TIME-SERIES TEMPERATURE-MEASUREMENTS AT HIGH-TEMPERATURE HYDROTHERMALVENTS, EAST PACIFIC RISE 9-DEGREES-49'-51'N - EVIDENCE FOR MONITORINGA CRUSTAL CRACKING EVENT

Temperature measurements of hydrothermal vent fluids provide an import ant indicator of the physical and chemical state of mid-ocean ridge cr est hydrothermal and magmatic systems, Changes in vent fluid temperatu re and chemistry can have dramatic effects on biological communities t hat inhabit these unique ecosystems. In an attempt to understand tempo ral variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrother mal vents on the East Pacific Rise crest between 9 degrees 49'N and 9 degrees 51'N were instrumented and sampled repeatedly during five year s following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9 degrees 50.2'N, has the most c omplete record of fluid temperatures from 1991 to 1997, including a co ntinuous temperature record of nearly three years (1994-1997). Bio9 ve nt fluids were 368 degrees C in 1991, increased to an estimated temper ature greater than or equal to 388 degrees C after a second volcanic e vent in 1992, and thereafter declined over the next similar to 2 years reaching a temperature of 365 degrees C in December 1993. Continuous temperature records and point measurements made by Alvin's thermocoupl e probe show Bio9 vent fluids were stable for similar to 15 months at 365 +/- 1 degrees C, until March 26, 1995. On March 26, an abrupt 7 de grees C increase occurred over a period of eight days at this vent, an d a maximum temperature of 372 +/- 1 degrees C persisted for 14 days. The vent fluid cooled gradually over similar to 3.5 months to 366 +/- 1 degrees C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 show s a 5 +/- 1 degrees C increase for the two-year period. The abrupt tem perature increase at Bio9 vent, and coincident changes in faunal commu nity structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Soh n et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Natur e (in press)], and the identification of a small earthquake swarm whic h occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a crackin g front penetrating into hot crustal rocks beneath the vent. (C) 1998 Elsevier Science B.V. All rights reserved.