PARADOXES IN THE PHASE RELATIONSHIPS CONSIDERING REDOX PATHWAYS   
FROM ANOXIC MUDS TO VARIABLY OXIC METAPELITIC SCHISTS
Pat WILDE
Pangloss Foundation
Berkeley, CA 94709
pat.wilde.td.57@aya.yale.edu

Mary S. QUINBY-HUNT
Lawrence Berkeley National Laboratory
Universtity of California
Berkeley CA 94720
mshunt@lbl.gov
Charles V. GUIDOTTI
Department of Geological Sciences
University of Maine
Orono, Maine 04469-5711
guidotti@maine.maine.edu


From Geological Society of America  1997 Annual Meeting:  Salt Lake City, v. 29, no. 6, A450

ABSTRACT

      In present marine environments peletic sediments are anoxic below a few microns from the surface, even if the overlying waters are oxic. 
During early diagenesis and dewatering processes anoxic condition are maintained by bacterial action. However, the geology record abounds 
with examples of clearly oxic mineral assemblages in metapelites (hematite and magnetite bearing rocks). Indeed, even typically reduced 
sulfide + graphite-bearing metapelites exhibit f(O2) values close to the QFM buffer. Such 'oxic' assemblages are found in closed-system 
metamorphic rocks with no evidence of further modification of bulk chemistry other than loss of H20 and C02 in response to changes of 
temperature and pressure. Such oxic metamorphic assemblages may be contrasted with anoxic metamorphic assemblages which ane generally 
graphite rich and also pyrite- or pyrrhotite-rich. In general, these are also characterized by a distinct suite of elements such as V, Zn which 
are associated in modern environments with organic-rich shales


	Apparently, between sedimentation and metamorphism routinely anoxic sedimentary conditions ane changed such that the 
metamorphosed equivalents are seemingly oxic. We propose by means of appropriate phase diagrams to illustrate the various pathways 
by which these seemingly conflicting conditions may be produced by combinations of pH, temperature, and pressure. This type of analysis 
may be geologically useful both for discerning and reconstructing subduction history as well as deducing initial paleo-oceanographic 
conditions from metamorphic rocks.