Rock‐Matrix Porosity and Permeability of the Hydrothermally Altered, Upper Oceanic Crust, Oman Ophiolite

Porosity and permeability are key controls on hydrothermal circulation and alteration in magmatically heated upper oceanic crust. However, the hydraulic properties of basalts altered above 200°C are largely unknown, leaving their role in high-temperature systems unclear. Here, we assess rock-matrix porosities and permeabilities of pervasively altered MORB-like basalts from outcrops in the sheeted dikes and axial lavas of the Semail ophiolite, Oman. The samples represent regional spilite alteration (chlorite–albite–quartz ± actinolite; 150–440°C) and localized epidosite alteration (epidote–quartz; 255–435°C). Porosity and permeability of spilitized rocks vary as follows: interpillow hyaloclastites (14–27 vol.%, 10−17–10−15 m2) > pillow cores (4–12 vol.%; ∼10−19–6 × 10−18 m2) > pillow rims (4–11 vol.%; ∼10−19–2 × 10−18 m2) > massive sheet flows (1–9 vol.%; ∼10−19 m2) ≥ dikes (1–5 vol.%; ∼10−19 m2). Pillow values fall within the ranges of existing data on fresh and low-temperature altered basalts in in situ crust. However, hyaloclastite permeabilities are 1–4 orders of magnitude higher and are clearly preferred flow paths. Epidosites have elevated porosity and permeability irrespective of rock morphology (18–26 vol.%; ∼10−16–10−14 m2). Pillow stacks have upscaled (∼104 m3) matrix porosities and permeabilities of ∼9 vol.% and ∼10−17–10−16 m2 when spilitized and 16 vol.% and up to ∼10−14 m2 when epidotized. Upscaled permeabilities of spilites meet minimum requirements for observed heat and fluid discharge from high-temperature seafloor systems even without fracture networks, and reflect strong flow through the rock-matrix

Reaction Mechanism and Water/Rock Ratios Involved in Epidosite Alteration of the Oceanic Crust

Epidosites are a prominent type of subseafloor hydrothermal alteration of basalts in ophiolites and greenstone belts, showing an end-member mineral assemblage of epidote + quartz + titanite + Fe-oxide. Epidosites are known to form within crustal-scale upflow zones and their fluids have been proposed as deep equivalents of black-smoker seafloor vent fluids. Proposals of the mass of fluid per mass of rock (W/R ratio) needed to form epidosites are contradictory, varying from 20 (Sr isotopes) to > 1,000 (Mg mobility). To test these proposals we have conducted a petrographic, geochemical and reactive-transport numerical simulation study of the chemical reaction that generates km3-size epidosite zones within the lavas and sheeted dike complex of the Samail ophiolite, Oman. At 250–400°C the modeled epidosite-forming fluid has near-neutral pH (∼ 5.2), high fO2, low sulfur and very low Fe (10−6 mol/kg) contents. These features argue against a genetic link with black-smoker fluids. Chemical buffering by the epidosite fluid enriches the precursor spilites in Ca and depletes them in Na and Mg. Completion of the spilite-to-epidosite reaction requires enormous W/R ratios of 700–∼40,000, depending on initial Mg content and temperature. Collectively, the variably altered rocks in the Samail epidosite zones record flow of ∼1015 kg of fluid through each km3 of precursor spilite rock. This fluid imposed on the epidosite an Sr-isotope signature inherited from the previous rock-buffered chemical evolution of the fluid through the oceanic crust, thereby explaining the apparently contradictory low W/R ratios based on Sr isotopes

Influence of in-situ temperatures and pressures on the permeability of hydrothermally altered basalts in the oceanic crust

Hydrothermal circulation through oceanic crust, and all the transfers of mass and heat caused by this circulation, are controlled by the evolving permeability of the altered basaltic rocks at in-situ pressures (P) and temperatures (T). Numerical models designed to elucidate these processes therefore rely on knowledge of the rock-matrix permeabilities as a function of P and T and degree of hydrothermal alteration. We provide the first measurements of changes in permeability of two major types of high-temperature alteration of basalts, i.e., spilites and epidosites, over the P–T range relevant to hydrothermally active oceanic crust. Experiments were performed on microfracture-poor samples of pervasively altered pillow lavas and sheeted dikes from the Semail (Oman) and Troodos (Cyprus) ophiolites. Gas permeameter measurements at room temperature show that compression to ~55 MPa effective pressure reduces the permeabilities of spilites and epidosites by approximately ~80%. Measurements at 50 MPa effective pressure using an oscillating flow apparatus show that heating spilites and epidosites from 25 to 450 °C reduces their permeabilities by ~40% and 50%, respectively. SEM observations before and after the experiments rule out formation of new microfractures. The small reduction in permeability upon heating is attributed to closure of the sparse pre-existing microfractures (> 200 μm) due to thermoelastic expansion of the crystalline matrix combined with local restriction of throats between interstitial pores due to anisotropic mineral expansion. Temperature cycling demonstrates that these changes in permeability are reversible. Duplicate measurements on cores drilled from the same rock samples reveal that all these effects are small compared to the natural heterogeneity of permeability in spilites and epidosites at the centimeter scale. Nevertheless, our quantitative temperature–permeability correlations at subseafloor pressures allow the changes in permeability for these alteration types to be incorporated into numerical simulations of water–rock interaction in the oceanic crust.ISSN:0040-195