries provide direct sampling of the composition and heterogeneity of the Earth's mantle. The chemical heterogeneities that have been observed in the mantle are usually attributed to recycling during subduction1-3, which allows for the addition of volatiles and incompatible elements into the mantle. Although many intraplate volcanoes sample deep-mantle reservoirs-possibly at the core-mantle boundary4-not all intraplate volcanoes are deep-rooted5, and reservoirs in other, shallower boundary layers are likely to participate in magma generation. Here we present evidence that suggests Bermuda sampled a previously unknown mantle domain, characterized by silica-undersaturated melts that are substantially enriched in incompatible elements and volatiles, and a unique, extreme isotopic signature. To our knowledge, Bermuda records the most radiogenic 206Pb/204Pb isotopes that have been documented in an ocean basin (with 206Pb/204Pb ratios of 19.9-21.7) using high-precision methods. Together with low 207Pb/204Pb ratios (15.5-15.6) and relatively invariant Sr, Nd, and Hf isotopes, the data suggest that this source must be less than 650 million years old. We therefore interpret the Bermuda source as a previously unknown, transient mantle reservoir that resulted from the recycling and storage of incompatible elements and volatiles6-8 in the transition zone (between the upper and lower mantle), aided by the fractionation of lead in a mineral that is stable only in this boundary layer, such as K-hollandite9,10. We suggest that recent recycling into the transition zone, related to subduction events during the formation of Pangea, is the reason why this reservoir has only been found in the Atlantic Ocean. Our geodynamic models suggest that this boundary layer was sampled by disturbances related to mantle flow. Seismic studies and diamond inclusions6,7 have shown that recycled materials can be stored in the transition zone11. For the first time, to our knowledge, we show geochemical evidence that this storage is key to the generation of extreme isotopic domains that were previously thought to be related only to deep recycling.

Ultramafic-hosted hydrothermal vent systems link the hydrosphere with the peridotitic mantle via serpentinization. Here, the fractionation and mobility of selected trace (Nd, Sm, Gd, Dy, Yb, Sr and Ba) and major cations (Si, Ca, Mg, Fe, Ni) during seawater-peridotite interaction was investigated through a series of experiments where natural olivine grains were reacted with an artificial seawater solution at a range of temperatures (15–90 °C) and grain size distributions. No evidence for any significant olivine dissolution or precipitation of carbonate and, Fe-oxy-hydroxide phases was observed in these experiments. Experimental data show a strong decoupling of REE (Nd, Sm, Gd, Dy, and Yb) from Sr and Ba under all experimental conditions, with Sr and Ba remaining quantitatively in solution. The REE were removed from the solution and were adsorbed onto olivine surface with kinetic rate constants (i.e. uptake over time) that increase with increasing temperature and increasing surface area (i.e. decreasing particle size). Dysprosium and Yb (heavy REE; HREE) were removed from solution with a faster rate than Nd and Sm (light REE; LREE). Gadolinium is decoupled from this trend, with a slower kinetic rate constant than Sm. The activation energies (Ea) of REE adsorption on olivine were higher for Nd and Sm than Dy and Yb. This suggests that the adsorbance of LREE is generally more dependent on temperature than the HREE. The Ea correlates well with the summed 1st, 2nd and 3rd ionization energies of REE suggesting a link between kinetic rates of element adsorption and electron configuration of the 4f-orbitals. Gadolinium has higher Ea than the other analyzed REE, consistent with the electron configuration of Gd3+ where all 4f-orbitals are filled with one electron each. These experimental data suggest that REE are adsorbed on the surface of olivine via inner sphere complexes under low-temperature hydrothermal conditions, when alteration processes are limited or extremely slow. Scavenging and fractionation of REE may occur within the recharge zone of peridotite-hosted hydrothermal systems at relatively low temperatures (

Spatial isotope variations in oceanic island basalts hold clues to the geochemical structure of mantle plumes. This signal is however susceptible to contributions from non-plume sources and modifications by physical processing in the upper mantle, i.e. source hybridization and segregation through gradients in melting conditions. A comprehensive survey of lava isotope systematics is therefore necessary to reveal spatiotemporal patterns at the scale of a single hotspot and disentangle plume sources from upper mantle physical and chemical contributions. We present Hf and Pb isotope data on 72 previously characterized onland and submarine lavas from the islands of Ni‘ihau and Kaua‘i, and from the North Arch Volcanic Field. These lavas cover the entire ∼ 5 Ma eruptive history of Ni‘ihau and Kaua‘i, allowing for a detailed assessment of the temporal geochemical evolution of Hawaiian volcanism. Furthermore, the broad lateral coverage of volcanic activity offered by these lavas constrains the spatial variability of sources across the volcanic chain, allowing investigation of the role of an upper asthenosphere contribution to plume volcanism. Early, shield-building (tholeiitic) lavas from Kaua‘i and Ni‘ihau overlap in Nd–Hf–Sr–Pb isotopes, contrasting with the across-plume asymmetry seen in younger shield lavas (i.e. the LOA-KEA double chain), and their isotope systematics suggest sampling of both LOA and KEA components. In contrast, late Kaua‘i and Ni‘ihau rejuvenated lavas do not overlap in isotope composition. They share common isotope characteristics with rejuvenated lavas from North Arch, Ka‘ula and O‘ahu, and together form two isotopically distinct, southwestern and northeastern groups. The two groups have steep slopes (>1.7) in a eNd − eHf plot and require distinct depleted components. The high eHf for a given eNd, high 87Sr/86Sr, and trace element systematics of the southwestern group (Ni‘ihau, Ka‘ula and O‘ahu) indicate a depleted component intrinsic to the plume. In contrast, lavas from the northeastern group (Kaua‘i and North Arch) indicate a source component isotopically identical to Pacific MORB. The shift from a spatially broad and homogeneous plume source in the shield stage of Kaua‘i and Ni‘ihau (6–3.5 Ma) to a bilaterally heterogeneous source during the rejuvenated stage (

