11The fate of crustal material returned to the convecting mantle by plate 12 tectonics is important for understanding the chemical and physical evolu-13 tion of the planet. Marked isotopic variability of Mo at the Earth's surface 14 o↵ers the promise of providing distinctive signatures of of such recycled ma-15 terial. However, characterisation of the behaviour of Mo during subduction 16 is needed to assess the potential of Mo isotope ratios as tracers for global 17 geochemical cycles. Here we present Mo isotope data for input and output 18 components of the archetypical Mariana arc: Mariana arc lavas, sediments 19 from ODP Sites 800, 801 and 802 near the Mariana trench and the altered 20 mafic, oceanic crust (AOC), from ODP Site 801, together with samples of 21 the deeper oceanic crust from ODP Site 1256. We also report new high pre-22 cision Pb isotope data for the Mariana arc lavas and a dataset of Pb isotope 23 ratios from sediments from ODP Sites 800, 801 and 802. The Mariana arc 24 lavas are enriched in Mo compared to elements of similar incompatibility 25 Email address: glxhf@bristol.ac.uk (Heye Freymuth)during upper mantle melting, and have distinct, isotopically heavy Mo (high 26 98 Mo/ 95 Mo) relative to the upper mantle, by up to 0.3 parts per thousand. 27 In contrast, the various subducting sediment lithologies dominantly host iso-28 topically light Mo. Coupled Pb and Mo enrichment in the Mariana arc lavas 29 suggests a common source for these elements and we further use Pb isotopes 30 to identify the origin of the isotopically heavy Mo. We infer that an aqueous 31 fluid component with elevated [Mo], [Pb], high 98 Mo/ 95 Mo and unradiogenic 32 Pb is derived from the subducting, mafic oceanic crust. Although the top few 33 hundred metres of the subducting, mafic crust have a high 98 Mo/ 95 Mo, as a 34 result of seawater alteration, tightly defined Pb isotope arrays of the Mariana 35 arc lavas extrapolate to a fluid component akin to fresh Pacific mid-ocean 36 ridge basalts. This argues against a flux dominantly derived from the highly 37 altered, uppermost mafic crust or indeed from an Indian-like mantle wedge. 38 Thus we infer that the Pb and Mo budgets of the fluid component are dom-39 inated by contributions from the deeper, less altered (cooler) portion of the 40 subducting Pacific crust. The high 98 Mo/ 95 Mo of this flux is likely caused 41 by isotopic fractionation during dehydration and fluid flow in the slab. As a 42 result, the residual mafic crust becomes isotopically lighter than the upper 43 mantle from which it was derived. Our results suggest that the continen-44 tal crust produced by arc magmatism should have an isotopically heavy Mo 45 composition compared to the mantle, whilst a contribution of deep recycled 46 oceanic crust to the sources of some ocean island basalts might be evident 47 from an isotopically light Mo signature.48 crust 50 2 portant tool to reconstruct paleo-redox conditions in the ocean (e.g. Siebert 53 et al., 2003; Arnold et al., 2004). Fractionation of Mo isotopes ...

Despite the key importance of altered oceanic mantle as a repository and carrier of light elements (B, Li, and Be) to depth, its inventory of these elements has hardly been explored and quantified. In order to constrain the systematics and budget of these elements we have studied samples of highly serpentinized (>50%) spinel harzburgite drilled at the Mid-Atlantic Ridge (FifteenTwenty Fracture zone, ODP Leg 209, Sites 1272A and 1274A). In-situ analysis by secondary ion mass spectrometry reveals that the B, Li and Be contents of mantle minerals (olivine, orthopyroxene, and clinopyroxene) remain unchanged during serpentinization. B and Li abundances largely correspond to those of unaltered mantle minerals whereas Be is close to the detection limit. The Li contents of clinopyroxene are slightly higher (0.44-2.8 lg g À1 ) compared to unaltered mantle clinopyroxene, and olivine and clinopyroxene show an inverse Li partitioning compared to literature data. These findings along with textural observations and major element composition obtained from microprobe analysis suggest reaction of the peridotites with a mafic silicate melt before serpentinization. Serpentine minerals are enriched in B (most values between 10 and 100 lg g À1 ), depleted in Li (most values below 1 lg g À1 ) compared to the primary phases, with considerable variation within and between samples. Be is at the detection limit. Analysis of whole rock samples by prompt gamma activation shows that serpentinization tends to increase B (10.4-65.0 lg g À1 ), H 2 O and Cl contents and to lower Li contents (0.07-3.37 lg g À1 ) of peridotites, implying that-contrary to alteration of oceanic crust-B is fractionated from Li and that the B and Li inventory should depend essentially on rock-water ratios. Based on our results and on literature data, we calculate the inventory of B and Li contained in the oceanic lithosphere, and its partitioning between crust and mantle as a function of plate characteristics. We model four cases, an ODP Leg 209-type lithosphere with almost no igneous crust, and a Semail-type lithosphere with a thick igneous crust, both at 1 and 75 Ma, respectively. The results show that the Li contents of the oceanic lithosphere are highly variable (17-307 kg in a column of 1 m  1 m  thickness of the lithosphere (kg/col)). They are controlled by the primary mantle phases and by altered crust, whereas the B contents (25-904 kg/col) depend entirely on serpentinization. In all cases, large quantities of B reside in the uppermost part of the plate and could hence be easily liberated during slab dehydration. The most prominent input of Li into subduction zones is to be expected from Semail-type lithosphere because most of the Li is stored at shallow levels in the plate. Subducting an ODP Leg 209-type lithosphere would mean only very little Li contribution from the slab. Serpentinized mantle thus plays an important role in B recycling in subduction zones, but it is of lesser importance for Li.

No abstract

No abstract

[1] Bulk rock lithium and oxygen isotope compositions from ODP Site 1256 were analyzed to investigate the seawater circulation in the upper oceanic crust formed at the East Pacific Rise (EPR Li principally reflects variations in water-rock ratio (w/r) together with a downhole increase of temperature. Seawater flow in the upper volcanic zone is likely to be channeled with generally small but variable w/r ratios. The w/r ratios increase rapidly with depth in the lower volcanic section into the sheeted dike complex indicating water dominated pervasive hydrothermal flow due to intensive upwelling of hydrothermal fluids.