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Geology; March 1988; v. 16; no. 3; p. 262-266; DOI: 10.1130/0091-7613(1988)016<0262:DOAZIT>2.3.CO;2
© 1988 Geological Society of America
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Distribution of alteration zones in the upper oceanic crust

Kathryn M. Gillis1 and Paul T. Robinson2

1 Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
2 Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

Aging of the upper oceanic crust produces alteration zones that have distinct mineralogical and geochemical characteristics. Five zones are recognized in the Troodos ophiolite and in situ oceanic crust: (1) a sea-floor weathering zone (SWZ), (2) a low-temperature zone (LTZ), (3) a transition zone (TZ), (4) an upper dike zone (UDZ), and (5) a mineralized zone (MZ). The MZ may be overlain by the SWZ, LTZ, or TZ, but in the known examples it is always underlain by the UDZ. The development of the SWZ is dependent upon the duration of cold-seawater circulation, which is dependent upon seafloor topography and the rate and nature of sedimentation. The boundary between the low- and high-temperature zones is largely controlled by permeability contrasts. Lateral variation in the depth of this boundary indicates that the thermal gradient is not uniform within the crust. This, along with the evidence for disequilibrium assemblages, implies that regional metamorphic zones are not appropriate to describe the distribution of alteration effects.




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