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 suite of sediment samples recovered from ODP Sites 1037 and 1038 (Leg 169, Escanaba Trough, Gorda Ridge spreading center, NE Pacific Ocean) has been carried out to assess the use of the sediment magnetic signal as a tracer of post-depositional alteration related to the discharge of hydrothermal fluids through these sediments. The uppermost (above 40 mbsf) section of three separate holes (Holes 1037 B, 1038 I and 1038 G) was sampled; previous sedimentological and geochemical studies indicated that these holes represent a transect from unaltered to strongly hydrothermally influenced sediments. Analysis of NRM, ARM, anisotropy of magnetic susceptibility (AMS), hysteresis parameters and low-temperature magnetic properties suggest increasing alteration of the magnetic minerals with increasing circulation of hydrothermal circulation of fluids. Fuzzy c-means clustering combined with non-linear mapping yields a more elaborate picture because similar alteration groupings at different holes can be compared. At Hole 1037 B, a sedimentary reference site 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 monoclinic pyrrhotite are detected. The detrital magnetic signal is still apparent at intermediate Hole 1038 I, but cluster analysis detects an increasing hydrothermal influence on the magnetomineralogy in the lower part of the hole. Strong hydrothermal alteration at Hole 1038 G results in a complete loss 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 few meters of the sediment column and the magnetic properties of the sediment can be used to trace even minor alterations of marine sediments related to the lateral and vertical advection of hydrothermal fluids which may easily be overlooked by more traditional methods.