A new olivine reference material – MongOL Sh11‐2 – for in situ analysis has been prepared from the central portion of a large (20 × 20 × 10 cm) mantle peridotite xenolith from a ~ 0.5 My old basaltic ...breccia at Shavaryn‐Tsaram, Tariat region, central Mongolia. The xenolith is a fertile mantle lherzolite with minimal signs of alteration. Approximately 10 g of 0.5–2 mm gem quality olivine fragments were separated under binocular microscope and analysed by EPMA, LA‐ICP‐MS, SIMS and bulk analytical methods (ID‐ICP‐MS for Mg and Fe, XRF, ICP‐MS) for major, minor and trace elements at six institutions world‐wide. The results show that the olivine fragments are sufficiently homogeneous with respect to major (Mg, Fe, Si), minor and trace elements. Significant inhomogeneity was revealed only for phosphorus (homogeneity index of 12.4), whereas Li, Na, Al, Sc, Ti and Cr show minor inhomogeneity (homogeneity index of 1–2). The presence of some mineral and fluid‐melt micro‐inclusions may be responsible for the inconsistency in mass fractions obtained by in situ and bulk analytical methods for Al, Cu, Sr, Zr, Ga, Dy and Ho. Here we report reference and information values for twenty‐seven major, minor and trace elements.
Olivine grains from a mantle xenolith were analysed by EPMA, LA‐ICP‐MS, SIMS, ID‐ICP‐MS, ICP‐MS and XRF in 6 institutions worldwide.
120 olivine grains tested showed uniform mass fractions of all elements except phosphorus.
Well‐characterised reference values were obtained for major (Si, Mg, Fe), minor (Ni, Mn) and trace elements (Li, Na, Al, Ca, Sc, Ti, V, Cr, Co, Cu, Zn, Y, Er, Tm, Yb, Lu).
Dacitic to rhyolitic glass shards from 80 widespread tephras erupted during the past 5 Mys from calderas in Kyushu, and SW, central, and NE Japan were analyzed. Laser ablation inductively coupled ...plasma mass spectrometry was used to determine 10 major and 33 trace elements and 207Pb/206Pb‐208Pb/206Pb isotope ratios. The tephras were classified into three major geochemical types and their source rocks were identified as plutonic, sedimentary, and intermediate amphibolite rocks in the upper crust. A few tephras from SW Japan were identified as adakite and alkali rhyolite and were regarded to have originated from slab melt and mantle melt, respectively. The Pb isotope ratios of the tephras are comparable to those of the intermediate lavas in the source areas but are different from the basalts in these areas. The crustal assimilants for the intermediate lavas were largely from crustal melts and are represented by the rhyolitic tephras. A large heat source is required for forming large volumes of felsic crustal melts and is usually supplied by the mantle via basalt. Hydrous arc basalt formed by cold slab subduction is voluminous, and its heat transfer with high water content may have melted crustal rocks leading to effective felsic magma production. Coincidence of basalt and felsic magma activities shown by this study suggests caldera‐forming eruptions are ultimately the effect of a mantle‐driven cause.
Key Points:
LA‐ICPMS analyzed major trace elements and Pb isotopes of glass shards
Geochemistries of late Cenozoic widespread tephras in Japan are described
Tephras are melts from crustal amphibolites, plutonic, and sedimentary rocks
Magmatism at some intraplate volcanoes and large igneous provinces (LIPs) in continental areas may originate from hydrous mantle upwelling (i.e. a plume) from the mantle transition zone (MTZ) at ...410-660 km depths in the Earth's deep interior. However, the ultimate origin of the magmatism, i.e. why mantle plumes could have been generated at the MTZ, remains unclear. Here, we study the buoyancy of a plume by investigating basalts from the Changbaishan volcano, beneath which a mantle plume from the hydrous MTZ is observed via seismology. Based on carefully determined water contents of the basalts, the potential temperature of the source mantle is estimated to be 1310-1400 °C, which is within the range of the normal upper mantle temperature. This observation suggests that the mantle plume did not have a significant excess heat, and that the plume upwelled because of buoyancy resulting from water supplied from the Pacific slab in the MTZ. Such a hydrous mantle plume can account for the formation of extremely hydrous LIP magmatism. The water was originally sourced from a stagnant slab and stored in the MTZ, and then upwelled irrespective of the presence or absence of a deep thermal plume.
