A well-exposed, oceanic hydrothermal rootzone in the Troodos ophiolite, Cyp
rus, is composed of the basal sheeted dikes, the upper 50-100 m of the gabb
roic sequence, and a contact aureole, interpreted as a fossil conductive bo
undary layer. The basal sheeted dikes and upper gabbros were pervasively al
tered at high temperatures (400 degrees -775 degreesC) and cut by a series
of broadly distributed mineralized faults that are oriented parallel to the
strike of the sheeted dike complex. Fluid inclusion and mineralogical data
for fault breccias show that circulating fluids were 350 degrees -400 degr
eesC, fairly oxidizing, and relatively H2S rich, with a range in salinity (
1.5-7.8 wt % NaCl equivalent). Field and petrological evidence suggest that
the rootzone migrated from the basal sheeted dikes into the upper gabbros
and back again, perhaps for several cycles. These migrations and P-T oscill
ations in the conductive boundary layer were likely caused by fluctuations
in thermal conditions imposed by the magmatic system at a spreading center.
The hydrothermal rootzone did not migrate to significant depths in the plu
tonic sequence until the waning stages of magmatism.