Imbalance in the oceanic strontium budget

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Abstract

Palmer and Edmond [Earth Planet. Sci. Lett. 92 (1989) 11–26] indicated that thermally plausible oceanic hydrothermal inputs of strontium to the oceans are not sufficient to balance the riverine input. It has recently been suggested that off-axis low-temperature hydrothermal circulation may reconcile this discrepancy [e.g. Butterfield et al., Geochim. Cosmochim. Acta 65 (2001) 4141–4153]. Strontium isotope alteration profiles are compiled for sampled in situ ocean and ophiolite crust to calculate a sustainable cumulative hydrothermal flux to the oceanic strontium budget. High-temperature circulation contributes ∼1.8×109 mol yr−1 of basaltic strontium to the oceans. Enhanced hydrothermal systems in arc-related spreading environments (10% of the crust) may increase this to ∼2.3×109 mol yr−1. It is shown that low-temperature flow cannot supply the remaining flux required to reconcile the oceanic strontium budget (∼8.7×109 mol yr−1) because this would require 100% exchange of seawater strontium for basaltic strontium over an 820 m section of MORB-like crust. Currently sampled in situ ocean crust is not altered to this extent. The isotopic alteration intensity of 120 Myr crust sampled in DSDP Holes 417D and 418A indicates that off-axis low-temperature flow may contribute up to ∼8×108 mol yr−1 of basaltic strontium (9% of that required). The ocean crust can sustain a total basaltic strontium flux of ∼3.1±0.8×109 mol yr−1 (87Sr/86Sr ∼0.7025) to the oceans. This is consistent with hydrothermal flux estimates, but remains less than a third of the flux required to balance the oceanic strontium budget. The ocean crust cannot support a higher hydrothermal contribution unless the average ocean crust is significantly more altered than current observation.

Introduction

Hydrothermal circulation at oceanic spreading centers modifies the thermal and igneous structure of the oceanic crust and the geochemical composition of the crust and oceans. Despite its significance to global geochemical cycles fundamental uncertainties remain. In particular, although the magnitude of the high-temperature water flux is reasonably well constrained from a range of thermal modeling and geochemical budget approaches (3–6±1.5×1013 kg yr−1[1]), our knowledge of low-temperature fluxes, geochemical exchange mechanisms, and the structural, temporal and spatial evolution of the systems remains poor.

Hydrothermal flux estimates based on geochemical budgets are founded on two alternative approaches: (1) mass balance of geochemical tracers in the oceans against the known river input, and (2) assessing the degree of alteration in oceanic crustal profiles. Mass balance of the oceanic strontium isotope budget should provide one of the more robust estimates of the hydrothermal water flux, because it is free of many of the assumptions inherent in the use of other geochemical tracers (see [1]). Palmer and Edmond [2] used ocean mass balance to calculate that a cumulative high-temperature hydrothermal water flux of ∼1.2×1014 kg yr−1 is required to keep the oceanic strontium budget near steady state. This is an order of magnitude greater than other high-temperature flux predictions [1]. They inferred that this is a high-temperature water flux because little significant strontium exchange is expected during low-temperature flow [3]. However, this flux would require a heat supply more than six times greater than the magmatic heat available [4]. Recently Butterfield et al. [5] and Mottl and Wheat [6], amongst others, have suggested that this discrepancy can be reconciled by significant low-temperature exchange in the flank environments.

The extent of alteration observed in ocean and ophiolite crustal profiles provides an important constraint on hydrothermal fluxes. If hydrothermal circulation satisfies the flux required to balance the oceanic strontium budget this will have a discernible impact on the alteration intensity of the crustal profile. In this paper, we compile strontium isotope alteration profiles for five ocean crust areas and two ophiolite complexes to quantify the degree of exchange displayed by the ocean crust from a range of ages and environments. By simple mass balance it is shown that the ocean crust is insufficiently altered by high-temperature fluids to achieve the hydrothermal strontium flux required to balance the oceanic strontium budget. Low-temperature flow cannot reconcile this, because it would require 100% isotopic exchange of basaltic strontium for seawater strontium over an 820 m section of ocean crust or proportionally less exchange over greater depths and such alteration is not observed.

