Rare earth element systematics in ancient and modem hydrothermal systems.
University of Southampton, Faculty of Science, School of Ocean and Earth Science,
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.
Actions (login required)