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Paleozoic and Mesozoic silica-rich seawater: Evidence from hematitic chert (jasper) deposits

Geology

By:
and
DOI: 10.1130/0091-7613(2003)031<0319:PAMSRS>2.0.CO;2

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Abstract

Laterally extensive beds of highly siliceous, hematitic chert (jasper) are associated with many volcanogenic massive sulfide (VMS) deposits of Late Cambrian to Early Cretaceous age, yet are unknown in analogous younger (including modern) settings. Textural studies suggest that VMS-related jaspers in the Ordovician L??kken ophiolite of Norway were originally deposited as Si- and Fe-rich gels that precipitated from hydrothermal plumes as colloidal silica and iron-oxyhydroxide particles. Rare earth element patterns and Ge/Si ratios of the jaspers reflect precipitation from plumes having seawater dilution factors of 103 to 104, similar to modern examples. We propose that silica in the ancient jaspers is not derived from submarine hydrothermal fluids-as suggested by previous workers-but instead was deposited from silic-rich sea-water. Flocculation and precipitation of the silica were triggered inorganically by the bridging effect of positively charged iron oxyhydroxides in the hydrothermal plume. A model involving amorphous silica (opal-A) precursors to the jaspers suggests that silica contents of Cambrian-Early Cretaceous oceans were at least 110 mg/L SiO2, compared to values of 40-60 mg/L SiO2 estimated in other studies. The evolution of ancient silica-rich to modern Fe-rich precipitates in submarine-hydrothermal plumes reflects a changeover from silica-saturated to silica-depleted seawater through Phanerozoic time, due mainly to ocean-wide emergence of diatoms in the Cretaceous.

Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
Paleozoic and Mesozoic silica-rich seawater: Evidence from hematitic chert (jasper) deposits
Series title:
Geology
DOI:
10.1130/0091-7613(2003)031<0319:PAMSRS>2.0.CO;2
Volume
31
Issue:
4
Year Published:
2003
Language:
English
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
Geology
First page:
319
Last page:
322