Discharge of hydrothermal fluids through sediment at the Escanaba Trough, Gorda Ridge (ODP Leg 169): assessing the effects on the rock magnetic signal

An integrated rock-magnetic and multivariate statistical analysis of a suit e of sediment samples recovered from ODP Sites 1037 and 1038 (Leg 169, Esca naba Trough, Gorda Ridge spreading center, NE Pacific Ocean) has been carri ed out to assess the use of the sediment magnetic signal as a tracer of pos t-depositional alteration related to the discharge of hydrothermal fluids t hrough these sediments. The uppermost (above 40 mbsf) section of three sepa rate holes (Holes 1037 B, 1038 I and 1038 G) was sampled; previous sediment ological and geochemical studies indicated that these holes represent a tra nsect from unaltered to strongly hydrothermally influenced sediments. Analy sis of NRM, ARM, anisotropy of magnetic susceptibility (AMS), hysteresis pa rameters and low-temperature magnetic properties suggest increasing alterat ion of the magnetic minerals with increasing circulation of hydrothermal ci rculation of fluids. Fuzzy c-means clustering combined with non-linear mapp ing yields a more elaborate picture because similar alteration groupings at different holes can be compared. At Hole 1037 B, a sedimentary reference s ite drilled away from the area of high heat flow: the rock-magnetic signal is a sensitive record of changes in detrital inputs. Magnetite and some mon oclinic pyrrhotite are detected. The detrital magnetic signal is still appa rent at intermediate Hole 1038 I, but cluster analysis detects an increasin g hydrothermal influence on the magnetomineralogy in the lower part of the hole. Strong hydrothermal alteration at Hole 1038 G results in a complete l oss of the detrital/diagenetic magnetic signal, which is replaced by a much weaker 'hydrothermal' signal composed of monoclinic pyrrhotite and pyrite. The alteration of the magnetic signal is not restricted to the uppermost f ew meters of the sediment column and the magnetic properties of the sedimen t can be used to trace even minor alterations of marine sediments related t o the lateral and vertical advection of hydrothermal fluids which may easil y be overlooked by more traditional methods. (C) 2000 Elsevier Science B.V. All rights reserved.