A certain type of deep‐sea sediment exhibits very high content of rare earth elements and yttrium (REY) and is therefore expected to serve as a novel resource for these industrially essential metals. ...In this paper, we statistically analyzed the bulk chemical composition of deep‐sea sediments collected from the western North Pacific Ocean. By applying independent component analysis to the multielemental data set, we extracted three independent components (ICs) that can be interpreted as the influence of Mn‐oxides (IC1), REY‐enriched biogenic calcium phosphate (IC2), and possibly a diagenetic effect involving Cu enrichment (IC3) on bulk sediment geochemistry. Subsequently, we selected representative samples based on the independent component analysis result, and implemented Sr–Nd–Pb isotopic analyses of bulk sediments. The results indicate that the extremely REY‐rich mud characterized by IC2 inherits the geochemical signature of deep Pacific seawater, whereas the non‐REY‐rich mud with less diagenetic alterations, characterized by IC3, implies an influence of terrigenous dust probably from the Taklimakan desert–Chinese loess plateau. IC1 may reflect the variation in sedimentation rates. Characteristic downhole variations of IC1 and IC3 scores imply the presence of hiatus and/or erosion of the sediment column across the REY content peak. The putative cause is an enhanced bottom current, which can physically separate coarse biogenic calcium phosphate grains with very high REY content and thus produce an extremely REY‐enriched sediment layer.
Plain Language Summary
Recent studies discovered that deep‐sea sediment in the western North Pacific Ocean is remarkably enriched in rare earth elements that are essential for modern high‐tech products. These sediments are thus expected to serve as a new resource for these metals. However, the origin of such high concentrations of rare earth elements in the abyss is unclear. In this paper, we implemented statistical and isotopic analyses of these sediments, and discovered that they are characterized by three components: manganese oxides, biogenic calcium phosphates concentrating rare earth elements from seawater, and a chemical process that moves copper in the sediment. We suggest that a strong bottom current is the cause of a selective deposition of biogenic calcium phosphates that produces mud with an extreme enrichment in rare earth elements.
Key Points
Multivariate analysis on geochemical data of deep‐sea sediments extracts key components associated with enrichment of rare earth elements
Sr–Nd–Pb isotope analyses indicate systematic changes regarding sources of elements in the western North Pacific sediment
Enhanced bottom current may cause significant enrichment of rare earth elements in the deep‐sea sediment
•Isotopically heavy K documented in Izu arc front.•Systematic shift to lighter K-isotope ratios in Izu rear arc.•Heavy K may reflect isotopic fractionation in association with slab ...dehydration.•Across-arc shift may reflect progressive depletion of slab in heavy K.•Potassium-isotope variations in arc lavas may be sensitive tracer of slab processes.
