Imbalance in the oceanic strontium budget
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]
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