Insights into Li and Li isotope cycling and sub-arc metasomatism from veined mantle xenoliths, Kamchatka

Harzburgitic xenoliths cut by pyroxenite veins from Avachinsky volcano, Kamchatka, are derived from the sub-arc mantle and record element transfer from the slab to the arc. Olivine and orthopyroxene in the harzburgites have Li isotopic compositions (δ7Li = +2.8 to +5.6) comparable to estimates of the upper mantle (δ7Li ~ +4 ± 2). The pyroxenite veins, which represent modal metasomatism and may therefore provide information about the metasomatic agent, have mantle-normalized trace element characteristics that suggest overprinting of their mantle source by an aqueous, slab-derived fluid. These include relative enrichments of Pb over Ce, U over Th and Sr over Nd. Li is enriched relative to the HREE, and ortho- and clinopyroxene from the veins are in Li elemental and isotopic equilibrium with each other and the surrounding harzburgite. Vein samples (δ7Li = +3.0 to +5.0) do not record a significant slab-derived δ7Li signature. These observations can be reconciled if slab Li diffusively re-equilibrates in the mantle wedge. Modeling demonstrates that Li equilibration of small (1–2 cm width) veins or melt conduits is achieved at mantle wedge temperatures within 101–105 years. We conclude that strongly fractionated Li isotopic signatures cannot be sustained for long periods in the sub-arc mantle, at least at shallow (<70 km) depths

An abrupt extinction in the Middle Permian (Capitanian) of the Boreal Realm (Spitsbergen) and its link to anoxia and acidification

The controversial Capitanian (Middle Permian, 262 Ma) extinction event is only known from equatorial latitudes, and consequently its global extent is poorly resolved. We demonstrate that there were two, severe extinctions amongst brachiopods in northern Boreal latitudes (Spitsbergen) in the Middle to Late Permian, separated by a recovery phase. New age dating of the Spitsbergen strata (belonging to the Kapp Starostin Formation), using strontium isotopes and d13C trends and comparison with better-dated sections in Greenland, suggests that the first crisis occurred in the Capitanian. This age assignment indicates that this Middle Permian extinction is manifested at higher latitudes. Redox proxies (pyrite framboids and trace metals) show that the Boreal crisis coincided with an intensification of oxygen depletion, implicating anoxia in the extinction scenario. The widespread and near-total loss of carbonates across the Boreal Realm also suggests a role for acidification in the crisis. The recovery interval saw the appearance of new brachiopod and bivalve taxa alongside survivors, and an increased mollusk dominance, resulting in an assemblage reminiscent of younger Mesozoic assemblages. The subsequent end-Permian mass extinction terminated this Late Permian radiation

Joint source-channel coding for a quantum multiple access channel

Suppose that two senders each obtain one share of the output of a classical, bivariate, correlated information source. They would like to transmit the correlated source to a receiver using a quantum multiple access channel. In prior work, Cover, El Gamal, and Salehi provided a combined source-channel coding strategy for a classical multiple access channel which outperforms the simpler "separation" strategy where separate codebooks are used for the source coding and the channel coding tasks. In the present paper, we prove that a coding strategy similar to the Cover-El Gamal-Salehi strategy and a corresponding quantum simultaneous decoder allow for the reliable transmission of a source over a quantum multiple access channel, as long as a set of information inequalities involving the Holevo quantity hold.Comment: 21 pages, v2: minor changes, accepted into Journal of Physics

Temporal evolution of proto-Izu–Bonin–Mariana arc volcanism over 10 Myr: Constraints from statistical analysis of melt inclusion compositions

