Hydrothermal alteration in the lower oceanic crust

Abstract

Hydrothermal circulation is a fundamental 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 modern oceanic crust and ophiolites, there remain major short-comings in our understanding of hydrothermal circulation in the oceanic crust, especially in the lower plutonic crust. In particular, it is not known whether fluid recharge and discharge occurs pervasively or if it is mainly channelled in discrete zones such as faults. This thesis integrates field observations with petrographic descriptions, mineral chemistry and Srisotope compositions of whole rock and mineral separates of variably altered gabbro samples collected throughout the lower ocean crust. The data are used to (i) constrain cross-cutting relationships of hydrothermal alteration from high to low temperatures, (ii) characterise observed zones of channelled fluid-flow, such as faults, and (iii) calculate elemental mass-changes related to intensive hydrothermal fluid-rock exchange in one lower crustal fault and extrapolate them to global hydrothermal fluxes. The Samail ophiolite in Oman offers a unique exposure of the lower ocean crust. Cross-cutting relationships of hydrothermal veins indicate a clear and continuous alteration sequence from temperatures of ~800 °C down to temperatures < 100-200 °C. Decimetres to metres-wide zones, fully re-equilibrated under greenschist facies conditions, are observed throughout the lower ocean crust and indicate that there was pathway for focussed hydrothermal discharge. Whole rock and epidote mineral separates sampled from these discharge zones yield Sr-isotope compositions (87Sr/86Sr=0.7043-0.7049) that are distinctly elevated relative to fresh layered gabbro and similar to compositions observed in epidosites of the sheeted dyke complex. No systematic decrease of the 87Sr/86Sr composition with depth was observed, therefore a channelled recharge, transporting hydrothermal fluids quickly and efficiently down to the base of the crust, is regarded as likely. Mass-change calculations from one chlorite-rich fault zone indicate that most elements were mobilised during intensive hydrothermal fluid-rock interaction and were either transported away by the passage of the hydrothermal fluid or were partially incorporated into secondary mineral phases. Chlorite thermometry indicate alteration temperatures of 300-350 °C at high fluid/rock ratios of up to 450:1-900:1, calculated from silica solubility. Elemental masschanges extrapolated to global hydrothermal fluxes result in significant, previously undocumented fluxes in- and out of the lower crust of most major elements as well as Sr, Cu, Zn and Cs. These results evidently highlight the importance of lower crustal fault zones for the global geochemical cycles.

More information

Identifiers

ORCID for Damon Teagle: orcid.org/0000-0002-4416-8409

Catalogue record

Export record