The amalgamation of five Paleo- to Mesoarchean protocontinent blocks [Singhbhum, Bastar, Dharwar, Aravalli and Bundelkhand] resulted in the present-day tectonic configuration of India. The Dongargarh Supergroup is a major volcano-sedimentary sequence in the Bastar craton, central India. For nearly six decades, it was presumed that the extensive lava pile in the Dongargarh Supergroup has accumulated over a protracted duration of magmatic eruptions spanning from early Paleoproterozoic (~2462 Ma) to mid-Mesoproterozoic (~1367 Ma). However, in the absence of any age information, this time-window remained speculative. The detrital zircon U-Pb depositional ages acquired for the three intercalated sedimentary units do not provide any such evidence of significantly prolonged magmatism in the Dongargarh Supergroup. The close proximity between the depositional ages and the similarities between the geochemical attributes of the basalts in the four volcanic formations [Pitapani, Sitagota, Mangikhuta and Kotima] of the Dongargarh Supergroup, suggest contemporaneous evolution spanning few tens of million years around ca. 2.5 Ga. The negative initial eNd(t=2.5 Ga) isotopic compositions in the bulk-rock basalt samples of Pitapani (−0.36 to +0.41) and Mangikhuta (−0.62 to −0.49) suggest interaction with ~3.5 Ga continental crust in the Bastar craton. On the contrary, near chondritic to positive initial eNd(t=2.5 Ga) isotopic compositions in the Kotima (+0.01 to +0.92) and Sitagota (+1.75 to +2.24) basalts are inconsistent with any significant crustal assimilation. Overall, the initial eHf(t=2.5 Ga) isotopic compositions in the four volcanic formations are positive and range from +0.31 to +6.26, which is consistent with their generation from juvenile mantle sources. On the basis of Nb concentrations and rare earth element systematics, the basalts in the Dongargarh Supergroup can be subdivided into two geochemical groups comprising of Sitagota + Kotima with Nb 4.5 ppm. The Zr/Nb and Nb/Yb systematics in these basalts suggest that their mantle sources were relatively fertile and enriched in high field strength elements, which is comparable to the source region of mid-oceanic ridge basalts, and basalts that are generated in the back-arc basins. Collectively, the Dongargarh basalts exhibit negative Nb and Ti anomalies on a primitive mantle normalized incompatible trace element variation diagram. The geochemical signatures in combination with the initial isotopic compositions in these basalts are inconsistent with their origin in a continental-rift setting, or from a mantle plume, as hypothesised by the previous evolutionary models proposed for the Dongargarh Supergroup. Moreover, it cannot be attributed to crustal assimilation sensu stricto. Instead, we interpret the evolution of the volcanic sequences in the Dongargarh Supergroup by the subduction of an oceanic spreading ridge beneath the continental lithosphere, presumably in a back-arc or marginal sea basin that has witnessed contemporaneous sedimentation, which led to the development of an Andean-type magmatic arc around ca. 2.5 Ga in the Bastar craton, central India.