In subduction zones, fluids rise from the slab to the mantle, causing metasomatism and flux melting of the mantle to produce arc magmas. The transfer of material from slab to mantle and, in turn, to arc crust is an important control on the long-term chemical evolution of the mantle and continental crust. In this study, we investigate the transport of K in subduction zones by exploring the systematics of K stable-isotope variations in lavas of the Izu arc. We find that the Izu lavas have isotopically heavy K relative to estimates for midocean ridge basalt (MORB)-source upper mantle. Moreover, the δ41K values of the lavas are clearly heavier than those of subducting sediments and are probably heavier than subducting altered ocean crust. An across-arc decrease in δ41K values is apparent. Arc-front lavas are heavier than the mantle by about 0.22‰ (median), whereas rear-arc lavas are heavier by only about 0.08‰ (median). The heavy K-isotope compositions of the arc lavas may arise from isotopic fractionation during slab dehydration, where light K is preferentially retained in phases such as phengite in the slab. The across-arc decrease in δ41K values may be due to progressive breakdown of these phases, and to associated depletion of the slab in heavy K. Variations in the relative contributions of different source materials—igneous ocean crust, sediment, and mantle peridotite—may also play a role. In particular, we explore a possibility, motivated by radiogenic-isotope studies, that the slab signal in K isotopes may be attenuated in the rear arc as a result of extensive fluid-mantle interaction. If K isotopes do track slab dehydration, then K isotopes provide insight into the transfer of K and similarly mobile elements out of the slab and into the upper mantle and arc crust. Lastly, we observe extreme isotopic variations in some of the lavas, which we interpret to result from crustal-level or Earth-surface processes that affect only a subset of the lavas.
We present a new numerical trace element mass balance model for adiabatic melting of a pyroxenite‐bearing peridotite for estimating mantle potential temperature, depth of melting column, and ...pyroxenite fraction in the source mantle for a primary ocean basalt/picrite. The Ocean Basalt Simulator version 1 (OBS1) uses a thermodynamic model of adiabatic melting of a pyroxenite‐bearing peridotite with experimentally/thermodynamically parameterized liquidus‐solidus intervals and source mineralogy. OBS1 can be used to calculate a sequence of adiabatic melting with two melting models, including (1) melting of peridotite and pyroxenite sources with simple mixing of their fractional melts (melt‐melt mixing model), and (2) pyroxenite melting, melt metasomatism in the host peridotite, and melting of the metasomatized peridotite (source‐metasomatism model). OBS1 can be used to explore (1) the fractions of peridotite and pyroxenite, (2) mantle potential temperature, (3) pressure of termination of melting, (4) degree of melting, and (5) residual mode of the sources. In order to constrain these parameters, the model calculates a mass balance for 26 incompatible trace elements in the sources and in the generated basalt/picrite. OBS1 is coded in an Excel spreadsheet and runs with VBA macros. Using OBS1, we examine the source compositions and conditions of the mid‐oceanic ridge basalts, Loihi‐Koolau basalts in the Hawaiian hot spot, and Jurassic Shatsky Rise and Mikabu oceanic plateau basalts and picrites. The OBS1 model shows the physical conditions, chemical mass balance, and amount of pyroxenite in the source peridotite, which are keys to global mantle recycling.
Key Points:
Source of ocean basalts originated from pyroxenite‐bearing peridotite
Thermodynamic adiabatic melting model
Experimental/thermodynamic parameterization of melting mineralogy
The Carnian Pluvial Episode (CPE) was a global environmental change and biotic crisis that occurred during the Carnian (Late Triassic). The climate during the CPE was characterized by a short-lived ...period of extreme rainfall, and an extinction of marine taxa is known to have occurred during the latest Julian (i.e. early Carnian). Although these events are considered to have been caused by the Wrangellia Flood Basalt (FB) volcanism, existing studies have found little direct evidence to support this. We investigated the temporal relationship between the eruption of Wrangellia FB and CPE using high-resolution microfossil biostratigraphy and paleo-seawater Os isotope data of an Upper Triassic bedded chert succession from an accretionary complex in Japan, which accumulated in a pelagic deep-sea environment in an equatorial region of the Panthalassa Ocean. Our biostratigraphic analysis, based on conodonts and radiolarians, and osmium isotope data show: (i) a continuous decline of initial Os isotope ratios (187Os/188Osi) in the early Julian; (ii) low 187Os/188Osi ratios during the late Julian; and (iii) an abrupt increase in 187Os/188Osi ratios at the end of the Julian. The decrease in 187Os/188Osi ratios throughout the Julian suggests an increased input of unradiogenic Os from the eruption of the Wrangellia FB into the ocean. Moreover, redox-sensitive elements, such as V and U, increased abruptly at the end of the Julian, which is the first evidence of reducing conditions during the CPE within the pelagic deep-sea Panthalassa Ocean. Marine anoxic event in the late Julian has been recognized from widespread deposition of black shales and organic-rich marls in intermediate to shallow water Tethyan sections. Thus, oxygen-depleted conditions occurred at the Tethyan shallow continental margin, as well as in the pelagic deep-sea Panthalassa Ocean, at the end of Wrangellia FB volcanism.
