Data report : trace element, Sr isotope, and Ge/SI composition of fluid and sediments in ridge-flank low-temperature hydrothermal environments

The data presented in this report demonstrate significant improvements in the ability to constrain trace element and Sr isotopic concentrations in sediments overlying ridge-flank hydrothermal systems. Improved sampling methods orchestrated by the Integrated Ocean Drilling Program (i.e., advanced piston coring and anoxic sample processing) enabled the collection of reactive pore water species with minimal alteration and sampling artifacts. Improved methods of high-resolution inductively coupled plasma–mass spectrometry trace element analysis, including the use of the 8-hydroxyquinoline functional group to extract and preconcentrate rare earth elements and other trace metals, were used to compile a data set of 28 trace element concentrations and ⁸⁷Sr/⁸⁶Sr ratios. From this extensive data set, we were able to increase the current understanding of how redox-reactive species respond to diagenic processes. Near-basement trends were used in combination with the known composition of hydrothermal fluids that exit Baby Bare Springs to asses our ability to predict basement fluid compositions using sediment pore water profiles collected by deep-sea drilling. The results show that prediction of basement fluid composition is possible for many trace elements, provided the near-basement concentration gradients are minimal. In order to place the Ge/Si systematics in a broader context, pore water and borehole fluid Ge and Si data are presented from additional sites across the Juan de Fuca Ridge flank and from two additional ridge-flank settings. These data show that Ge concentrations and Ge/Si ratios are much higher in the basement fluids than in the basal sediments because of increased mobilization of Ge relative to Si within the basement hydrothermal reservoir. Solid-phase sediment data are presented, highlighted by the occurrence of Mn- and carbonate-rich layers

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

Workshop report: Exploring deep oceanic crust off Hawai‘i

For more than half a century, exploring a complete sequence of the oceanic crust from the seafloor through the Mohorovičić discontinuity (Moho) and into the uppermost mantle has been one of the most challenging missions of scientific ocean drilling. Such a scientific and technological achievement would provide humankind with profound insights into the largest realm of our planet and expand our fundamental understanding of Earth's deep interior and its geodynamic behavior. The formation of new oceanic crust at mid-ocean ridges and its subsequent aging over millions of years, leading to subduction, arc volcanism, and recycling of some components into the mantle, comprise the dominant geological cycle of matter and energy on Earth. Although previous scientific ocean drilling has cored some drill holes into old (> 110 Ma) and young (< 20 Ma) ocean crust, our sampling remains relatively shallow (< 2 km into intact crust) and unrepresentative of average oceanic crust. To date, no hole penetrates more than 100 m into intact average-aged oceanic crust that records the long-term history of seawater–basalt exchange (60 to 90 Myr). In addition, the nature, extent, and evolution of the deep subseafloor biosphere within oceanic crust remains poorly unknown. To address these fundamentally significant scientific issues, an international workshop “Exploring Deep Oceanic Crust off Hawai`i” brought together 106 scientists and engineers from 16 countries that represented the entire spectrum of disciplines, including petrologists, geophysicists, geochemists, microbiologists, geodynamic modelers, and drilling/logging engineers. The aim of the workshop was to develop a full International Ocean Discovery Program (IODP) proposal to drill a 2.5 km deep hole into oceanic crust on the North Arch off Hawai`i with the drilling research vessel Chikyu. This drill hole would provide samples down to cumulate gabbros of mature (∼ 80 Ma) oceanic crust formed at a half spreading rate of ∼ 3.5 cm a−1. A Moho reflection has been observed at ∼ 5.5 km below the seafloor at this site, and the workshop concluded that the proposed 2.5 km deep scientific drilling on the North Arch off Hawai`i would provide an essential “pilot hole” to inform the design of future mantle drilling

Hydrothermal alteration of the ocean crust: insights from Macquarie Island and drilled in situ ocean crust

Hydrothermal circulation is a fundamental process in the formation and aging of the ocean crust, influencing its structure, physical and chemical properties, and the composition of the oceans and the mantle. The impact of hydrothermal circulation on mid-ocean ridge processes depends on the composition and volume of circulating hydrothermal fluids, and the extent of partitioning between high temperature axial- and low temperature ridge flank- systems, but these processes remain poorly constrained. This study uses whole rock and secondary mineral chemistries of altered ocean crust to (i) assess the extent of fluid-rock exchange during hydrothermal circulation, and (ii) determine the compositions of axial and ridge flank hydrothermal fluids.Sub Antarctic Macquarie Island is a unique sub-aerial exposure of a complete section of ocean crust in the ocean basin in which it formed. Sr and O isotope analyses from Macquarie Island, combined with stratigraphic reconstructions provide the first isotopic profiles through a complete section of normal ocean crust. Tracer transport mass balance calculations indicate that a time-integrated fluid flux of 4 ± 1 x 106 kg/m2 is required to produce the observed shift in Sr-isotopic composition. This can be supported by the available mid-ocean ridge magmatic heat and is similar to estimates for sections of in situ ocean crust, but a factor of 10 lower than estimates for ophiolites indicating a fundamental difference between the hydrothermal cooling of mid-ocean ridge and supra-subduction zone ocean crust.Heat flow studies indicate that hydrothermal circulation persists for tens of millions on the ridge flanks, with approximately two-thirds of hydrothermal heat loss occurring off-axis at significantly lower-temperatures than in axial hydrothermal systems. Consequently a much larger volume of fluid is required and only small deviations in fluid compositions may result in significant contributions to ocean chemical budgets. Direct sampling of in situ basement fluids is extremely difficult, and can only be applied to active systems. Here, methods to calculate the compositions of ridge flank fluids from the compositions of secondary mineral precipitates are presented and applied to basalt-hosted calcium carbonate veins. Veins from the eastern flank of the Juan de Fuca Ridge record a temperature dependent fluid evolution, similar to that of near-basement pore fluids sampled by borehole studies. Carbonate veins from the Juan de Fuca Ridge and Ocean Drilling Program Site 1256 record a sufficient decrease in the fluid Sr-isotopic composition with temperature to balance the global ocean Sr budget, however, this result cannot be reconciled with the observation of Davis et al. (2003) that the studied ocean crust has exchanged insufficient Sr with the oceans to balance the global Sr budget. This suggests that these areas cannot be typical of the ocean crust as a whole

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 (&gt;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