Over the last ~3 Ma, the volcanic complex of Milos Island has evolved from a shallow submarine into a subaerial edifice. It has erupted almost the entire range of calc-alkaline series compositions, but silicic units are volumetrically dominant (Fytikas et al., 1986; Stewart & McPhie, 2006). Although numerous studies have been published, data on the mineral record of the magmatic processes are absent. We examined amphiboles from 3 explosive and 4 effusive units, ranging from andesite to rhyolite, to gain insights into the structure and evolution of the plumbing system. Like many arc volcanoes worldwide, Milos products contain bimodal amphibole populations, often present within the same unit. Mg-hornblende (6.79-7.22 a.p.f.u. Si) forms macro-crysts (>600 μm; often partly decomposed) and crystal clots with plagioclase (An47-51), orthopyroxene (Wo1-2En61-62Fs37-38), and magnetite in the effusive units and phenocrysts (300-600 μm) in more evolved pumices. Mg-hastingsite occurs in effusive units as: (1) pristine micro-phenocrysts (2En68Fs30) rimmed by clinopyroxene (Wo43En47Fs10), plagioclase (An47), and magnetite; and (3) framework-forming crystals in quenched enclaves; and (4) the only amphibole (6.29-6.59 a.p.f.u. Si) phenocrysts in andesitic scoria.Temperature (T) and pressure (P) conditions were calculated by applying hornblende-plagioclase (Holland and Blundy, 1994) and amphibole composition (Ridolfi and Renzulli, 2012) thermo-barometers. Amphibole compositions and calculated P-T conditions are in good agreement with experimentally grown amphiboles. Mg-hornblende compositions and their petrographic context are consistent with cold storage (780±24°C) in a near-solidus, upper crustal (1.7-2.8 kbar) silicic mush. This scenario is further supported by the rhyolitic (74±3.6 wt.% SiO2) compositions of calculated melts in equilibrium with Mg-hornblende, in contrast with the less evolved bulk compositions of the host effusive units. Although the explosive eruptions likely originated from differentiated, crystal-poor melt pockets in the mush, the more common effusions of hybrid andesite-dacite magmas resulted from interaction between mafic recharge magma and the silicic mush. This interaction is preserved in the disequilibrium textures affecting both Mg-hornblendes and Mg-hastingsites, coupled with the growth of high-T (960-885°C) post-recharge Mg-hastingsite. Most of the recharge magmas in Milos are effectively dispersed, trapped, and hybridized in the upper crust, although in rare cases magmas from a deeper crustal storage region (T~960-885°C;P~3.8-5.1 kbar) erupted after limited interaction with the upper crustal storage system.The mineral chemistry reveals that a large, shallow, silicic reservoir has been the dominant component of the Pliocene plumbing system beneath Milos. Magma inputs from deeper crustal sources are preserved in enclaves and volumetrically minor explosive products. The plumbing system of Milos shares similarities with other Aegean arc volcanoes, where magmas experience storage, differentiation, and assimilation in different crustal levels, like Methana (Popa et al., 2020).AcknowledgementsThe research work was supported by the Hellenic Foundation for Research and Innovation (HFRI) under the HFRI PhD Fellowship grant (Fellowship Number: 364).ReferencesFytikas, M. et al. (1986). JVGR,28(3-4),297-317.Holland, T., & Blundy, J. (1994).CTMP,116(4),433-447.Popa, R.G. et al. (2020).JVGR, 106884.Ridolfi, F., &Renzulli, A. (2012).CTMP,163(5),877-895.Stewart, A.L., & McPhie, J. (2006).BulV,68(7-8),703-726.

The Cook-Austral volcanic lineament extends from Macdonald Seamount (east) to Aitutaki Island (west) in the South Pacific Ocean and consists of hotspot-related volcanic islands, seamounts, and atolls. The Cook-Austral volcanic lineament has been characterized as multiple overlapping, age-progressive hotspot tracks generated by at least two mantle plumes, including the Arago and Macdonald plumes, which have fed volcano construction for ∼20 m.y. The Arago and Macdonald hotspot tracks are argued to have been active for at least 70 m.y. and to extend northwest of the Cook-Austral volcanic lineament into the Cretaceous-aged Tuvalu-Gilbert and Tokelau Island chains, respectively. Large gaps in sampling exist along the predicted hotspot tracks, complicating efforts seeking to show that the Arago and Macdonald hotspots have been continuous, long-lived sources of hotspot volcanism back into the Cretaceous. We present new major- and trace-element concentrations and radiogenic isotopes for three seamounts (Moki, Malulu, Dino) and one atoll (Rose), and new clinopyroxene 40Ar/39Ar ages for Rose (24.81 ± 1.02 Ma) and Moki (44.53 ± 10.05 Ma). All volcanoes are located in the poorly sampled region between the younger Cook-Austral and the older, Cretaceous portions of the Arago and Macdonald hotspot tracks. Absolute plate motion modeling indicates that the Rose and Moki volcanoes lie on or near the reconstructed traces of the Arago and Macdonald hotspots, respectively, and the 40Ar/39Ar ages for Rose and Moki align with the predicted age progression for the Arago (Rose) and Macdonald (Moki) hotspots, thereby linking the younger Cook-Austral and older Cretaceous portions of the long-lived (>70 m.y.) Arago and Macdonald hotspot tracks.

The 11th Symposium on Polar Science/Interdisciplinary sessions: [IN] New insights of ship-based interdisciplinary study in Soutnern Ocean, Thu. 3 Dec.

The origin, transport pathway, and spatial variability of total organic carbon (OC) in the western Himalayan glaciers are poorly understood compared to those of black carbon (BC) and dust, but it is critically important to evaluate the climatic role of OC in the region. By applying the distribution of OC activation energy; 14C activity; and radiogenic isotopes of 208Pb/204Pb, 207Pb/204Pb, and 206Pb/204Pb in glacial debris and atmospheric particulate matter (PM10 size fraction), we demonstrate that 98.3 ± 1.6 and 1.7 ± 1.6% of OC in western Himalayan glaciers are derived from biomass and petrogenic sources, respectively. The δ13C and N/C composition indicates that the biomass is a complex mixture of C3 vegetation and autochthonous photoautotrophic input modified by heterotrophic microbial activity. The data set reveals that the studied western Himalayan glacier has negligible contributions from fossil-fuel-derived particles, which contrasts to the central and eastern Himalayan glaciers that have significant contributions from fossil fuel sources. We show that this spatial variability of OC sources relates to regional differences in air mass transport pathways and precipitation regimes over the Himalaya. Moreover, our observation suggests that biomass-derived carbon could be the only primary driver of carbon-induced glacier melting in the western Himalaya.

