Chloride depletions and enrichments in seafloor hydrothermal fluids: Constraints from experimental basalt alteration studies

W. E. Seyfried, Michael E. Berndt, D. R. Janecky

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Abstract

Na-K-Ca-Cl fluid was reacted with diabase at 400° and 425°C, 400 bars and fluid rock mass ratio of 0.5 to assess the relative mobility of dissolved Cl. Fluids from the present experiments reveal relatively large time and temperature dependent decreases in Cl; temperature increase enhances Cl removal, whereas reaction progress has the opposite effect. These changes are not due to boiling and/or phase separation. At 400° and 425°C, 400 bars, Cl removal from solution is accompanied by Ca and Na fixation, respectively. Both experiments produced mixed-layer chlorite/smectite, clinozoisite and albite(?). Olivine was not detected by X-ray diffraction or petrographic analysis of run products from either experiment, however, we propose that the time dependent changes in dissolved Cl result from olivine replacement by an Fe-hydroxy chloride phase followed by hydration effects. Quenched and washed alteration products, however, did not reveal anomalous Cl, which suggests that the Cl-bearing phase is characterized by retrograde solubility. This is consistent with the relative magnitude of Cl fixation observed for experiments at both temperatures and from results of an isobaric cooling experiment. The experiments provide evidence for the non-conservative behavior of Cl during hydrothermal alteration of basalt. The data are particularly important in light of the Cl-depleted nature of some ridge crest hot spring fluids, and suggest temperatures of formation for these fluids of approximately 410°C, assuming sub-seafloor pressure of 400 bars. In addition, the retrograde solubility of the Cl-bearing phase responsible for Cl fixation during high temperature basalt alteration, may help to explain Cl enrichment in hot spring fluids which have conductively cooled below 350°C.

Original languageEnglish (US)
Pages (from-to)469-475
Number of pages7
JournalGeochimica et Cosmochimica Acta
Volume50
Issue number3
DOIs
StatePublished - Mar 1986

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