Rare earth element systematics in ancient and modern hydrothermal systems
The geochemical significance of on-axis diffuse fluids, in addition to those formed during the waning phases of hydrothermal systems and off-axis crustal ageing processes, has been investigated through a comparison of the rare earth element (REE) systematics of hydrothermal materials from the ore-forming hydrothermal systems of the TAG vent field, 26 N Mid-Atlantic Ridge, and 90 Myr Troodos Ophiolite, Cyprus. Ophiolites integrate a long (c. 20 Myr) history of seafloor alteration, which reflects both , axial and off-axis hydrothermal processes. Spatially-resolved laser ablation inductively coupled plasma-mass spectrometric (LA ICP-MS) REE analyses of individual alteration phases within stockwork-mineralised Troodos lavas have been used to deconvolve the complex alteration processes associated with crustal generation at an oceanic spreading ridge. REE mobility was associated with the development of both high- (-200 to 350 C) and lowtemperature (<100 C) secondary phases which precipitated within contrasting alteration regimes (discharge- and recharge-dominated respectively). Low-temperature alteration phases are the major repository for the REEs in lavas which are depleted in the light REE Eu relative to pristine volcanic glass compositions. These data indicate that much of the REE signature of the alteration pipe is a post-mineralisation overprint acquired in the waning stages of hydrothermal activity and during the protracted alteration of the oceanic basement, rather than on-axis greenschist facies hydrothermal alteration. Submersible and drilling studies of the TAG mound have led to the development of models of mound growth and fluid evolution within an actively-forming seafloor sulphide deposit. The REEs have been used to test the applicability of these models to processes of sulphide mound and metalliferous sediment formation which occurred within the Troodos ophiolite. The REE patterns of umber, ochre and sulphide sampled from a section through the top of the Skouriotissa ore body clearly demonstrate extensive seawater ingress and circulation throughout the upper ore body during waning hydrothermalism and cooling of the mound, which has resulted in the overprinting of any original hydrothermal signatures in both mound sediments and sulphides. This study has demonstrated that the geochemistry of the sulphide mound deposits continues to evolve following the peak of hydrothermal activity, and that the seawater overprinting of the Skouriotissa deposit is the end product of a process which we only see the initiation of on the modern seafloor. At TAG, the origin of far-field Mn and Fe-rich oxide crusts has remained controversial over 25 years of investigations of the vent field. The REE and Nd isotope data presented in this thesis indicate these ferromanganese deposits are forming by a combination of sedimentation of Mn-rich particulates from the TAG hydrothermal plume, and direct precipitation from diffuse hydrothermal fluids seeping from the rift valley wall. The REE data reveal that the separation of manganese from other hydrothermally-derived metals at TAG is due to both plume processes and the spatial distribution of diffuse flow within the vent field. The studies presented in this dissertation have demonstrated the use ofREEs as tracers of chemical processes in ancient and modem hydrothermal systems on a wide range of temporal and spatial scales.