The driving processes responsible for producing the Central Atlantic Magmatic Province, the Large Igneous Province associated with end-Triassic rifting of Pangea, remain largely debated. Because their compositions encompass most of the Central Atlantic basalt spectrum, tholeiites from southern Eastern North America are considered pivotal for identifying magma origins. New(176)Hf/Hf-177 measurements for 201 Ma Eastern North American tholeiites dominantly record a local petrogenetic history. Their epsilon(Hf)ratios, corrected to an emplacement age of 201 Ma (-7.85 to +5.86), form a positive but shallowly sloped array slightly deviating from the terrestrial array on a epsilon(Hf)versus epsilon(Nd)diagram. Comparison of(176)Hf/Hf-177 to other isotope ratios and trace elements helps to rule out several petrogenetic scenarios, particularly mixing of melts from global depleted or enriched mantle components. In contrast, partial melting of subduction-metasomatized mantle can explain the parental magma composition for southern Eastern North America. Such metasomatism likely occurred during Paleozoic subduction around Pangea and may have been dominated by sediment-derived fluid reactions. The observed(176)Hf/Hf-177 versus(143)Nd/Nd-144 array may reflect subsequent assimilation of lower continental crust, perhaps together with limited direct melting of recycled continental crust in the asthenosphere. The proposed recycling scenario does not specifically support or preclude a mantle plume origin for the Central Atlantic Magmatic Province but instead points toward the presence of a distinct local mantle source and crustal assimilation processes during magma transport. Detailed understanding of these local effects is needed in order to more accurately understand the origins of Large Igneous Provinces.

Water influences geodynamic processes such as melting, deformation and rheology, yet its distribution in the oceanic upper mantle is primarily known indirectly from melt inclusions and glasses of erupted mantle melts (i.e. mid-ocean ridge and ocean island basalts). To better constrain the mechanisms influencing the distribution of H2O in the mantle, particularly regarding the role of metasomatism, we analyzed 15 peridotite xenoliths from Savai‘i and two dunite xenoliths from Ta‘ū (Samoa) for structural H2O (by polarized Fourier transform infrared spectroscopy), and major and trace element concentrations. Clinopyroxenes from the Ta‘ū dunites show trace element concentrations consistent with equilibration with their host lavas, but lower H2O contents than expected. Savai‘i peridotites are highly depleted harzburgites (melt depletion ≥17 %). They show strong evidence of transient metasomatism by both carbonatite and silicate melts, with highly variable Ti and Zr depletions and light rare earth element enrichments. However, despite metasomatism the H2O concentrations in olivines (0 − 4 ppm H2O) and orthopyroxenes (17 − 89 ppm H2O) are among the lowest reported in oceanic xenoliths, but higher than expected for the estimated degree of depletion. In general, H2O concentrations vary less than those of other incompatible trace elements in these samples. Transects across mineral grains show generally homogeneous distributions of H2O, indicating no significant H2O loss or gain during ascent. Raman spectroscopy on inclusions in minerals shows the presence of CO2 but an absence of molecular H2O. This agrees with the absence of H2O concentration variations between inclusion-rich and -poor domains in minerals. The above data can be explained by transient metasomatism along grain boundaries, now recorded as planes of inclusions within annealed grains. Fast diffusion of hydrogen (but not lithophile elements) from the inclusions into the host mineral phase will simultaneously enrich H2O contents across the grain and lower them in the inclusion-rich domains. The result is highly variable metasomatism recorded in lithophile elements, with smaller magnitude H2O variations that are decoupled from lithophile element metasomatism. Comparison with xenoliths from Hawai‘i shows that evidence for metasomatism from lithophile elements alone does not imply rehydration of the oceanic lithosphere. Instead, H2O concentrations depend on the overall amount of H2O added to the lithosphere through metasomatism, and the proximity of sampled material to areas of melt infiltration in the lithosphere.

Rare high-(3)He/(4)He signatures in ocean island basalts (OIB) erupted at volcanic hotspots derive from deep-seated domains preserved in Earth’s interior. Only high-(3)He/(4)He OIB exhibit anomalous (182)W—an isotopic signature inherited during the earliest history of Earth—supporting an ancient origin of high (3)He/(4)He. However, it is not understood why some OIB host anomalous (182)W while others do not. We provide geochemical data for the highest-(3)He/(4)He lavas from Iceland (up to 42.9 times atmospheric) with anomalous (182)W and examine how Sr-Nd-Hf-Pb isotopic variations—useful for tracing subducted, recycled crust—relate to high (3)He/(4)He and anomalous (182)W. These data, together with data on global OIB, show that the highest-(3)He/(4)He and the largest-magnitude (182)W anomalies are found only in geochemically depleted mantle domains—with high (143)Nd/(144)Nd and low (206)Pb/(204)Pb—lacking strong signatures of recycled materials. In contrast, OIB with the strongest signatures associated with recycled materials have low (3)He/(4)He and lack anomalous (182)W. These observations provide important clues regarding the survival of the ancient He and W signatures in Earth’s mantle. We show that high-(3)He/(4)He mantle domains with anomalous (182)W have low W and (4)He concentrations compared to recycled materials and are therefore highly susceptible to being overprinted with low (3)He/(4)He and normal (not anomalous) (182)W characteristic of subducted crust. Thus, high (3)He/(4)He and anomalous (182)W are preserved exclusively in mantle domains least modified by recycled crust. This model places the long-term preservation of ancient high (3)He/(4)He and anomalous (182)W in the geodynamic context of crustal subduction and recycling and informs on survival of other early-formed heterogeneities in Earth’s interior.

