2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM
Paper No. 292-10
Presentation Time: 10:30 AM-10:45 AM

Onset and Progression of Two-Stage Serpentinization and Magnetite Formation In Olivine-Rich Troctolite, Core 227, IODP Hole U1309D

BEARD, James S., Dept. of Earth Sciences, Virginia Museum of Natural History, 21 Starling Avenue, Martinsville, VA 24112, jbeard@nsf.gov, FROST, B. Ronald, Department of Geology and Geophysics, University of Wyoming, Laramie, 82071, and FRYER, Patricia, SOEST/HIGP, U Hawaii, Honolulu, HI 96822

Early serpentinization in core 227, IODP Hole U1309D is an approximately isochemical (except for water) replacement of olivine by a mixture of antigorite (Mg#92) and Fe-rich (Mg#65) brucite. The early serpentinite veins (type 1) are thin (<0.05mm) and exploit pre-existing cracks in olivine. Early serpentinization was a high-temperature (>300oC), rock-dominated, event. The second (main) episode of serpentinization produced through-going, mm-scale lizardite(Mg#96)-magnetite veins (type 2). Type 2 veins reflect open system serpentinization whose mineralogy and bulk chemistry requires the addition of silica. Magnetite forms one or more distinct bands in the interior of type 2 veins. At the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine) a brucite-serpentine mixture, similar to that found in type 1 veins, is present. The magnetite-lizardite cores of type 2 veins form by oxidation of the Fe component of this marginal brucite to yield magnetite combined with silicification of the Mg component of brucite to yield a more magnesian serpentine. However, the brucite-out reaction alone cannot account for the composition of the serpentine in the type 2 veins; desilication of ferrous serpentine must also occur.

In the core 227 samples olivine in contact with seawater initially reacts to form the low aSiO2 assemblage Fe-rich brucite plus serpentine. Unless isolated from seawater (as in type 1 veins), the brucite then reacts to form magnetite and a magnesian serpentine. Hence, magnetite and olivine are never in contact in these samples. The desilication of serpentine in the type 2 veins is a reflection of the instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine coexisting with magnetite in serpentinites is a function of serpentine-magnetite and not serpentine-olivine equilbria.

2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM
General Information for this Meeting
Session No. 292
Discovering Petrologic Truth in Minerals I: In Honor of Bernard W. Evans
George R. Brown Convention Center: 351AD
8:00 AM-12:00 PM, Wednesday, 8 October 2008

Geological Society of America Abstracts with Programs, Vol. 40, No. 6, p. 452

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