Characterising low temperature alteration and oxidation of the upper oceanic crust

Rutter, Jennifer (2015) Characterising low temperature alteration and oxidation of the upper oceanic crust. University of Southampton, Ocean and Earth Science, Doctoral Thesis, 329pp.

Record type: Thesis (Doctoral)

Abstract

The circulation of low temperature hydrothermal fluids through the oceanic crust has significant effects on the geochemical composition of the oceans and crust. Quantification of the magnitude and extent of these changes is essential for determining the crustal inputs to the mantle at subduction zones, the sub-sea floor microbial biomass, and the net changes in basement and seawater geochemistry. A highly heterogeneous upper crustal permeability structure, and variabilities in sediment thickness, lithology type, and spreading rate, means that fluid pathways and the extent of alteration are difficult to constrain.

The effects of low temperature hydrothermal alteration in basement rocks are quantified at two hydrological end-member locations: a 0.97 - 3.59 Myr basement transect across the Juan de Fuca (JdF) Ridge Flank in the East Pacific Ocean, and at 8 Myr basement Hole 395A, located at 23N on the western flank of the Mid-Atlantic Ridge. The influences of regional hydrology are demonstrated in observed variabilities in the character of alteration. Generally, the JdF basement rocks are characterised by orange, green, and grey alteration halos, and net geochemical changes associated with alteration occurring under oxidising conditions. Hole 395A basalts are orange and grey, and net geochemical changes reflect a change from open to partially restricted circulation at 400 mbsf. At both sites, whole rock ⁸⁷Sr/⁸⁶Sr, Fe³⁺/Fe_TOT, and secondary mineral abundances show no trends of increasing alteration intensity with depth. The magnitude of alteration is similar in JdF and North Pond Holes (~10 %), implying that the oceanic crust is altered up to average values early; within the first million years of formation.

A global study of the oxidation extent of the oceanic crust supports these findings. The average Fe³⁺/Fe_TOT ratio for 3000 new and compiled analyses are weighted according to alteration type, and show that the oxidation extent is raised to an average of 34 % within the first 1 Myr of crustal formation, and does not increase as a function of crustal age. Additionally, results show that flow type has only minor influence on the oxidation extent of basalts. Downhole plots of Fe³⁺/Fe_TOT show that a sharp transition to lower oxidation ratios with depth is only evident in Hole 504B. New data from Hole 1256D show that elevated oxidation ratios are maintained to as deep as 1500 mbsf. A new estimate for the total oxidation extent of young and old ocean crust is 21 plus or minus 4 % to 23 plus or minus 4 %, respectively. An age representative calculation of the total net flux of ferric iron transported to the mantle at modern subduction zones is 1.20 x 10¹³ mol^Fe3+/yr.