Cryoconite holes are depressions in glacial ice surfaces filled with dark debris that reduce albedo. The relative contributions of combustion and microbial carbon to cryoconite carbon are currently not known. To constrain cryoconite organic carbon composition and carbon sources to microorganisms living on glacier surfaces, measurements of bulk organic carbon and microbial phospholipid fatty acids (PLFAs) from supraglacial cryoconite sediment within the ablation zones of Spencer and Matanuska glaciers in southern Alaska were coupled with radiocarbon (14C) analyses. The 14C content of bulk cryoconite organic carbon on both glaciers was depleted relative to the modern atmosphere, while the PLFAs contained carbon that was recently in equilibrium with the atmosphere. Because the bulk cryoconite material is isotopically distinct from the PLFAs, these results indicate that cryoconite organic carbon is not bioavailable to the microbes. Instead, modern carbon in the microbes suggests that carbon is being quickly cycled by them within the cryoconite. Biomarker and stable isotope analyses of cryoconite organic carbon points to fresh carbon inputs to cryoconite and indicates that combusted fossil carbon is a not a major component of cryoconite organic carbon. Trace element analyses of the bulk cryoconite show crustal inputs and no evidence for excess metals associated with recent combustions sources, indicating that the aged bulk cryoconite organic carbon is likely of rock origin (e.g., shale). This study highlights that microbes in cryoconite holes on these glaciers are fixing atmospheric carbon and not using the aged carbon surrounding them.

© 2018 Elsevier Ltd Observed differences in Mg isotope ratios between bulk magmatic rocks are small, often on a sub per mill level. Inter–mineral differences in the 26Mg/24Mg ratio (expressed as δ26Mg) in plutonic rocks are on a similar scale, and have mostly been attributed to equilibrium isotope fractionation at magmatic temperatures. Here we report Mg isotope data on minerals in spinel peridotite and garnet pyroxenite xenoliths from the rejuvenated stage of volcanism on Oahu and Kauai, Hawaii. The new data are compared to literature data and to theoretical predictions to investigate the processes responsible for inter–mineral Mg isotope fractionation at magmatic temperatures. Theory predicts up to per mill level differences in δ26Mg between olivine and spinel at magmatic temperatures and a general decrease in Δ26Mgolivine-spinel (=δ26Mgolivine – δ26Mgspinel) with increasing temperature, but also with increasing Cr# in spinel. For peridotites with a simple petrogenetic history by melt depletion, where increasing depletion relates to increasing melting temperatures, Δ26Mgolivine-spinel should thus systematically decrease with increasing Cr# in spinel. However, most natural peridotites, including the Hawaiian spinel peridotites investigated in this study, are overprinted by variable extents of melt-rock reaction, which disturb the systematic primary temperature and compositionally related olivine–spinel Mg isotope systematics. Diffusion, subsolidus re-equilibration, or surface alteration may further affect the observed olivine–spinel Mg isotope fractionation in peridotites, making Δ26Mgolivine-spinel in peridotites a difficult–to–apply geothermometer. The available Mg isotope data on clinopyroxene and garnet suggest that this mineral pair is a more promising geothermometer, but its application is restricted to garnet–bearing igneous (garnet pyroxenites) and metamorphic rocks (eclogites). Although the observed δ26Mg variation is on a sub per mill range in bulk magmatic rocks, the clearly resolvable inter–mineral Mg isotope differences imply that crystallization or preferential melting of isotopically distinct minerals such garnet, spinel, and clinopyroxene should cause Mg isotope fractionation between bulk melt and residue. Calculated Mg isotope variations during partial mantle melting indeed predict differences between melt and residue, but these are analytically resolvable only for melting of mafic lithologies, that is, garnet pyroxenites. Contributions from garnet pyroxenite melts may thus account for some of the isotopically light δ26Mg observed in ocean island basalts and trace lithological mantle heterogeneity. Consequently, applications for high-temperature Mg isotope fractionations are promising and diverse, and recent advances in analytical precision may allow the full petrogenetic potential inherent in the sub per mill variations in δ26Mg in magmatic rocks to be exploited.

We present 176Hf/177Hf isotope ratios on 41 previously well-characterized subaerial and submarine samples from the Azores islands of Sao Miguel, Terceira, Graciosa, Faial, Pico and the Joao de Castro seamount (on the Terceira Rift). In eNd–eHf isotope space all Azores lavas fall below the mantle array reference line and do not overlap the proximal Atlantic MORB. Lavas from Sao Miguel and Joao de Castro form two distinct and well defined arrays extending below the mantle array, which has not been previously documented in other oceanic magmatic provinces. The Nd-Hf isotope compositions of Joao de Castro overlap those of HIMU type lavas, yet they lack the characteristically radiogenic Pb isotope ratios of HIMU. The combined Nd-Hf-Pb-Sr isotope systematics of both Sao Miguel and Joao de Castro endmembers can be explained by recycling of a single package of heterogeneous oceanic crust ranging from D-MORB to E-MORB in composition, with an age between 2.5 and 3.0 Ga, with no requirement for parent-daughter ratio modification during subduction. In contrast the Nd-Hf-Pb isotope systematics of lavas from Sao Jorge, Terceira, Graciosa, Pico and Faial are consistent with the presence of younger (