International Ocean Discovery Program (IODP) Expedition 351 ‘Izu–Bonin–Mariana (IBM) Arc Origins’ drilled Site U1438, situated in the northwestern region of the Philippine Sea. Here volcaniclastic sediments and the igneous basement of the proto-IBM volcanic arc were recovered. To gain a better understanding of the magmatic processes and evolution of the proto-IBM arc, we studied melt inclusions hosted in fresh igneous minerals and sampled from 30–40 Myr old deposits, reflecting the maturation of arc volcanism following subduction initiation at 52 Ma. We performed a novel statistical analysis on the major element composition of 237 representative melt inclusions selected from a previously published dataset, covering the full age range between 30 and 40 Ma. In addition, we analysed volatiles (H2O, S, F and Cl) and P2O5 by secondary ion mass spectrometry for a subset of 47 melt inclusions selected from the dataset. Based on statistical analysis of the major element composition of melt inclusions and by considering their trace and volatile element compositions, we distinguished five main clusters of melt inclusions, which can be further separated into a total of eight subclusters. Among the eight subclusters, we identified three major magma types: (1) enriched medium-K magmas, which form a tholeiitic trend (30–38 Ma); (2) enriched medium-K magmas, which form a calc-alkaline trend (30–39 Ma); (3) depleted low-K magmas, which form a calc-alkaline trend (35–40 Ma). We demonstrate the following: (1) the eruption of depleted low-K calc-alkaline magmas occurred prior to 40 Ma and ceased sharply at 35 Ma; (2) the eruption of depleted low-K calc-alkaline magmas, enriched medium-K calc-alkaline magmas and enriched medium-K tholeiitic magmas overlapped between 35 and 38–39 Ma; (3) the eruption of enriched medium-K tholeiitic and enriched medium-K calc-alkaline magmas became predominant thereafter at the proto-IBM arc. Identification of three major magma types is distinct from the previous work, in which enriched medium-K calc-alkaline magmas and depleted low-K calc-alkaline magmas were not identified. This indicates the usefulness of our statistical analysis as a powerful tool to partition a mixture of multivariable geochemical datasets, such as the composition of melt inclusions in this case. Our data suggest that a depleted mantle source had been replaced by an enriched mantle source owing to convection beneath the proto-IBM arc from >40 to 35 Ma. Finally, thermodynamic modelling indicates that the overall geochemical variation of melt inclusions assigned to each cluster can be broadly reproduced either by crystallization differentiation assuming P = 50 MPa (∼2 km deep) and ∼2 wt% H2O (almost saturated H2O content at 50 MPa) or P = 300 MPa (∼15 km deep) and ∼6 wt% H2O (almost saturated H2O content at 300 MPa). Assuming oxygen fugacity (fO2) of log fO2 equal to +1 relative to the nickel–nickel oxide (NNO) buffer best reproduces the overall geochemical variation of melt inclusions, but assuming more oxidizing conditions (log fO2 = +1 to +2 NNO) probably reproduces the geochemical variation of enriched medium-K and calc-alkaline melt inclusions (30–39 Ma)

The arc arises: The links between volcanic output, arc evolution and melt composition

Subduction initiation is a key process for global plate tectonics. Individual lithologies developed during subduction initiation and arc inception have been identified in the trench wall of the Izu–Bonin–Mariana (IBM) island arc but a continuous record of this process has not previously been described. Here, we present results from International Ocean Discovery Program Expedition 351 that drilled a single site west of the Kyushu–Palau Ridge (KPR), a chain of extinct stratovolcanoes that represents the proto-IBM island arc, active for ∼25 Ma following subduction initiation. Site U1438 recovered 150 m of oceanic igneous basement and ∼1450 m of overlying sediments. The lower 1300 m of these sediments comprise volcaniclastic gravity-flow deposits shed from the evolving KPR arc front. We separated fresh magmatic minerals from Site U1438 sediments, and analyzed 304 glass (formerly melt) inclusions, hosted by clinopyroxene and plagioclase. Compositions of glass inclusions preserve a temporal magmatic record of the juvenile island arc, complementary to the predominant mid-Miocene to recent activity determined from tephra layers recovered by drilling in the IBM forearc. The glass inclusions record the progressive transition of melt compositions dominated by an early ‘calc-alkalic’, high-Mg andesitic stage to a younger tholeiitic stage over a time period of 11 Ma. High-precision trace element analytical data record a simultaneously increasing influence of a deep subduction component (e.g., increase in Th vs. Nb, light rare earth element enrichment) and a more fertile mantle source (reflected in increased high field strength element abundances). This compositional change is accompanied by increased deposition rates of volcaniclastic sediments reflecting magmatic output and maturity of the arc. We conclude the ‘calc-alkalic’ stage of arc evolution may endure as long as mantle wedge sources are not mostly advected away from the zones of arc magma generation, or the rate of wedge replenishment by corner flow does not overwhelm the rate of magma extractionPAB thanks IODP Germany for supporting his participation on Expedition 351, ANZIC for funding of the analytical work through a grant to RJA, and the Alexander von Humboldt Foundation for his Feodor Lynen Research Fellowship. Contributions by KMM and KJ were supported by grants from the National Science Foundation (OCE-1503694) and from the Consortium for Ocean Leadership US Science Support Program. IPS thanks UK IODP (NE/M007782/1) for funding

