RAPID HIGH-TEMPERATURE METAMORPHISM OF EAST PACIFIC RISE GABBROS FROMHESS DEEP

Metamorphosed oceanic gabbros provide a record of the cooling history of the lower crust near mid-ocean ridges, but the temperature range, r ate, and location of subsolidus events are poorly known. We combine ho rnblende-plagioclase thermometry, statistical analysis, and thermal mo dels to estimate precisely the temperature, time, distance from axis, and duration of metamorphism in East pacific Rise gabbros from Hess De ep, ODP Hole 894G. Metamorphic hornblende and plagioclase, which forme d during microfracturing and sea water penetration, equilibrated at a mean temperature of 716+/-8 degrees C (90% confidence level). Comparis on of the properties of the observed temperature distribution with tho se of model events indicates that metamorphism spanned less than or eq ual to 60 degrees C. When combined with thermal models of fast-spreadi ng centers, this implies that metamorphism was rapid (less than or equ al to 6000 yr) and occurred 1-4 km off axis. Application of this appro ach to other gabbros will allow comparison of spatial and temporal cha racteristics of deformation and fluid flow in the lower oceanic crust as a function of ridge setting.