Picrites with anomalous high-Fe (ferropicrites), have erupted from the Archean to recent, however, their incidences are extremely limited. Particularly, post-Archean occurrences are scarce. Experimental studies indicate that ferropicrites are derived from partial melting of the upper mantle at moderate pressures (≤4 GPa). The origin of the anomalous Fe-enrichment in their source, however, remains elusive, and in part attributed to the presence of silica-deficient pyroxenite. In this contribution we present a detailed account of ferropicrites in the Chitrangi Formation of the Paleoproterozoic Mahakoshal supracrustal belt in the Central India Tectonic Zone. The Lu-Hf isotope systematics of these rocks imply an age between ∼2.3 Ga and ∼1.8 Ga. Compared to the Lu-Hf isotopic system, the Sm-Nd isotopic system appears to be disturbed. Nevertheless, the combined Hf and Nd depleted mantle model ages of these lavas are consistent at ∼2.1 Ga, which we propose as the maximum age. Geochemically, the rocks have moderate to high concentrations of Fe 2 O 3 (T) (13–19 wt%) and MgO (6–18 wt%), with moderate to low SiO 2 (49–42 wt%) and Al 2 O 3 (14–9 wt%) contents, and low Al 2 O 3 /TiO 2 ( N ∼10. The primitive mantle normalized positive Nb anomalies in combination with key trace element ratios, and their Hf, Nd isotope systematics preclude contamination by Archean upper continental crust. Further, their juvenile Hf, Nd isotope systematics are inconsistent with long lived recycled crustal or sedimentary components in the mantle source of the Mahakoshal ferropicrites. In contrast, the enrichments in the incompatible lithophile elements suggest that the source of the Mahakoshal ferropicrites was refertilized by melts, possibly from an upwelling mantle plume shortly prior to eruption. The preserved pillow structures imply that magmatism took place in an ocean basin. Therefore, we argue for the existence of a proto-ocean prior to the amalgamation of the northern cratonic block consisting of Aravalli and Bundelkhand proto-continents, and the southern cratonic block consisting of Singhbhum, Bastar and Dharwar proto-continents, which were still adrift at ∼2.1 Ga.

Few long-term environmental records are available for southern Africa where shifts in atmospheric circulation and changes in sea surface temperatures interact to influence regional climate dynamics. We present downcore grain size and inorganic geochemistry data covering the last ~ 23,000 years from a peatland on the east coast of South Africa and examine links between shifts in regional wind activity and palaeoclimatic variability. Our record documents substantial variations in aeolian flux associated with changes in regional climate and wind patterns that reflect larger scale atmospheric circulation patterns. Substantially higher fluxes observed during the Last Glacial Maximum (LGM) are linked to widespread aridification and an expansion in local source areas brought about by a clear shift to dry and cool conditions. Variations in grain size distribution reveal that the aeolian record from Mfabeni comprises two dominant end-members; locally-derived coarse-grained material and a more fine-grained dust component. Marked changes in composition and modal grain size suggest that hydrological shifts in the region during the LGM were accompanied by an increase in storm frequency and wind strength that we link to a northward displacement in the westerly wind belt and a strengthening in wind intensity. Coupling between a rapid increase in sea surface temperature (SST) and an approximate three-fold decrease in aeolian activity after 15 kcal yr BP suggests that changes in SST and its effect on the position and intensity of the westerlies in the Southern Ocean was the dominant climatic driver in the region during deglaciation. Substantially lower aeolian activities through the early Holocene indicate a warming in regional climate and the establishment of more humid conditions under the influence of enhanced tropical easterly flow. Our record also documents more subtle changes in climate over the mid to late Holocene and provides support for an arid phase in southern African climate 6–4 kcal yr BP, as well as an increase in climate variability associated with a strengthening in El Nino–Southern Oscillation (ENSO) activity ~ 2 kcal yr BP. The study contributes to current knowledge of atmospheric circulation patterns in the Southern Hemisphere and provides new insight into links between aeolian activity, regional wind patterns and climatic variability over glacial-interglacial timescales for a region where existing palaeoclimate records are scarce.

Earth’s mantle has cooled since the Archaean. Geochemical identification of anomalously hot lavas formed above the Galapagos Plume 89 million years ago, however, implies that a hot mantle reservoir may have persisted for billions of years.

Pliocene alkali basalts from the Bakony-Balaton Highland Volcanic Field (BBHVF) in the western Pannonian Basin carry mantle xenoliths comprising hydrous and anhydrous spinel peridotites. The studied mantle xenoliths from Szentbekalla, near Lake Balaton, Hungary, are fine- and coarse-grained fertile to depleted spinel lherzolites, spinel harzburgites and dunites, with protogranular, porphyroclastic, and secondary protogranular and mosaic equigranular textures. Melt pockets, with shapes resembling amphibole, are common in a number of samples, whereas other samples have thin films of intergranular glass. Bulk-rock major element abundances show that the mantle lithosphere beneath this area experienced variable degrees (up to 20%) of partial melting. The clinopyroxene trace elements systematics retain a record of melt depletion and metasomatic processes attributed to subduction-related melt/fluids or to the infiltration of percolating undersaturated melts in the Pannonian lithospheric mantle. The radiogenic isotopes of Sr, Nd and Hf in clinopyroxene suggest that this metasomatism was a relatively recent event. Textural evidence suggests that the calcite filling up the vesicles in the melt pockets and in veinlets cross-cutting the constituent minerals is of epigenetic nature and not due to carbonatite metasomatism. The non-metasomatized primitive mantle normalized clinopyroxene REE abundances mimic those, but at higher values, of their bulk-rock REE patterns. Bulk-rock and clinopyroxene REE with upward and downward LREE respectively, indicate up to 2.4% host basalt infiltration. The calculated bulk composition of the melt pockets is identical to small amphibole relics found as inclusions in second generation clinopyroxene within the melt pockets, suggesting incongruent melting of amphibole, without the need for additional metasomatic melt/fluids to initiate the amphibole breakdown. The heat for the temperature increase necessary for amphibole breakdown was derived from the host basalt. The estimated time for diffusive Ca exchange between matrix olivine and olivine overgrowth in contact with the melt pockets is very short, ranging between 21 and 200 days, indicating that amphibole breakdown took place immediately before or during the xenolith entrainment in the alkali basalt.