Basalt derived from highly refractory mantle sources during early Izu-Bonin-Mariana arc development

The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs

Origin of depleted basalts during subduction initiation and early development of the Izu-Bonin-Mariana island arc: Evidence from IODP expedition 351 site U1438, Amami-Sankaku basin

The Izu-Bonin-Mariana (IBM) island arc formed following initiation of subduction of the Pacific plate beneath the Philippine Sea plate at about 52 Ma. Site U1438 of IODP Expedition 351 was drilled to sample the oceanic basement on which the IBM arc was constructed, to better understand magmatism prior to and during the subduction initiation event. Site U1438 igneous basement Unit 1 (150 m) was drilled beneath 1460 m of primarily volcaniclastic sediments and sedimentary rock. Basement basalts are microcrystalline to fine-grained flows and form several distinct subunits (1a-1f), all relatively mafic (MgO = 6.5–13.8%; Mg# = 52–83), with Cr = 71–506 ppm and Ni = 62–342 ppm. All subunits are depleted in non-fluid mobile incompatible trace elements. Ratios such as Sm/Nd (0.35–0.44), Lu/Hf (0.19–0.37), and Zr/Nb (55–106) reach the highest values found in MORB, while La/Yb (0.31–0.92), La/Sm (0.43–0.91) and Nb/La (0.39–0.59) reach the lowest values. Abundances of fluid-mobile incompatible elements, K, Rb, Cs and U, vary with rock physical properties, indicating control by post-eruptive seawater alteration, but lowest abundances are typical of fresh, highly depleted MORBs. Mantle sources for the different subunits define a trend of progressive incompatible element depletion. Inferred pressures of magma segregation are 0.6–2.1 GPa with temperatures of 1280–1470 °C. New 40Ar/39Ar dates for Site U1438 basalts averaging 48.7 Ma (Ishizuka et al., 2018) are younger that the inferred age of IBM subduction initiation based on the oldest ages (52 Ma) of IBM forearc basalts (FAB) from the eastern margin of the Philippine Sea plate. FAB are hypothesized to be the first magma type erupted as the Pacific plate subsided, followed by boninites, and ultimately typical arc magmas over a period of about 10 Ma. Site U1438 basalts and IBM FABs are similar, but Site U1438 basalts have lower V contents, higher Ti/V and little geochemical evidence for involvement of slab-derived fluids. We hypothesize that the asthenospheric upwelling and extension expected during subduction initiation occurred over a broad expanse of the upper plate, even as hydrous fluids were introduced near the plate edge to produce FABs and boninites. Site U1438 basalts formed by decompression melting during the first 3 Ma of subduction initiation, and were stranded behind the early IBM arc as mantle conditions shifted to flux melting beneath a well-defined volcanic front.This research was supported by grants from the Consortium for Ocean Leadership to R. Hickey-Vargas and G. Yogodzinski, and collaborative National Science Foundation grants OCE1537861 to R. Hickey-Vargas and OCE1537135 to G. Yogodzinski and M. Bizimis. O. Ishizuka acknowledges Grant-in-Aid (B) (No. 25287133) for sample preparation, and I.P. Savov acknowledges support from the UK-IODP and NERC NE/M007782/1. The authors thank the International Ocean Discovery Program for this opportunity and gratefully acknowledge the input and efforts of all Expedition 351 shipboard scientists, IODP staff and crew of the JOIDES Resolution. R. Hickey-Vargas thanks Dr. Tatiana Trejos and Dr. Jose Almirall of FIU’s Trace Evidence Analysis Facility for use of the ICP-mass spectrometers and for sharing their analytical expertise

Implications of Eocene-age Philippine Sea and forearc basalts for initiation and early history of the Izu-Bonin-Mariana arc

