Comparative mineralogy and geochemistry of hydrothermal iron‐rich crusts from the Pitcairn, Teahitia‐mehetia, and Macdonald hot spot areas of the S. W. pacific

Abstract

Hydrothermal iron-rich crusts have been recovered from a number of hot spot volcanos including Crough Seamount, Pitcairn Volcanos 2 and 1, Cyana Seamount, Teahitia, Moua Pihaa, and Macdonald Seamount in the S. W. Pacific. Mineralogically, the crusts consists of ferrihydrite with traces of the weathering products of volcanic ash (feldspar, nontronite, pyroxene, and serpentinite). The iron oxyhydroxide phase has a mean particle size of 3-4 nm indicating rapid deposition. Electron microprobe studies have revealed the presence of filamentous iron-silica deposits within the crusts reflecting the possible bacterial oxidation of iron from the Downloaded by [North Carolina State University] at 11:41 12 April 2015 46 P. Stoffers et al. hydrothermal fluids. The crusts display wide variability in composition both between individual sampling stations and between seamounts. Endmember analysis shows that the compositional data can be resolved into three endmembers: a Fe-rich endmember, a light and heavy rare earth element endmember, and a Ba (barite)-rich endmember. The Fe-rich endmember appears to contain very low concentrations of most trace elements. For bulk samples, the composition of the iron-rich crusts reflects dilution of the iron oxyhydroxide phase by volcanic ash and, to a lesser extent, a hydrogenous component. This is illustrated by the wide variability in SiO 2 (11.1-71.3%) and Mn (0.01-1.21%) contents of the crusts. For iron-rich crusts containing greater than 40% Fe, the Pitcairn crusts display lower contents of Pb, Ba, Mo, U, Th, As, and rare earth elements (REE) and lower cerium anomalies than those from Teahitia. REE profiles of crusts from each of the hot spot volcanoes are characterized by small negative cerium anomalies but pronounced positive europium anomalies. The low average La/Fe ratios of the crusts from the various seamounts (47-572 X 10 -6 ) and positive Eu anomalies of the crusts suggest rapid deposition of the iron oxyhydroxide near the hydrothermal vent. The high Fe/Mn ratio of vent fluids at hot spot volcanoes (8.5-5.6) may account for the formation of these iron-rich crusts. The present data indicate that there may be differences in the nature of the iron-rich crusts based on the depth of occurrence. This influences the temperature of the venting hydrothermal fluids and the possibility of occurrence of submarine phreatomagmatic eruptions.

Neutrophilic Fe-Oxidizing Bacteria Are Abundant at the Loihi Seamount Hydrothermal Vents and Play a Major Role in Fe Oxide Deposition

“…Specific examples include the finding of abundant tubelike microbial filaments with a very high Fe content (up to 42% by weight) in the Coriolis troughs in the southwestern Pacific (19). Abundant evidence of Fe mats dominated by putative Fe-oxidizing bacteria have been found in investigations of the Franklin Seamount in the Woodlark Basin and the MacDonald Seamount located off the coast of New Guinea (3,33,35).…”

Low‐temperature magnetic behavior of ferrihydrite

“…It has been reported in the water column of lakes [Tipping and Ohstad, 1981], in sediments such as red bed deposits [Chukhrov, 1973], in iron-rich plumes at the East Pacific Rise [Dill et al, 1994], in iron-rich crusts at hot spots on the East Pacific Rise [Stoffers et al, 1993], and in chondrites [Brearley and Prinz, 1992]. Additionally, ferrihydrite has been found to be present in soils, such as spodosols [Shoji and Yamada, 1991], hydromorphic soils [Campbell and Schwertmann, 1984], and vertisols [Fitzpatrick, 1988].…”

Four-Hundred-and-Ninety-Million-Year Record of Bacteriogenic Iron Oxide Precipitation at Sea-Floor Hydrothermal Vents

“…The filaments are usually between 1 and 5 µmindiameter, and 10s to 100s µm long. Many have distinctive morphologies, including twisted ribbons, hollow sheaths and dendritic filaments (Alt 1988;Juniper and Fouquet 1988;Stoffers et al 1993;Halbach et al 2001;Thorseth et al 2001;Boyd and Scott 2001;Emerson and Moyer 2002;Kennedy et al 2003aKennedy et al , 2003bKennedy et al , 2003c. Several authors have noted the similarity of these morphologies with structures formed by neutrophilic Fe-oxidizing bacteria, including Gallionella ferruginea, which grows Fe encrusted twisted stalks (e.g., Hanert 1973Hanert , 2002, and Leptothrix ochracea, which forms Fe oxide encrusted sheaths (e.g., Emerson and Revsbech 1994).…”

“…The mineralogy of these Fe oxide deposits is dominated by poorly ordered Fe oxyhydroxides (Two-XRDline ferrihydrite and goethite), often with significant amounts of amorphous silica (up to 73 wt%) and Mn (up to 14 wt%) (Alt 1988;Juniper and Fouquet 1988;Binns et al 1993;Hekinian et al 1993;Stoffers et al 1993;Fortin, Ferris, and Scott 1998;Iizasa et al 1998;Boyd and Scott 2001;Emerson and Moyer 2002;Kennedy, Scott, and Ferris 2003a).…”

“…Fe oxide deposits are commonly found at hydrothermal vent sites at mid-ocean ridge and back-arc sea floor spreading centers, seamounts associated with these spreading centers, and intraplate seamounts (e.g., Barrett, Taylor, and Lugowski 1987;Alt 1988;Binns et al 1993;Hekinian et al 1993;Stoffers et al 1993;Bogdanov et al 1998;Iizasa et al 1998;Halbach, Halbach, and Lüders 2002). The formation of such deposits can be attributed to several abiogenic processes: sedimentation from hydrothermal plumes (e.g., Barrett et al 1987;Mills, Elderfield, and Thomson 1993); in situ precipitation from diffuse low-temperature flow through sediments (e.g., Koski et al 1985;Alt 1988), typically 20-100…”