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School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton SO14 3ZH, UK
Rare earth element (REE) mobility has been investigated in stockwork mineralized lavas and dykes that lie in a zone of former black smoker fluid upflow within Troodos oceanic lithosphere. Most lavas from the Pitharokhoma alteration pipe have REE compositions similar to Troodos volcanic glasses, indicating that on- and off-axis hydrothermal alteration processes have not induced significant net REE mobilization. A degree of REE mobility is inferred for some lavas which are depleted in the light REE ± Eu. Within the lavas, high- and low-temperature hydrothermal alteration phases analysed by laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) display contrasting REE patterns (variably light REE enriched and depleted respectively). Thus during high- and low-temperature hydrothermal alteration of the lavas there may have been some relative loss or gain of light and heavy REEs, that did not necessarily cause any substantial net REE mobilization. Low-temperature hydrothermal phases have been identified as the major repository for the REEs in many of the altered lavas. These data suggest that much of the REE signature of the alteration pipe is not a relict of on-axis greenschist facies alteration, but a lower-temperature post-mineralisation overprint acquired in the waning stages of hydrothermal activity and during subsequent ageing of the oceanic basement.