Understanding the evolution of passive margins requires knowledge of temporal and chemical constraints on magmatism following the transition from super-continent to rifting, to post-rifting evolution. The Eastern North American Margin (ENAM) is an ideal study location as several magmatic pulses occurred in the 200 My following rifting. In particular, the Virginia-West Virginia region of the ENAM has experienced two post-rift magmatic pulses at ∼152 Ma and 47 Ma, and thus provides a unique opportunity to study the long-term magmatic evolution of passive margins. Here we present a comprehensive set of geochemical data that includes new 40Ar/39Ar ages, major and trace-element compositions, and analysis of radiogenic isotopes to further constrain their magmatic history. The Late Jurassic volcanics are bi-modal, from basanites to phonolites, while the Eocene volcanics range from picrobasalt to rhyolite. Modeling suggests that the felsic volcanics from both the Late Jurassic and Eocene events are consistent with fractional crystallization. Sr-Nd-Pb systematics for the Late Jurassic event suggests HIMU and EMII components in the magma source that we interpret as upper mantle components rather than crustal interaction. Lithospheric delamination is the best hypothesis for magmatism in Virginia/West Virginia, due to tectonic instabilities that are remnant from the long-term evolution of this margin, resulting in a “passive-aggressive” margin that records multiple magmatic events long after rifting ended. This article is protected by copyright. All rights reserved.

Platinum Group Element (PGE) pollution on the Indian subcontinent is a growing concern because vehicle sales in India have rapidly increased over the last decade, and it is well known that automobile catalytic converters are one of the major source of anthropogenic PGE in the environment. Despite the rapid growth of the Indian automobile industry, the sources and magnitude of PGE contamination in Indian airborne particles are unknown. In this study we report PGE and mercury (Hg) concentrations, as well as osmium isotope ratios (187Os/188Os) of airborne particles (PM10) collected in Kanpur, a large industrial city in India. We estimate that 61 ± 22%, 32 ± 24%, and 7 ± 3% of the total Os fraction are derived from eroding upper continental crust, catalytic converters fitted in the exhaust system of motor vehicles, and fossil fuel combustion, respectively. Only one sample had a ten times higher (∼76%) than average contribution from fossil fuel. Unlike Os, Pt is predominantly (84 ± 10%) derived from anthropogenic sources. Platinum Group Element and Hg concentrations are not well correlated. However, the highest concentration of particulate Hg corresponds to the most radiogenic 187Os/188Os isotope ratios (4.6). Our results further indicated that PGE/Ir ratios could be successfully used to quantify the relative proportions of natural and anthropogenic PGE sources in aerosol samples. Since PGE and Hg data on Indian environmental samples are scarce, this study provides an interpretive framework that calls for additional assessments of PGE and Hg concentrations in environmental samples from India.

Serpentinized abyssal peridotites are the product of the chemical interaction between the Earth's oceans and the peridotitic upper mantle. This reaction is dominated by the addition of water and fluid mobile elements to the peridotite, with implications on the cycling of water and fluid mobile elements in subduction zones. Rare earth elements (REE) were generally thought as immobile during serpentinization. However the identification of Nd (and Sr) isotopic disequilibrium between fresh clinopyroxene and their host bulk serpentinized peridotite suggest addition of Nd and other light REE to the peridotite during serpentinization ( Frisby et al. (2016) ). As with the REE, the high-field strength elements (HFSE) have also been considered immobile and non-reactive during serpentinization. However, this has yet to be explicitly tested. Here we present a modified dissolution and extraction method that improves on the yields of REE and HFSE from highly depleted ultramafic rocks (peridotites) through the use of B(OH)3 during digestion dry-down steps, Fe-precipitation for REE-HFSE sequestration, and liquid–liquid exchange for Fe removal. Using this method we report on the Nd and Hf isotope compositions of bulk rock serpentinized abyssal peridotites from the South-West Indian Ridge (SWIR), Mid-Atlantic Ridge (MIR) and Mid-Caymen Rise (MCR) and on samples where their clinopyroxene Hf, Nd isotopes were previously reported. The bulk rock Hf isotopes are equal to clinopyroxene to within 1 eHf, while Nd isotopes in the bulk peridotite are up to 8 eNd units less radiogenic than their clinopyroxene. Melt-rock reaction between the depleted peridotite endmember and local enriched lavas cannot generate the observed decoupling between Hf and Nd isotopes. Our data is instead consistent with seawater — peridotite interaction as this results in significant shift in Nd isotopes with little to no change in Hf isotopes until extremely high water/rock mass ratios of > 100,000. Our data shows that Hf isotopes are not affected by serpentinization and that Hf isotopes in bulk peridotites better record the magmatic composition of peridotites than Nd or Sr. The largest Nd isotopic difference between clinopyroxene and bulk peridotite is observed in the most LREE-depleted samples, while refertilized samples show little change. Based on the bulk rock (serpentinized) and clinopyroxene (magmatic value) Nd isotope compositions we estimate that 1–34% of the bulk rock Nd budget is seawater derived, but only 1–6% of Sm and