Section snippets

Geological setting of selected sites

Strontium isotope data have been compiled for the five best studied areas of oceanic crust (Table 1). The selection of ocean crustal sites was limited to areas where there are sufficient strontium isotope data, and where a significant proportion of the crustal profile can be observed.

Strontium isotope data have also been compiled for the Troodos Ophiolite, Cyprus and the Semail Ophiolite, Oman (Table 1), two of the best studied and most complete ophiolites. However, these ophiolites formed in

Strontium isotopic alteration profiles through ophiolite and ocean crust

Fig. 1 shows the compilation of published strontium isotope data plotted against approximate stratigraphic depth, for the five ocean crust sites (Fig. 1a,b) and the two ophiolite assemblages (Fig. 1c,d). The amount of isotopic shift from primary basaltic compositions towards the seawater value represents the degree of fluid–rock interaction and exchange in the system. For comparability this is expressed as a percentage exchange of the sampled profile (87Sr/86SrROCK) relative to unaltered rock (

Discussion

These profiles present a time-integrated record of hydrothermal alteration and indicate that ophiolite crust is significantly more altered than in situ ocean crust. This raises two questions: (1) is the ocean crust sufficiently altered to balance the oceanic strontium budget, and (2) does tectonic environment extend a primary control on the magnitude of hydrothermal fluxes? To address these questions we investigate the hydrothermal basaltic strontium flux required to balance the oceanic

The oceanic hydrothermal contribution to the oceanic strontium budget

We estimate that the ocean crust can sustain a maximum cumulative hydrothermal basaltic strontium flux of ∼3.1±0.8×109 mol yr−1. This is less than a third of the hydrothermal basaltic strontium budget required to maintain the oceanic strontium budget. The estimate is constrained by: (1) the alteration intensity of sampled oceanic crust, (2) currently observed hydrothermal fluid characteristics, (3) 20% isotopic exchange of the upper 550 m profile by low-temperature fluids, and (4) the maximum

How can the oceanic strontium budget be reconciled?

It is possible that the discrepancies in the oceanic strontium budget arise because our present sampling of modern ocean basement is not representative of global in situ crust. For example, ODP Hole 504B forms the oceanic reference section but has recovery rates of less than 20% which may preferentially bias sampling of less altered crust. Ocean crustal profiles exposed by tectonic disruption are by definition atypical and probably more altered than average in situ crust. Also the suite of

Conclusions

Hydrothermal circulation cannot supply the basaltic strontium flux required to balance the oceanic strontium budget. Crustal alteration profiles and modern hydrothermal fluid characteristics indicate that combined high- and low-temperature strontium isotopic exchange (with allowance for increased hydrothermal activity associated with arc-related systems) may contribute up to a third of the hydrothermal basaltic strontium flux required. The ocean crust is not sufficiently altered to sustain a

Acknowledgements

Chris McLeod introduced A.D. and M.J.B. to the geology of the Oman ophiolite, Laurence Coogan provided helpful comments and Mike Mottl and Claude Allègre are thanked for thoughtful reviews. A.D. acknowledges support from a NERC PhD studentship.[BW]

References (80)

  • J.L. Bischoff et al.

    Phase relations and adiabats in boiling seafloor geothermal systems

    Earth Planet. Sci. Lett.

    (1985)
  • H. Kojitani et al.

    Melting enthalpies of mantle peridotite: calorimetric determinations in the system CaO-MgO-Al2O3-SiO2 and application to magma generation

    Earth Planet. Sci. Lett.

    (1997)
  • J. Hess et al.

    Assessing seawater/basalt exchange of strontium isotopes in hydrothermal processes on the flanks of mid-ocean ridges

    Earth Planet. Sci. Lett.

    (1991)
  • P.A. Baker et al.

    Large-scale lateral advection of seawater through oceanic crust in the Central Equatorial Pacific

    Earth Planet. Sci. Lett.

    (1991)
  • H. Elderfield et al.

    Fluid and geochemical transport through oceanic crust a transect across the eastern flank of the Juan de Fuca Ridge

    Earth Planet. Sci. Lett.

    (1999)
  • C.G. Wheat et al.

    Composition of pore and spring waters from Baby Bare global implications for geochemical fluxes from a ridge flank hydrothermal system

    Geochim. Cosmochim. Acta

    (2000)
  • J.A. Crisp

    Rates of magma emplacement and volcanic output

    J. Volcanol. Geotherm. Res.

    (1984)
  • P. Louvat et al.

