Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D

publisher: Elsevier articletitle: Hydrothermal cooling of the ocean crust: Insights from ODP Hole 1256D journaltitle: Earth and Planetary Science Letters articlelink: http://dx.doi.org/10.1016/j.epsl.2017.01.010 content_type: article copyright: © 2017 The Author(s). Published by Elsevier B.V

Hydrothermal calcium-carbonate veins reveal past ocean chemistry

Records of past ocean chemistry provide an integrated history of fundamental Earth processes, including the evolution of its continents, climate, and life. Here, we describe a recent dramatic shift in appreciation of the value and the application of studies of ocean crustal hydrothermal processes, which can be used to both reconstruct records of past ocean chemistry and decipher the past changes to global conditions responsible for any variations in these records. In particular, we describe a recently developed method for the determination of past seawater cation ratios using hydrothermal calcium-carbonate veins precipitated from seawater-derived fluids in the upper ocean crust

Channelling of hydrothermal fluids during the accretion and evolution of the upper oceanic crust: Sr isotope evidence from ODP Hole 1256D

ODP Hole 1256D in the eastern equatorial Pacific is the first penetration of a complete section of fast spread ocean crust down to the dike–gabbro transition, and only the second borehole to sample in situ sheeted dikes after DSDP Hole 504B. Here a high spatial resolution record of whole rock and mineral strontium isotopic compositions from Site 1256 is combined with core observations and downhole wireline geophysical measurements to determine the extent of basalt–hydrothermal fluid reaction and to identify fluid pathways at different levels in the upper ocean crust.The volcanic sequence at Site 1256 is dominated by sheet and massive lava flows but the Sr isotope profile shows only limited exchange with seawater. However, the upper margins of two anomalously thick (>25 m) massive flow sequences are strongly hydrothermally altered with elevated Sr isotope ratios and appear to be conduits of lateral low-temperature off-axis fluid flow. Elsewhere in the lavas, high 87Sr/86Sr are restricted to breccia horizons. Mineralised hyaloclastic breccias in the Lava–Dike Transition are strongly altered to Mg-saponite, silica and pyrite, indicating alteration by mixed seawater and cooled hydrothermal fluids. In the Sheeted Dike Complex 87Sr/86Sr ratios are pervasively shifted towards hydrothermal fluid values (?0.705). Dike chilled margins display secondary mineral assemblages formed during both axial recharge and discharge and have higher 87Sr/86Sr than dike cores, indicating preferential fluid flow along dike margins. Localised increases in 87Sr/86Sr in the Dike–Gabbro Transition indicates the channelling of fluids along the sub-horizontal intrusive boundaries of the 25 to 50 m-thick gabbroic intrusions, with only minor increases in 87Sr/86Sr within the cores of the gabbro bodies.When compared to the pillow lava-dominated section from Hole 504B, the Sr isotope measurements from Site 1256 suggest that the extent of hydrothermal circulation in the upper ocean crust may be strongly dependent on the eruption style. Sheet and massive flow dominated lava sequences typical of fast spreading ridges may experience relatively restricted circulation, but there may be much more widespread circulation through pillow lava-dominated sections. In addition, the Hole 1256D sheeted dikes display a much greater extent of Sr-isotopic exchange compared to dikes from Hole 504B. Because seawater-derived hydrothermal fluids must transit the dikes during their evolution to black smoker-type fluids, the different Sr-isotope profiles for Holes 504B and 1256D suggest there are significant variations in mid-ocean ridge hydrothermal systems at fast and intermediate spreading ridges, which may impact geochemical cycles of elements mobilised by fluid–rock exchange at different temperatures

(Table 1) Geochemistry of carbonate veins from basalts of the eastern flank of the Juan de Fuca Ridge, ODP Leg 168

