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Abstract

Petrographic observations within oceanic gabbros drilled by Ocean Drilling Program (ODP) at Leg 153
from the Mid-Atlantic Ridge, Kane Fracture Zone (MARK) area (23N) in combination with experimental
findings suggest that the late stage magmatic evolution of these gabbros is characterized by pervasive
hydrous partial melting. Water-saturated melting experiments on a variety of natural gabbros between 900
and 1000C at 200 MPa produced newly formed minerals forming a characteristic paragenesis consisting
of plagioclase, orthopyroxene, and pargasitic amphibole ± clinopyroxene. The An content of the new
plagioclases is uniformly higher than that of the protolith. Olivine and clinopyroxene primocrysts react to
form neoblastic orthopyroxene and pargasitic amphibole. These features can also be observed in the
gabbros from the MARK area. Here we found zones at plagioclase grain boundaries showing a strong
enrichment in An component, with An contents up to 20 to 25 mol % higher than those of the host
plagioclase. Primary olivines and clinopyroxenes in contact with such zones react to orthopyroxene and
pargasitic amphibole. These phases rim olivine and clinopyroxene and grow ‘‘interstitially,’’ typical
petrographic characteristics of a late stage magmatic phase. The observed late stage microstructures in the
gabbros thus appear to be the results of partial melting processes triggered by water-rich fluids and are not
crystallization products of a percolating differentiated late melt. Here we demonstrate that hydrothermal
circulation within the gabbroic layer starts at much higher temperatures (900–1000C) than up to now
believed. Water-rich fluids propagate on grain boundaries in a ductile regime, causing hydrous partial
melting on a large scale. A cracks system, a prerequisite in current models for enabling hydrothermal
circulation, is not necessary. The observed process has the potential for transfer of heat and mass between
the upper and lower oceanic crust. Provided that the water-rich fluids triggering the partial melting process
are seawater derived, this process may have a significant influence on the cooling of the deep oceanic crust
which is in concordance with new thermal models implying that high-temperature hydrothermal circulation
is regarded to play an important role in transport of heat in the deep oceanic crust.

Item Type:	Article
Authors/Creators:	Koepke, J and Feig, ST and Snow, J
Keywords:	gabbro; oceanic crust; ODP; partial melting; plagioclase
Journal or Publication Title:	Geochemistry Geophysics Geosystems
ISSN:	1525-2027
DOI / ID Number:	https://doi.org/10.1029/2004GC000805
Additional Information:	©2005. American Geophysical Union
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