ALBERT

Description: Several meters of unconsolidated hydrothermal sediment were recovered from the Snake Pit hydrothermal field during ODP Leg 106. Polymetallic sulfides comprise most of the sediment with minor fragments of massive sulfide, organic debris, clay minerals, and fresh glass shards. Trace element and Sr-isotope contents of hydrothermal clays and sulfides from Holes 649B and 649G indicate that these minerals precipitated from a mixed hydrothermal fluid-seawater solution. Evaluation of the REE mineral data and the Snake Pit hydrothermal fluids shows that the REE distribution coefficients between the hydrothermal fluids and clay-sulfide mixes range from 100-500. This indicates that hydrothermal fluids originating in the root-zone of the Snake Pit hydrothermal system may be modified by the precipitation of hydrothermal minerals, either in the shallow subsurface or within chimney structures. Contrasting REE profiles of clay-sulfide aggregates and massive sulfides from Holes 649B and 649G may be accounted for by spatial and/or temporal variations in redox conditions in the plumbing system.

Description: Sedimentary sections recovered from the Tonga platform and forearc during Ocean Drilling Program Leg 135 provide a record of the sedimentary evolution of the active margin of the Indo-Australian Plate from late Eocene time to the Present. Facies analyses of the sediments, coupled with interpretations of downhole Formation MicroScanner logs, allow the complete sedimentary and subsidence history of each site to be reconstructed. After taking into account the water depths in which the sediments were deposited and their subsequent compaction, the forearc region of the Tofua Arc (Site 841) can be seen to have experienced an initial period of tectonic subsidence dating from 35.5 Ma. Subsidence has probably been gradual since that time, with possible phases of accelerated subsidence, starting at 16.2 and 10.0 Ma. The Tonga Platform (Site 840) records only the last 7.0 Ma of arc evolution. However, the increased accuracy of paleowater depth determinations possible with shallow-water platform sediments allows the resolution of a distinct increase in subsidence rates at 5.30 Ma. Thus, sedimentology and subsidence analyses show the existence of at least two, and possibly four, separate subsidence events in the forearc region. Subsidence dating from 35.5 Ma is linked to rifting of the South Fiji Basin. Any subsidence dating from 16.2 Ma at Site 841 does not correlate with another known tectonic event and is perhaps linked to localized extensional faulting related to slab roll back during steady-state subduction. Subsidence from 10.0 Ma coincides with the breakup of the early Tertiary Vitiaz Arc because of the subduction polarity reversal in the New Hebrides and the subsequent readjustment of the plate boundary geometry. More recently, rapid subsidence and deposition of a upward-fining cycle from 5.30 Ma to the Present at Site 840 is thought to relate to rifting of the Lau Basin. Sedimentation is principally controlled by tectonic activity, with variations in eustatic sea level playing a significant, but subordinate role. Subduction of the Louisville Seamount Chain seems to have disrupted the forearc region locally, although it had only a modest effect on the subsidence history and sedimentation of the Tonga Platform as a whole.

Description: A series of six holes in the Lau Basin and two in the Tonga Platform were drilled during Leg 135 of the Ocean Drilling Program. The volcaniclastic sands found within the sedimentary cover are typically dominated by fragments of dacite glass, although basaltic andesite glass, calcic plagioclase, and ortho- and clinopyroxene grains are also identified. Total silica contents of individual glass grains indicate a bimodal spread of values at sites close to the Eastern Lau Spreading Center (ELSC), whereas those adjacent to the remnant arc (Lau Ridge) showed a complete spread of compositions. Analyses from the Tonga Platform (Site 840) indicate a prerift phase of arc volcanism (7.0-5.0 Ma) when a complete spectrum of silica values was erupted before backarc basin rifting occurred and the locus of volcanism switched to intrabasinal seamounts producing very similar island-arc tholeiites and their differentiates. The principal mode of deposition of volcanic sediment in the Lau Basin is thought to be by proximal mass-flows and turbidity currents from submarine, intrabasinal seamounts. Continuous volcaniclastic sedimentation throughout the opening of the basin indicates that volcanism during the initial stages of basin rifting occurred in the form of intrabasinal submarine edifices, before the reestablishment of a fixed chain of arc volcanoes, adjacent to the Tonga Platform at approximately 3.0 Ma in the southern Lau Basin. Renewal of arc volcanism approximately coincided with the propagation of backarc spreading centers into that area. Trace and rare earth element analyses of basaltic grains (SiO2 = 45%-55%) from the Tonga Platform (Site 840) before and after rifting show systematic trends in the abundances of incompatible elements and in incompatible element ratios that are consistent with either progressive depletion and then reenrichment of a mantle source, or an equivalent progressive increase and then decrease in the degree of melting. On top of this, there is some indication of a systematic shift in the character of enrichment towards a greater abundance of mobile incompatibles associated with the flux from the slab. The culmination of this cycle can be considered to be the modern Tofua Arc. These trends are most reasonably attributed to a thinning of the arc lithosphere before rifting and the associated increase in the height of the melting column within the subarc asthenosphere. Subsequent magmatic underplating after rifting causes a thickening of the arc lithosphere and a fall in the degree of partial melting, with a progressive slab-flux component being added to the source. The incompatible element ratio trends are correlated whether they are high-field-strength elements, rare-earth elements, or mobile large-ion-lithophile elements, implying that they relate to incompatibilities between source and melt, even though the absolute values in the basic rocks are typically arc-like rather than MORB-like, particularly in their enrichment in Ba and depletion in Nb. The behavior of Nb in particular appears to have important implications for the process by which arc-tholeiites acquire their distinctive trace-element characteristics.