INTRODUCTION

1. The role of recycled crust in the mantle composition, geodynamics andglobal environmental catastrophes • A.V. Sobolev (1,2), S.V. Sobolev (3), A.W. Hofmann (2), D.V. Kuzmin (2), K.P. Jochum (2) et al, • Institut des Sciences de la Terre (ISTerre) Université J. Fourier-CNRS, Grenoble, France; • Max-Planck-Institut für Chemie, J.-J.-Becher-Weg 27, D-55128 Mainz, Germany; • Deutsches GeoForschungsZentrum GFZ, Telegrafenberg, 14473, Potsdam, Germany. INTRODUCTION 30 years ago, Hofmann and White (1982) proposed the radical hypothesis that crust from ocean basins cycled deep through the mantle to reappear, eons later, in mantle plumes. Recently we developed a method to estimate quantitatively the amount and the age of the recycled crust (Sobolev et al, 2007, 2008, 2011a). This formed the basis of thermomechanical modelling of the Siberian LIP (Sobolev et al, 2011b). The model explained many hitherto puzzling aspects of the mantle geodynamics associated with this event. In addition, by invoking gas release through melting of recycled crust, an explanation was proposed for the well-known but poorly understood coincidence between LIPs and mass extinctions. Thus Hofmann and White’s hypothesis has evolved into the concept that substantial amounts of recycled crust in the source of LIPs lie at the root of their dramatic geodynamic and environmental impact. . YOUNG SOURCE OF HAWAIIAN PLUME: Sobolev A.V., Hofmann A.W., Jochum K.P. et al, (2011) Nature,476, 434-437 We report the first data on 87Sr/86Sr ratios and 208Pb/207Pb/206Pb for 138 melt inclusions in olivine phenocrysts from lavas of Mauna Loa shield volcano, Hawaii, indicating enormous mantle source heterogeneity. Melt fractions with different compositions were mixed up during olivine crystallization producing typical Mauna Loa lavas, which thus do not indicate mantle source heterogeneity LINKING MANTLE PLUMES, LARGE IGNEOUS PROVINCES AND ENVIRONMENTAL CATASTROPHES: Sobolev S.V., Sobolev A.V, Kuzmin D.V. et al, (2011) Nature,477, 312-316. Large igneous provinces (LIPs) are known for their rapid production of enormous volumes of magma (up to several million cubic kilometers in less than a million years), for marked thinning of the lithosphere, often ending with a continental break-up, and for their links to global environmental catastrophes. Despite the importance of LIPs, controversy surrounds even the basic idea that they form through melting in the heads of thermal mantle plumes. Fig.1.Petrological constraints. a, Geological map of the Siberian Traps. Dark green areas are lavas, light green areas are tuffs. The three studied regions are Norilsk (N), Putorana plateau (P) and Maymecha–Kotuy province (M). White numbers stand for the potential mantle temperature b, FeO/ MnO ratios of averaged per sample olivine phenocrysts over normalized Gd/Yb ratios of host lavas. The green oval is the reference for the almost pure shallow peridotitic mantle source. GA, garnet in the mantle source. c, The proportions of pyroxenite-derived melt calculated independently of Mn deficiency (XpxMn) and Ni excess (Xpx Ni). d, Integrated lava section for Siberian Traps based on the Norilsk section. Xpx is the proportion of pyroxenite-derived melt, calculated as the average of XpxMn and Xpx Ni for high-forsterite olivines and as XpxMn for low-forsterite olivines,. Small black dots show lavas of the Norilsk section. Per, peridotite derived melt component. APPROACH The crux of this approach is the quantitative estimate of the fraction, composition and age of recycled oceanic crust in the mantle source. The basic approach consists of measured by EPMA the minor element contents of the more magnesian olivines in each LIP. To obtain information on the age and composition of recycled crust requires analysis of trace elements and isotopic compositions by laser-source ICPMS. Studies of melt inclusions yield information about volatile contents and PT conditions of primary magmas and their sources. We present petrological evidence for a large amount (15 wt%) of dense recycled oceanic crust in the head of the plume. Fig. 1. Relation between compositions of Puu Wahi, Mauna Loa, melt inclusions and host olivine (Fo). Red field illustrates range of compositions of recent Mauna Loa lavas according to GEOROC database. Error bars: one standard error. Outlined are ultra-depleted melt inclusions (UDM). Fig. 2. Composition of melt inclusions and matrix glass in euhedral olivine crystals of sample (K97-15b), Puu Wahi scoria cone, Mauna Loa, Hawaii. a–d, 87Sr/86Srratios are plotted against Rb/Sr (a),207Pb/206Pb (b), Sr (c) and Cl (d). Compositions of bulk Mauna Loa lavas from the GEOROC database are shown (in orange) for comparison. The compositions of average UDM inclusions are outlined. Error bars indicate s.e.m. Fig. 2.Model. a.Maximum pre-magmatic surface uplift (H) atop a spreading mantle plume with an excess temperature of 250 oC.b, c, Temperature distributions (oC) in the model cross-section at model times of 0.15 Myr (b) and 0.5 Myr (c). d, Snapshots of the plume breaking through the lithosphere in the domain shown by the white rectangle in f. Colors show concentrations of the pyroxenitic component in the plume or in the crystallized melt. e, f, Distribution of the pyroxenite component in the plume (Cpx) or in the crystallized melt in the model cross-section at model times of 0.15 Myr (e) and 0.5 Myr (f). The solid line marks the boundary of the depleted lithosphere. We show that highly radiogenic strontium in severely rubidium-depleted melt inclusions matches the isotopic composition of 200–650-Myr-old seawater. We infer that such seawater must have contaminated the Mauna Loa source rock, before subduction, imparting a unique ‘time stamp’ on this source Fig. 3. 