UK-IODP Moratorium Co-Chief Support for Expedition 393: Quantifying the geochemical impacts of ocean crustal ageing.

The mid-ocean ridges are a range of submarine ridge that spiral around Earth like the seams on a baseball. This is where new ocean crust is constructed from magma formed from the partial melting of the upper mantle that then upwells to be intruded into and erupted onto the ocean floor. More that 3 square kilometres of ocean crust are formed each year. On average the ocean crust is approximately 6.5 +/- 1 km thick, and consequently more than 20 cubic kilometres of magma moves from the mantle to make new crust every year, making this process the dominant mechanism for heat and mass transfer from the interior of the Earth to the surface of our planet. Geophysical surveys demonstrate that the ocean crust is completely solidified within a kilometre or so of the spreading ridge. The cooling and crystallisation of these magmas is facilitated by the circulation of seawater-derived "black-smoker" hydrothermal fluids (upto 400 degrees C) that vent on the seafloor at spectacular black smoker chimneys, form volcanic-hosted massive sulphide base metal deposits (Cu, Zn), and provide energy and environments for diverse communities of exotic chemosynthetic organisms and microbes. As the ocean crust spreads away from the ridge axis further reactions with seawater-derived fluids occur on the vast ridge flanks that cover more than 60 percent of the surface of our planet. This circulation is driven by the cooling of the new plate-the lithosphere-as it matures away from the ridge. Seafloor measurements indicate that compared to models on average there is a discernible conductive heat flow deficit out to crust of ~65 million years, indicating the advection of heat by the circulation of hydrothermal fluids. However, in contrast to the high temperature systems at the ridge axis, hydrothermal circulation on the ridge flanks occurs at low temperatures generally much less than 100 degrees and generally at only a few tens of degrees or lower. This leads to the alteration of volcanic rocks and the precipitation of secondary minerals such as clays, iron oxides, carbonates, sulphides and zeolites. Because of their vast scale, these ridge flank reactions involve huge volumes of seawater-derived hydrothermal fluids and are important components in the global geochemical budgets of important elements and compounds such as U, K, CO2, H2O, S, Mg, Ca and Fe3+/Fetotal (oxidation). Most of these reactions occur in the volcanic lava flows of the uppermost crust.

Integrated Ocean Discovery Program (IODP) Expedition 390/393 are the first experiment to systematically quantify the evolving styles and extents of seawater-basalt hydrothermal exchange across the flank of a mid-ocean ridge. Importantly it will drill sites with normal thickness sediment cover in contrast to earlier ocean drilling that has tended to sample sites with anomalously thick sediment cover. It will drill through the sedimentary overburden and sample 200 to 300 metres of the lava sequences in ocean crust of 6.6, 15.2, 30.6, 49.2 and 61.2 million years old on the western flank of the slow to medium spreading rate Mid-Atlantic Ridge across an east-west transect at approximately 30 deg South. At the oldest 61.2 million-year-old crust IODP Expedition 390/393 will drill two sites - one with only a thin sediment cover (<300 m) to contrast and compare with ocean crust of the same age that has been buried by a much thicker sediment cover (~600 m).

Careful quantitative core description, mineral identification and geochemical analyses of fresh, little altered ocean basalt and representative samples of more strongly altered rocks and minerals from lava flows, breccias and veins will enable us to understand the evolving styles of hydrothermal fluid-rock reactions and quantify the progressive extents of chemical exchange between seawater and the ocean crust. These analyses will also reveal information about changes in the composition of ocean bottom waters over the past 60 million years.