•An Os isotopic profile of the Carnian chert succession in Japan was determined.•Os isotope data indicate the Wrangellia LIP formed during the Julian.•Two negative δ13Corg shifts were detected in the Julian 2 in the study section.•Enrichment of redox-sensitive elements suggests oxygen-depleted conditions occurred in the Panthalassa in the Julian 2.
Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB‐like tholeiitic basalts crop out over large areas. These “fore‐arc ...basalts” (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near‐trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu‐Bonin‐Mariana (IBM) back‐arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid‐ocean ridges and back‐arc basins. Some FAB also have higher concentrations of fluid‐soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near‐trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water‐rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near‐trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.
We present a new method for estimating the composition of water‐bearing primary arc basalt and its source mantle conditions. The PRIMACALC2 model uses a thermodynamic fractional crystallization model ...COMAGMAT3.72 and runs with an Excel macro to examine the mantle equilibrium and trace element calculations of a primary basalt. COMAGMAT3.72 calculates magma fractionation in 0–10 kb at various compositions, pressure, oxygen fugacity, and water content, but is only applicable for forward calculations. PRIMACALC2 first calculates the provisional composition of a primary basalt from an observed magma. The basalt composition is then calculated by COMAGMAT3.72 for crystallization. Differences in elemental concentrations between observed and the closest‐match calculated magmas are then adjusted in the primary basalt. Further iteration continues until the calculated magma composition converges with the observed magma, resulting in the primary basalt composition. Once the fitting is satisfied, back calculations of trace elements are made using stepwise addition of fractionated minerals. Mantle equilibrium of the primary basalt is tested using the Fo‐NiO relationship of olivine in equilibrium with the primary basalt, and thus with the source mantle. Source mantle pressure, temperature, and degree of melting are estimated using petrogenetic grids based on experimental data obtained in anhydrous systems. Mantle melting temperature in a hydrous system is computed by adjusting T with a parameterization for a water‐bearing system. PRIMACALC2 can be used either in dry or water‐bearing arc magmas and is also applicable to mid‐ocean ridge basalts and nonalkalic ocean island basalts.
Key Points
Model calculation of primary arc basalt magma
Thermodynamic fractional crystallization model
Mantle equilibrium determined using petrogenetic grids
Potential risks of supply shortages for critical metals including rare-earth elements and yttrium (REY) have spurred great interest in commercial mining of deep-sea mineral resources. Deep-sea mud ...containing over 5,000 ppm total REY content was discovered in the western North Pacific Ocean near Minamitorishima Island, Japan, in 2013. This REY-rich mud has great potential as a rare-earth metal resource because of the enormous amount available and its advantageous mineralogical features. Here, we estimated the resource amount in REY-rich mud with Geographical Information System software and established a mineral processing procedure to greatly enhance its economic value. The resource amount was estimated to be 1.2 Mt of rare-earth oxide for the most promising area (105 km
× 0-10 mbsf), which accounts for 62, 47, 32, and 56 years of annual global demand for Y, Eu, Tb, and Dy, respectively. Moreover, using a hydrocyclone separator enabled us to recover selectively biogenic calcium phosphate grains, which have high REY content (up to 22,000 ppm) and constitute the coarser domain in the grain-size distribution. The enormous resource amount and the effectiveness of the mineral processing are strong indicators that this new REY resource could be exploited in the near future.
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