Publication Date:
2015-12-11
Description:
Oxygen isotope stratigraphy was established, and various geochemical and mineralogical
investigations were performed on Mid-Atlantic ridge crest sediments at 37-39°N in the vicinity
of the Lucky Strike and Menez Gwen hydrothermal sites, in order to characterize sediment
sources in time and space and to describe and quantify hydrothermal input in sediments.
Multivariate statistical analysis of geochemical data demonstrated that volcaniclastic,
ultramafic, detrital and hydrothermal sediment sources are present in addition to biogenic carbonate,
which dominates the chemical composition of most samples. The hydrothermal contribution
includes metals derived from hydrothermal solutions (Fe, Mn, Cu) and elements scavenged
from the water column by hydrothermal oxyhydroxides (P, V, As). The distribution of hydrothermal
factor scores and the pattern of metal accumulation rates suggest maximum hydrothermal
input at 3rN south of the Lucky Strike vent field, which may include contributions from
other presently undiscovered vent fields south of the area of the present study. Metal accumulation
rates are significantly higher than at abyssal plains remote from the ridge axis, and comparable
to other hydrothermally influenced environments. While the downcore distribution of
hydrothermal tracers was obviously modified by early diagenetic processes, time-integrated
average fluxes clearly indicate continuous hydrothermal influence in the study area at least down
to 55,000 years B.P ..
Barium appears to be dominantly of hydrothermal origin in sediments on the flanks of
the Lucky Strike seamount, where it may be derived from redeposited hydrothermal precipitates
containing abundant barite. A minor contribution of plume-derived fine-grained sulfides and
possibly barite may be present in distal sediments based on the occurrence of a cubic phase,
probably sphalerite, in gravity concentrates (〉3.0g/cm1
). However, biogenic sources seem to
predominate in these sediments. Ba maxima, particularly during early Tennination I, were
tentatively interpreted as indicators of productivity spikes which could result from a combination
of incursions of cold surface water and enhanced terrigenous input.
An allochthonous chrysotile-dominated serpentine layer is intercalated within pclagic sediments
in a basin in the central part of the 38°05'N fracture zone. This material was derived
from alteration of ultramafic rocks and may represent a low-temperature hydrothermal deposit
associated with this rock type. It was apparently transported down the walls of the fracture zone
in this area of steep relief. Cr/Ni-ratios suggest an olivine-enriched protolith, which is in accord
with the results of previous petrologic studies of pcridotites at the Mid-Atlantic Ridge, and
related to the proximity of the Azores hot spot. The predominance of chrysotilc suggests that
serpentinization took place at temperatures 〈200°C. Smectite/chlorite mixed layer phases and
chlorite formed at the same or slightly higher temperatures. The accessory phases talc and
tremolite indicate greenschist facies conditions, which may have occurred prior to serpentinization
during a history of retrograde metamorphosis. Alternatively, talc and tremolite were
derived from another source area on the walls of the fracture zone.
Type:
Thesis
,
NonPeerReviewed
Format:
text
Permalink
