CHARACTER OF ACTIVE HYDROTHERMAL MOUNDS AND NEARBY ALTERED HEMIPELAGIC SEDIMENTS IN THE HYDROTHERMAL AREAS OF MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA-RIDGE - DATA ON SHALLOW CORES
Rjw. Turner et al., CHARACTER OF ACTIVE HYDROTHERMAL MOUNDS AND NEARBY ALTERED HEMIPELAGIC SEDIMENTS IN THE HYDROTHERMAL AREAS OF MIDDLE VALLEY, NORTHERN JUAN-DE-FUCA-RIDGE - DATA ON SHALLOW CORES, Canadian Mineralogist, 31, 1993, pp. 973-995
The Area of Active Venting hydrothermal field (AAV) occurs within the
Middle Valley sediment-filled rift on northern Juan de Fuca Ridge. The
AAV is a rhomb-shaped zone of hydrothermally altered sediment 800 m b
y 350 m with moderate temperature venting (<276-degrees-C), associated
with anhydrite chimneys on hydrothermal mounds, and biota-rich areas
associated with diffuse venting. Ten push-core samples (< 30 cm depth)
were collected in and near the AAV by the submersible ALVIN during 19
90 to examine the surficial nature of altered hemipelagic sediment and
hydrothermal mounds. Within the AAV but away from active vent areas,
weakly altered sediment is grey colored and indurated due to dissoluti
on of calcareous microfossils and an increase in authigenic calcite an
d pyrite. Proximal to vent areas, intensely altered sediment is green
colored, with an increased abundance of Mg-smectite, iron-bearing illi
te-smectite, pyrite, organic carbon and barite. Altered sediments cont
ain elevated contents of MgO, total Fe2O3, MnO, S, Ba, Zn, Cu, Pb, As,
Sb and Se, and are depleted in CaO and CO2 relative to unaltered sedi
ment. Element enrichments and depletions increase toward active vent a
reas on hydrothermal mounds. Tilted hydrothermal crusts, and horizonta
l Mg-silicate veins seen in the cores, suggest that hydrothermal mound
s grow by inflation, whereas chimney growth and collapse contribute a
veneer of hydrothermal sediment. Sediment adjacent to an active chimne
y has layers of anhydrite, sulfide and nodular Mg-silicate. The sedime
nt contains a high content of MgO and reduced S, and is composed of se
rpentine, saponite, talc, pyrrhotite, pyrite, gypsum, anhydrite, marca
site, sphalerite, chalcopyrite, isocubanite and galena. The upper laye
r reflects recent chimney collapse, whereas the sulfide layer represen
ts a residual accumulation of past collapse events. The abundance of M
g-silicates and limited biota suggest that recharge occurs over much o
f the mound surface, whereas discharge is focused at chimneys. Hydroth
ermal sediments in areas of diffuse venting are characterized by stron
g H2S odor, abundant quartz, amorphous silica, barite, smectite, organ
ic matter and biota. They differ from mound sediments by higher conten
ts of Ba and SiO2, and tower contents of S and most chalcophile elemen
ts. These areas may represent an incipient stage of hydrothermal mound
growth.