Leg 168 of the Ocean Drilling Program (ODP) investigated the heat flow, fluid chemistry and crustal alteration associated with ridge flank hydrothermal systems. Ten sites were drilled on the eastern flank of the Juan de Fuca Ridge, along an 80 km transect, between 20 and 100 km east of the spreading centre. Recovered cores consisted of 100-500 m of sediment with shallow penetration (1.7-48.1 m) into the underlying igneous basement (0.8-3.6Ma). Here we use the composition of calcium carbonate minerals, from veins within the upper basement, to reconstruct the evolving chemistry of hydrothermal fluids with increasing crustal age and sediment cover thickness. We show for the first time a clear link between the alteration of the basement rocks as recorded by secondary minerals, and the nearbasement sedimentary pore fluids, which are often assumed to be representative of the basement fluids responsible for low temperature alteration of the upper crust. Carbonates precipitated from basement fluids that ranged in strontium isotopic composition from near-modern seawater (87Sr/86Sr =~ 0.70918) to the near-basement pore fluid values at any one site. 87Sr/86Sr ratios are independent of mineralogy with both aragonite and calcite precipitating from variably evolved fluids with the range in carbonate 87Sr/86Sr increasing with crustal age. A parallel geochemical evolution of basement fluids and sediment porewaters is shown since 87Sr/86Sr ratios of near-basement pore fluids decrease from 0.709013 to 0.707108 away from the ridge axis. A correlation exists between 87Sr/86Sr ratios and d18O-calculated fluid temperatures, with more geochemically evolved carbonates having precipitated from warmer fluids. Basement fluid compositions, calculated from carbonate Sr, Mg, Fe and Mn concentrations combined with suitable partition coefficients, are also temperature-dependent. Given an observed increase in basement temperature with age, from 16°C to 64°C along the transect, a progressive chemical development of basement fluid is demonstrated. Carbonate veins in volcanic basement from ODP Holes 504B and 896A, on the Costa Rica Rift, record the same temperature compositional evolution of basement fluid as those from the Juan de Fuca Ridge flank. Although these locations have different thermal histories and therefore must have experienced different temporal geochemical evolution of basement fluid, basement temperature appears to be the dominant control on basement fluid composition

Hydrothermal fault zones in the lower oceanic crust: An example from Wadi Gideah, Samail ophiolite, Oman

Hydrothermal circulation is a key process for chemical and isotopic exchange between the solid Earth and oceans, and for the extraction of heat from newly accreted crust at mid-ocean ridges. However, due to a dearth of samples from intact oceanic crust, or continuous sample suites from ophiolites, there remain major shortcomings in our understanding of hydrothermal circulation in the oceanic crust, especially in the lower plutonic crust. In particular, it remains unknown whether fluid recharge and discharge occurs pervasively or if it is mainly channelled within discrete zones such as faults. Here, we describe a hydrothermally-altered fault zone that crops out in the Wadi Gideah in the layered gabbro section of the Samail ophiolite of Oman. A one metre thick normal fault comprising deformed chlorite ± epidote fault rock with disseminated chalcopyrite and pyrite, offsets gently dipping layered olivine gabbros. The chlorite-rich fault rocks surround strongly altered clasts of layered gabbro, 50 to 80 cm in size. Layered gabbros in the hanging wall and the footwall are partially altered and abundantly veined by epidote, prehnite, laumontite and calcite veins. In the wall rocks, igneous plagioclase (An82±2%) is partially altered towards more albitic compositions (An75-81), and chlorite + tremolite partially replaces plagioclase and clinopyroxene. Clinopyroxene is moderately overgrown by Mg-hornblende. Whole rock mass change calculations show that the chlorite-rich fault rocks are enriched in Fe, Mn, H2O + CO2, Co, Cu, Zn, Ba, and U, but have lost significant amounts of Si, Ca, Na, Cr, Ni, Rb, Sr, Cs, light rare earth elements (LREE), Eu, and Pb. Gabbro clasts within the fault zone as well as altered rock from the immediate hanging wall show enrichments in Na, volatiles, Sr, Ba and U and depletions of Si, Ti, Al, Fe, Mn, Mg, Ca, Cu, Zn, Rb, Cs, LREE, and Pb. Chlorite thermometry suggests a formation temperature of 300–350 °C for the fault rock and based on the Si loss and solubility of silica in hydrothermal fluids the intensity of alteration requires a fluid to rock ratio of up to 900:1. Strontium isotope whole rock data of the fault rock yield 87Sr/86Sr ratios of 0.7043–0.7048, which is considerably more radiogenic than fresh layered gabbro from this locality (87Sr/86Sr = 0.7030–0.7033), and similar to black smoker hydrothermal signatures based on epidote, measured from epidote veins in the footwall and elsewhere in the ophiolite (87Sr/86Sr = 0.7043–0.7051). Altered gabbro clasts within the fault zone show similar values with 87Sr/86Sr ratios of ~0.7045–0.7050. In contrast, the hanging and footwall gabbros display values only slightly more radiogenic than fresh layered gabbro. The elevated strontium isotope composition of the fault rock and clasts together with the observed secondary mineral assemblages and calculated mass changes strongly supports the intense interaction with seawater-derived up-welling hydrothermal fluids, active during oceanic spreading. Assuming that such a fault zone is globally representative of faulting in the lower crust, an extrapolation of our results from mass change calculations to elemental fluxes, shows a significant contribution to the global hydrothermal budgets of Si, Ti, Fe, Mn, Mg, Ca, H2O, Cu, Zn, Sr and Cs.</p

