Two spatial scales of fluid-rock interaction in an ophiolite suite are
revealed by oxygen isotope and hydrogen isotope studies of metagabbros
on the island of Tinos (Cyclades, Greece). Sequentially formed mineral
generations in the metagabbros include relict igneous augite, hornblende
of sub-seafloor hydrothermal origin, and actinolite and albite formed by
regional greenschist-facies metamorphism during orogenesis. With the
exception of augite (delta O-18 = 4.4-5.6 parts per thousand), the
metagabbros are characterized by unusually high delta O-18 values:
hornblende (5.8-7.4 parts per thousand), actinolite (6.5-10.2 parts per
thousand.), feldspar (14.6-14.9 parts per thousand )and whole rocks
(7.0-10.5 parts per thousand). Hornblende deltaD values range from -57
to -66 parts per thousand. The high delta O-18 values and the deltaD
range of the hornblendes are compatible with interaction of oceanic
gabbro with seawater that had previously been enriched in O-18/O-16
(delta O-18 = 6.5-8 parts per thousand) by isotopic exchange at moderate
to high temperatures. The high degree of oceanic alteration in the
layered gabbros, mass balance calculations of isotopic exchange, and
field evidence for early oceanic thrusting suggest that seawater could
have penetrated deeply into the ocean crust, becoming O-18/O-16-enriched
through isotopic exchange with gabbros at progressively increasing
temperature. Upward, down-temperature flow of the high-delta O-18 water
would be very effective in elevating the delta O-18 values of gabbros.
The regional greenschist metamorphic overprint of the ophiolite,
possibly the result of continued thrusting and piling up of nappes
during obduction, is characterized by localized fluid-rock exchange.
Actinolite in massive gabbroic layers has delta O-18 values (6.5-7.2
parts per thousand) close to those of the hornblende, whereas in
deformed meter-sized gabbroic blocks the amphiboles have significantly
higher values (8.4-10.2 parts per thousand). Likewise, albite in the
gabbroic blocks has high delta O-18 values of ca. 15 parts per thousand
that are ascribed to meter-scale exchange with O-18-rich fluids derived
from dehydration reactions in low-temperature hydrothermally altered
basaltic host rock enclosing the blocks. Deformation-enhanced
permeability facilitated fluid infiltration in gabbroic blocks, whereas
the relatively undeformed, and therefore less permeable, massive gabbros
experienced minor interaction with fluids. The orogenic fluid-rock
interaction thus represents local-scale redistribution of hydrous
mineral components introduced during seafloor hydrothermal exchange. (C)
2001 Elsevier Science BN. All rights reserved.