COMPARATIVE MINERALOGY AND GEOCHEMISTRY OF HYDROTHERMAL IRON-RICH CRUSTS FROM THE PITCAIRN, TEAHITIA-MEHETIA, AND MACDONALD HOT-SPOT AREAS OF THE SW PACIFIC

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 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 sample's, 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 SiO2 (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.