We report on the bulk-rock geochemistry and Lu-Hf isotope systematics, and infer the petrogenesis of the ultramafic rocks in the Neoarchean Veligallu greenstone terrane, eastern Dharwar craton, India. The ultramafics along with the basalts and adakites yield a bulk-rock Lu-Hf isochron age of 2.696 ± 0.054 Ga, consistent with the available SIMS zircon U-Pb age of 2.697 ± 5 Ma, reported for the felsic volcanics, in the literature. The rocks have positive initial eHf(2.696 Ga) = +3.0 to +6.5, consistent with an origin from a long term depleted mantle source relative to a chondritic reservoir at ∼2.7 Ga. Geochemically, the ultramafic rocks are characterized by high MgO = 24–34 wt.%, Mg# ∼ 89, Cr = 2290–3855 ppm and Ni = 604–966 ppm contents; moderate to low SiO2 = 52–49 wt.%, Al2O3 = 9.2–4.7 wt.%, and TiO2 = 0.30–0.14 wt.%. To first order, these chemical compositions broadly resemble komatiites. The rocks exhibit higher than chondrite Al2O3/TiO2 (21–37) ratio, depletions in their middle-REE relative to the heavy-REE i.e. GdN/YbN

The margin of the western North Pacific Ocean releases redox-active elements like Mn, Co, and Ce into the water column to undergo further transformation through oxide formation, scavenging, and reductive dissolution. Near the margin, the upper ocean waters enriched in these elements are characterized by high dissolved oxygen, low salinity, and low temperature, and are a source of the North Pacific Intermediate Water. High dissolved concentrations are observed across the Western Subarctic Gyre, with a rapid decrease in concentrations away from the margin and across the subarctic-subtropical front. The particulate concentrations of Mn, Co, and Ce are also high in the subarctic surface ocean and enriched relative to Ti and trivalent rare earth elements. Furthermore, the particles enriched in Mn, Co, and Ce coincide at the same depth range, suggesting that these elemental cycles are coupled through microbial oxidation in the subarctic gyre as the waters travel along the margin before being subducted at the subarctic-subtropical front. Away from the margin, the Mn, Co, and Ce cycles decouple, as Mn and Ce settle out as particles while dissolved Co is preserved and transported within the North Pacific Intermediate Water into the central North Pacific Ocean.

A. M. acknowledges support from the Swiss National Science Foundation (grant 200020/135511 to Othmar Muntener and grant P2LAP2_171819 to A. M.) and Swiss IODP. This work was also supported by grants to G. M. Y. and M. B. from NSF (OCE‐1537135) and IODP.

Sanitary sewer overflows (SSOs) are a common problem across the United States. An estimated 23,000–75,000 SSOs occurred annually in 2004 discharging between 11 and 38 billion liters of untreated wastewater to receiving waters. SSOs release many contaminants, including engineered nanomaterials (ENMs), to receiving water bodies. Measuring ENM concentrations in environmental samples remains a key challenge in environmental nanotechnology and requires the distinction between natural and engineered particles. This distinction between natural and engineered particles is often hampered by the similarities in the intrinsic properties of natural and engineered particles such as particle size, composition, density, surface chemistry, and by the limitations of the available nanometrology tools. To overcome these challenges, we applied a multi-method approach to measure the concentrations and properties of TiO(2) engineered particles (e.g., ENMs and pigments) including 1) multi-element single particle-inductively coupled plasma-mass spectrometry (ME-SP-ICP-MS) to identify elemental associations and to determine elemental ratios in natural particles, 2) total elemental concentrations and ratios calculated from total metal concentrations measured following total sample digestion to estimate engineered particle concentrations, and 3) transmission electron microscopy (TEM) to characterize engineered particle size and morphology. ME-SP-ICP-MS analysis revealed that natural TiO(2) particles are often associated with at least one of the following elements Al, Fe, Ce, Si, La, Zr, Nb, Pb, Ba, Th, Ta, W and U, and that elemental ratios of Ti to these elements is typical of riverine particulates and the average crustal ratios, except for Pb likely due to anthropogenic Pb contamination. High TiO(2) engineered particle concentrations up to 100 μg L(−1) were found in SSOs-impacted surface waters. TEM analysis demonstrated the presence of regular-shape TiO(2) particles in SSOs-impacted surface waters. This study provides a comprehensive approach for measuring TiO(2) engineered particle concentrations in surface waters. The quantitative data produced in this work can be used as input for modeling studies and pave the road toward routine monitoring of ENMs in environmental systems, validation of ENM fate models, and more accurate ENM exposure and risk assessment.