Reconstructing past seawater Mg/Ca and Sr/Ca from mid-ocean ridge flank calcium carbonate veins

Proxies for past seawater chemistry such as Mg/Ca and Sr/Ca ratios provide a record of the dynamic exchanges of elements between the solid Earth, atmosphere and hydrosphere, and the evolving influence of life. Here, we estimate past oceanic Mg/Ca and Sr/Ca ratios from suites of 1.6 to 170-million-year-old calcium carbonate veins precipitated from seawater-derived fluids in ocean ridge flank basalts. Our data indicate that prior to the Neogene, oceanic Mg/Ca and Sr/Ca were lower than in the modern ocean. Decreased ocean spreading since the Cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early Tertiary may explain the recent rise in these ratios

Controls on thallium uptake during hydrothermal alteration of the upper ocean crust

Hydrothermal circulation is a fundamental component of global biogeochemical cycles. However, the magnitude of the high temperature axial hydrothermal fluid flux remains disputed, and the lower temperature ridge flank fluid flux is difficult to quantify. Thallium (Tl) isotopes behave differently in axial compared to ridge flank systems, with Tl near-quantitatively stripped from the intrusive crust by high temperature hydrothermal reactions, but added to the lavas during low temperature reaction with seawater. This contrasting behavior provides a unique approach to determine the fluid fluxes associated with axial and ridge flank environments. Unfortunately, our understanding of the Tl isotopic mass balance is hindered by poor knowledge of the mineralogical, physical and chemical controls on Tl-uptake by the ocean crust.Here we use analyses of basaltic volcanic upper crust from Integrated Ocean Drilling Program Hole U1301B on the Juan de Fuca Ridge flank, combined with published analyses of dredged seafloor basalts and upper crustal basalts from Holes 504B and 896A, to investigate the controls on Tl-uptake by mid-ocean ridge basalts and evaluate when in the evolution of the ridge flank hydrothermal system Tl-uptake occurs.Seafloor basalts indicate an association between basaltic uptake of Tl from cold seawater and uptake of Cs and Rb, which are known to partition into K-rich phases. Although there is no clear relationship between Tl and K contents of seafloor basalts, the data do not rule out the incorporation of at least some Tl into the same minerals as the alkali elements. In contrast, we find no relationship between the Tl content and either the abundance of secondary phyllosilicate minerals, or the K, Cs or Rb contents in upper crustal basalts. We conclude that the uptake of Tl and alkali elements during hydrothermal alteration of the upper crust involves different processes and/or mineral phases compared to those that govern seafloor weathering. Furthermore, a correlation between the Tl and S concentrations of upper crustal basalts from Holes U1301B, 504B and 896A indicates that Tl is primarily incorporated into secondary sulfides. Given that some of these secondary sulfides formed as a result of microbial sulfate reduction, microbial action is at least indirectly responsible for Tl-uptake.Thallium-enrichment of ridge flank basalts requires a Tl-bearing fluid and physical, chemical and microbial conditions that favor secondary sulfide formation. Uptake of Tl occurs in reducing environments in the background rocks away from fluid flow pathways during early ‘open’ circulation of oxidizing seawater but more pervasively throughout the system during later ‘restricted’ circulation of reducing fluids. The Tl-isotope system is therefore a useful tracer of the fluid flux through both the ‘open’ and ‘restricted’ ridge flank hydrothermal regimes