    Present denudation rates at Reunion island determined by river geochemistry: basalt weathering and mass budget between chemical and mechanical erosions

    Geochim. Cosmochim. Acta

    (1997)
  • C. Dessert et al.

    Erosion of Deccan Traps determined by river geochemistry: impact on the global climate and the 87Sr/86Sr ratio of seawater

    Earth Planet. Sci. Lett.

    (2001)
  • J. Gaillardet et al.

    Global silicate weathering and CO2 consumption rates deduced from the chemistry of large rivers

    Chem. Geol.

    (1999)
  • H. Kojitani et al.

    Melting enthalpies of mantle peridoitite calorimetric determinations in the system CaO-MgO-Al2O3-SiO2 and application to magma generation

    Earth Planet. Sci. Lett.

    (1997)
  • B.-M. Jahn et al.

    Nd and Sr isotopic compositions and REE abundances of Cretaceous MORB (holes 471D and 418A, legs 51, 52 and 53)

    Earth Planet. Sci. Lett.

    (1980)
  • H. Staudigel et al.

    Alteration of the oceanic-crust processes and timing

    Earth Planet. Sci. Lett.

    (1981)
  • S.R. Hart et al.

    The fingerprint of seawater circulation in a 500-meter section of ocean crust gabbros

    Geochim. Cosmochim Acta

    (1999)
  • W. Bach et al.

    The geochemical consequences of late-stage low-grade alteration of lower ocean crust at the SW Indian Ridge: Results from ODP Hole 735B (Leg 176)

    Geochim. Cosmochim. Acta

    (2001)
  • P.M. Holm

    Sr, Nd and Pb isotopic composition of in situ lower crust at the Southwest Indian Ridge results from ODP Leg 176

    Chem. Geol.

    (2002)
  • H.J. Chapman et al.

    87Sr enrichment of ophiolitic sulphide deposits in Cyprus confirms ore formation by circulating seawater

    Earth Planet. Sci. Lett.

    (1977)
  • E.T.C. Spooner et al.

    Strontium isotopic contamination and oxidation during ocean floor hydrothermal metamorphism of the ophiolitic rocks of the Troodos massif, Cyprus

    Geochim. Cosmochim. Acta

    (1977)
  • M.T. McCulloch et al.

    Nd, Sr and oxygen isotopic study of the Cretaceous Semail ophiolite and implications for the petrogenesis and seawater-hydrothermal alteration of oceanic crust

    Earth Planet. Sci. Lett.

    (1980)
  • H. Elderfield et al.

    Mid-ocean ridge hydrothermal fluxes and the chemical composition of the ocean

    Annu. Rev. Earth Planet. Sci.

    (1996)
  • M.E. Berndt et al.

    Hydrothermal alteration processes at mid-ocean ridges experimental and theoretical constraints from Ca and Sr exchange reactions and Sr isotope ratios

    J. Geophys. Res.

    (1988)
  • N.H. Sleep

    Hydrothermal circulation, anhydrite precipitation, and thermal structure at ridge axes

    J. Geophys. Res.

    (1991)
  • W.H. Burke et al.

    Variation of seawater 87Sr/86Sr throughout Phanerozoic time

    Geology

    (1982)
  • J.L. Morton et al.

    A mid-ocean ridge thermal model constraints on the volume of axial hydrothermal heat flux

    J. Geophys. Res.

    (1985)
  • R.S. White et al.

    Oceanic crustal thickness from seismic measurements and rare earth element inversions

    J. Geophys. Res.

    (1992)
  • K.L. Von Damm, Controls on the chemistry and temporal variability of fluids, Geophys. Monogr. 91...
  • D.S. Kelley et al.

    Volcanoes, fluids and life at mid-ocean ridge spreading centres

    Annu. Rev. Earth Planet. Sci.

    (2002)
  • U. Fehn et al.

    Numerical models for the hydrothermal field at the Galapagos spreading center

    J. Geophys. Res.

    (1983)
  • T. Jupp et al.

    A thermodynamic explanation for black smoker temperatures

    Nature

    (2000)
  • J.G. Sclater et al.

    The heat flow through oceanic and continental crust and the heat loss of the Earth

    Rev. Geophys. Space Phys.

    (1980)
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