Secular variation in seawater chemistry and the origin of calcium chloride basinal brines
- 1Department of Geological Sciences and Environmental Studies, State University of New York, Binghamton, New York 13902, USA
- 2Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA
- 3Department of Geological Sciences and Environmental Studies, State University of New York, Binghamton, New York 13902, USA
Abstract
CaCl2 basinal brines, which are present in most Phanerozoic sedimentary basins, inherited their chemistries and salinities from evaporated paleoseawaters when the world oceans were Ca rich and SO4 poor (CaCl2 seas). CaCl2 seas coincided with periods of rapid seafloor spreading, high influxes of mid-ocean-ridge brines rich in CaCl2, and elevated sea levels, conditions that favored accumulation of marine CaCl2 brines in marginal and interior continental basins. Typical basinal brines in Silurian–Devonian formations of the interior Illinois basin, United States, show the same compositional trends as those of progressively evaporated CaCl2-rich Silurian seawater. Chemical deviations can be accounted for quantitatively by brine-rock reactions during burial (dolomitization, dolomite and K-feldspar cement). This explanation for the origin of CaCl2 basinal brines contrasts with others that assume constancy of seawater chemistry and involve more complex brine-rock interactions.
Footnotes
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↵GSA Data Repository item 2003130, Table DR1, Ca2+ and
in ancient seawater (0–544 Ma) used in Figure 1, and Table DR2, mineral mass-balance calculations, is available online at www.geosociety.org/pubs/ft2003.htm, or on request from editinggeosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.
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- Accepted June 29, 2003.
- Received April 8, 2003.
- Revision received June 25, 2003.
- Geological Society of America
