Mineralogy of serpentine muds of ODP Leg 125 holes (Table 1)

Large serpentinite seamounts are common in the forearc regions between the trench axis and the active volcanic fronts of the Mariana and Izu-Bonin intraoceanic arcs. The seamounts apparently form both as mud volcanoes, composed of unconsolidated serpentine mud flows that have entrained metamorphosed ultramafic and mafic rocks, and as horst blocks, possibly diapirically emplaced, of serpentinized ultramafics partially draped with unconsolidated serpentine slump deposits and mud flows. The clayand silt-sized serpentine recovered from three sites on Conical Seamount on the Mariana forearc region and from two sites on Torishima Forearc Seamount on the Izu-Bonin forearc region is composed predominantly of chrysotile, brucite, chlorite, and clays. A variety of accessory minerals attest to the presence of unusual pore fluids in some of the samples. Aragonite, unstable at the depths at which the serpentine deposits were drilled, is present in many of the surficial cores from Conical Seamount. Sjogrenite minerals, commonly found as weathering products of serpentine resulting from interaction with groundwater, are found in most of the samples. The presence of aragonite and carbonate-hydroxide hydrate minerals argues for interaction of the serpentine deposits with fluids other than seawater. There are numerous examples of sedimentary serpentinite deposits exposed on land that are very similar to the deposits recovered from the serpentine seamounts drilled on ODP Leg 125. We suggest that Conical Seamount may be a type locality for the study of in situ formation of many of these sedimentary serpentinite bodies. Further, we suggest that both the deposits drilled on Conical Seamount and on Torishima Forearc Seamount demonstrate that serpentinization can continue in situ within the seamounts through interaction of the serpentine deposits with both seawater and subduction-related fluids.

(Table 1) Composition of paraffin hydrocarbons at DSDP Site 56-434

Geochemical investigation of 18 samples of sediments from Site 434 involved determining the content of organic carbon, of bitumoid A (The chloroform A-chl and alcohol-benzene A-alb extracts) and its various fractions, and of individual hydrocarbons as well as the structural group composition of resins. We identified certain samples that differed sharply from the rest by their increased bitumen content and relatively low molecular hydrocarbons and by the fact that their resinous components were more neutral and aliphatic in composition. The distribution of bitumoid and its components seems to reflect migration processes in operation during the early stages of the transformation of organic matter.

(Table 1) Geochemistry of ash layers at DSDP Leg 57 Holes

The study of the main characteristics of ash layers in Leg 57 cores shows that they are suitable for an analysis of the effect on eruptive activity of their distribution. We found (1) sediment recovery good and ash layers numerous; (2) sedimentary environment generally free from terrigenous clastic material; (3) reworking limited; (4) volcanic glass very acidic, ranging from rhyolitic to rhyodacitic composition; and (5) alteration and diagenesis negligible above the lower Miocene. The curves of explosive volcanic activity in Holes 438, 439, and 440 display two stages of high activity: an early one around 16 m.y. and a late one starting 5 m.y. B.P., both stages being separated by an upper Miocene quiescence. Detail in these results is limited by the chemical composition of the glass and accounts only for trends in explosive acid volcanism. Nevertheless, results are roughly in agreement with other data from the Northwest Pacific, although some discrepancies in the correlation of intensity of the episodes occur. The data from Leg 57 support the hypothesis of synchronous pulses in explosive volcanism.