Whole-rock isotope ratio (Hf, Nd, Pb, Sr) and trace element data for basement rocks at ocean drilling Sites U1438, 1201 and 447 immediately west of the KPR (Kyushu-Palau Ridge) are compared to those of FAB (forearc basalts) previously interpreted to be the initial products of IBM subduction volcanism. West-of-KPR basement basalts (drill sites U1438, 1201, 447) and FAB occupy the same Hf-Nd and Pb-Pb isotopic space and share distinctive source characteristics with εHf mostly > 16.5 and up to εHf = 19.8, which is more radiogenic than most Indian mid-ocean ridge basalts (MORB). Lead isotopic ratios are depleted, with 206Pb/204Pb = 17.8–18.8 accompanying relatively high 208Pb/204Pb, indicating an Indian-MORB source unlike that of West Philippine Basin plume basalts. Some Sr isotopes show affects of seawater alteration, but samples with 87Sr/86Sr 8.0 appear to preserve magmatic compositions and also indicate a common source for west-of-KPR basement and FAB. Trace element ratios resistant to seawater alteration (La/Yb, Lu/Hf, Zr/Nb, Sm/Nd) in west-of-KPR basement are generally more depleted than normal MORB and so also appear similar to FAB. At Site U1438, only andesite sills intruding sedimentary rocks overlying the basement have subduction-influenced geochemical characteristics (εNd ∼ 6.6, εHf ∼ 13.8, La/Yb > 2.5, Nd/Hf ∼ 9). The key characteristic that unites drill site basement rocks west of KPR and FAB is the nature of their source, which is more depleted in lithophile trace elements than average MORB but with Hf, Nd, and Pb isotope ratios that are common in MORB. The lithophile element-depleted nature of FAB has been linked to initiation of IBM subduction in the Eocene, but Sm-Nd model ages and errorchron relationships in Site U1438 basement indicate that the depleted character of the rocks is a regional characteristic that was produced well prior to the time of subduction initiation and persists today in the source of modern IBM arc volcanic rocks with Sm/Nd > 0.34 and εNd ∼ 9.0.This work was supported by grants from the Consortium for Ocean Leadership to co-authors who participated in Exp. 351 of the International Ocean Discovery Program (GMY, RHV, AM, IPS, OI, and RA). This work was also supported by National Science Foundation grants OCE1537135 to GMY and MB, and OCE-1537861 to RHV; A Swiss National Science Foundation grant to O. Mu¨ntener (grant 200020/135511); a UK NERC grant (NE/ M007782/1) to IPS; and a Grant-in-Aid (B) to OI (No. 25287133) for sample preparation

Age of Izu-Bonin-Mariana arc basement

Documenting the early tectonic and magmatic evolution of the Izu–Bonin–Mariana (IBM) arc system in the Western Pacific is critical for understanding the process and cause of subduction initiation along the current convergent margin between the Pacific and Philippine Sea plates. Forearc igneous sections provide firm evidence for seafloor spreading at the time of subduction initiation (52 Ma) and production of “forearc basalt”. Ocean floor drilling (International Ocean Discovery Program Expedition 351) recovered basement-forming, low-Ti tholeiitic basalt crust formed shortly after subduction initiation but distal from the convergent margin (nominally reararc) of the future IBM arc (Amami Sankaku Basin: ASB). Radiometric dating of this basement gives an age range (49.3–46.8 Ma with a weighted average of 48.7 Ma) that overlaps that of basalt in the present-day IBM forearc, but up to 3.3 m.y. younger than the onset of forearc basalt activity. Similarity in age range and geochemical character between the reararc and forearc basalts implies that the ocean crust newly formed by seafloor spreading during subduction initiation extends from fore- to reararc of the present-day IBM arc. Given the age difference between the oldest forearc basalt and the ASB crust, asymmetric spreading caused by ridge migration might have taken place. This scenario for the formation of the ASB implies that the Mesozoic remnant arc terrane of the Daito Ridges comprised the overriding plate at subduction initiation. The juxtaposition of a relatively buoyant remnant arc terrane adjacent to an oceanic plate was more favourable for subduction initiation than would have been the case if both downgoing and overriding plates had been oceanic.OI and YK appreciate JAMSTEC and J-DESC for their funding to join the expedition and post cruise research. IPS thanks UK-NERC for support for participation of the IODP cruise and part of the postcruise research. OI also used Grant-in-Aid (B) (No. 25287133) for shore-based research