87Sr/86Sr ratios of the two most radiogenic Mauna Loa melt inclusions superimposed on the time evolution of Sr isotopic composition of seawater (Shields and Veizer, 2002). a, Seawater evolution for the past 2 Gyr. b, Detailed Sr evolution for the past 1 Gyr. The green diamond indicates seawater composition at 90 Myr corresponding to the age of lithosphere under the Big Island, Hawaii. The horizontal dashed line represents the pooled average 87Sr/86Srratio of 21 analyses of two UDM inclusions. The dotted line and heavy black line indicate the upper and lower limits of range of two standard errors of the average 87Sr/86Sr, respectively. We suggest that the radiogenic Sr in UDMs is derived from ancient seawater in the Mauna Loa source (see the text). The blue field represents the acceptable age range for the recycled seawater component. We reject ages marked by the red field because we have ruled out recent seawater contamination. We consider the lower limit of the shown range (87Sr/86Sr= 0.7075, indicated on both panels) to be a minimum estimate for the composition of the seawater component in the Mauna Loa source, because its isotopic composition must have been diluted by less radiogenic components from the original unaltered basalt and source peridotite. The most likely age of recycled seawater Sr is in the range 200–550 Myr. Fig. 2. Reflected light image of melt inclusion in olivine exposed on the surface illustrating all applied in-situ analytical techniques. Three large (100 μm) laser pits are for Pb isotopes, three (50 μm) for Sr isotopes, and one (60 μm) for trace elements. The position of two ion probe pits (ca 20 μm) for B analysis is indicated by red ovals. Three electron probe spots of 3 μm for analysis of major elements and Cl and S are almost invisible in the center of inclusion We develop a thermomechanical model that implies extensive plume melting and heterogeneous erosion of the thick cratonic lithosphere over the course of a few hundred thousand years and predicts no pre-magmatic uplift and requires no lithospheric extension. Fig. 1. Puu Wahi, Mauna Loa, Hawaii euhedral olivine (OL) with melt inclusions (MI) and surrounded glass (GL). Melt inclusions consist of fresh glass and contain spherical shrinkage bubbles (B), chromium spinel crystals (Sp) and occasionally small droplets of sulfide melt. Fig. 3 Production of volatiles and its consequences for mass extinctions. a, Plot of modelledCO2 (left axis) and HCl (right axis) amounts extracted from the plume against model time (lower axis). Solid curves show the minimum estimate and dashed curves the maximum estimate of CO2 and HCl extracted from the plume. The grey rectangle shows the estimated range of the released CO2 during the Permo-Triassic mass extinction. The green area shows time dependence of the normalized volume of the magma crossing the 50-km depth, calculated for the re-fertilized lithosphere. On the top axis we show geological time and a possible model for triggering the Permo-Triassic mass extinction. GBT, gases break through. Also shown is U–Pb dating of the extinction event and U–Pb dating of main-phase Siberian basalts26 and intrusions. b, Plot of mass extinction intensity (light blue field) with major LIPs (circles) against geological time, together with the timing of different ocean modes. Circle colors denote the timing of LIPs relative to ocean modes: blue, ‘Cretan’ mode; red ‘Neritan’ mode; blue and red together, transition mode. The scale of circle sizes is in millions of cubic kilometers. CAMP, Central Atlantic Magmatic Province; NAMP, Northern Atlantic Magmatic Provinces, OJP, Ontong Java; CP, Caribbean Plateaux; CR, Columbian River basalts. The presence of 200–650-Myr-old oceanic crust in the source of Hawaiian lavas implies a timescale of general mantle circulation with an average rate of about 2 cm/yr, much faster than previously thought. • Sobolev, A.V., Hofmann, A.W., Jochum, K.-P. Kuzmin, D.V. and B. Stoll (2011a). A young source for the Hawaiian plume. Nature 476 (7361), 434-437. • Sobolev S.V., Sobolev A.V., Kuzmin D.V., Krivolutskaya N.A., Petrunin A.G., Arndt N.T., Radko V.A., VasilievYu.R. (2011b). Linking mantle plumes, large igneous provinces and environmental catastrophes. Nature 477 (7364), 312-316. The model suggests that massive degassing of CO2 and HCl, mostly from the recycled crust in the plume head, could alone trigger a mass extinction and predicts it happening before the main volcanic phase, in agreement with stratigraphic and geochronological data for the Siberian Traps and other LIPs. New Wave UP 193 laser ablation system Electron probe microanalyser (EPMA) Jxa 8230 Thermo Scientific ELEMENT 2 ICP-MS