Micro-scale Chemical Effects of Low Temperature Weathering of DSDP Basaltic Glasses
Micro-scale Chemical Effects of Low Temperature Weathering of DSDP Basaltic Glasses
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Date
1979
Authors
Ailin-Pyzik, Iris Blanche
Advisor
Sommer, Sheldon E.
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Abstract
Unaltered deep-sea basaltic g lasses are believed to
be the best record of initial magma composition , and as
such are important in the study of petrogenesis. However ,
these glasses are altered by their long contact with seawater,
becoming hydrated and undergoing chemical exchange.
This chemical exchange affects the composition of seawater
and plays a role in the chemical equilibrium of t he oceans.
A study of the trace metal and major element alteration
of glasses from Deep Sea Drilling Project Site 396B
has been conducted, using a selected area x - ray fluorescence
technique (developed for this study) for the trace
metal analyses, and the electron rnicroprobe for the major
elements. The samples included sections of pillow basalt
rinds, hyaloclastite s, and a few crystalline sections.
The glasses were found to release a bout o ne- half
the original Si and Al, two- thirds of the Mg and Na , and
over 90% of the Ca originally present, during alteration
to palagonite. Fe and Ti were found to be immobile, and
K was increased 40-fold by concentration from seawater.
For the trace metals, over one-quarter of the Zn, Cu and
Ni were released, 40% of the Mn, and over 10% of the Cr.
These changes apply only to the conversion of fresh glass
(sideromelane ) to palagonite (smectite), and do not include
the effects of authigenic phillipsite and calcite
reprecipitated locally. Differences between the effects
of low temperature weathering on the crystalline basalts
and the glasses appear to be primarily a function of the
susceptibility of the primary mineral phases to attack,
with the glass, being the least stable phase, being the
most altered.