Igneous geochemistry of OP Leg 126 samples

Major element, trace element, and radiogenic isotope compositions of samples collected from Ocean Drilling Program Leg 126 in the Izu-Bonin forearc basin are presented. Lavas from the center of the basin (Site 793) are high-MgO, low-Ti, two-pyroxene basaltic andesites, and represent the products of synrift volcanism in the forearc region. These synrift lavas share many of the geochemical and petrographic characteristics of boninites. In terms of their element abundances, ratios, and isotope systematics they are intermediate between low-Ti arc tholeiites from the active arc and boninites of the outer-arc high. These features suggest a systematic geochemical gradation between volcanics related to trench distance and a variably depleted source. A basement high drilled on the western flank of the basin (Site 792) comprises a series of plagioclase-rich two-pyroxene andesites with calc-alkaline affinities. These lavas are similar to calc-alkaline volcanics from Japan, but have lower contents of Ti, Zr, and low-field-strength elements (LFSE). Lavas from Site 793 show inter-element variations between Zr, Ti, Sr, Ni, and Cr that are consistent with those predicted during crystallization and melting processes. In comparison, concentrations of P, Y, LFSE, and the rare-earth elements (REE) are anomalous. These elements have been redistributed within the lava pile, concentrating particularly in sections of massive and pillowed flows. Relative movement of these two-element groupings can be related to the alteration of interstitial basaltic andesite glass to a clay mineral assemblage by a post-eruptive process. Fluid-rock interaction has produced similar effects in the basement lavas of Site 792. In this sequence, andesites and dacites have undergone a volume change related to silica mobility. As a result of this process, some lithologies have the major element characteristics of basaltic andesite and rhyolite, but can be related to andesitic or dacitic precursors by silica removal or addition.

Composition of ODP Leg 126 sandstones

Early diagenesis in Leg 126 forearc and backarc sands/sandstones is characterized by the dissolution of intermediate to mafic brown glass, the alteration of colorless rhyolitic glass to clay minerals, precipitation of thin clay-mineral rim cements, and minor precipitation of clinoptilolite cements. Later, more intense diagenesis is restricted to Oligocene forearc basin sediments at Sites 787,792, and 793. In these sections, the effects of early diagenesis have been intensified and overprinted by later diagenetic effects including (1) large-scale dissolution of feldspar and pyroxene crystals, (2) further dissolution of vitric components, (3) precipitation of minor carbonate cements, and (4) pervasive, multiple-staged zeolite cementation. Zeolite minerals present include analcite, mordenite, natrolite, heulandite, wairakite, chabazite, erionite, herschelite, and phillipsite. The latest diagenetic events appear to be the minor dissolution of zeolite cements and the precipitation of minor carbonate and potassium feldspar(?) cements. Observed porosity types include primary interparticles; primary intraparticles in vesicular glass and foraminifers; primary interparticles reduced by compaction and cementation; secondary intraparticles produced by dissolution of feldspar, nonopaque heavy minerals, volcanic glass, and foraminifer tests; and secondary interparticles produced by the dissolution of zeolite cements. Within forearc Oligocene sections at Sites 787 and 792, diagenetic effects appear to decrease with depth in the Oligocene section; however, at Site 793 the majority of samples are intensely altered.

(Table 1) Summary of maximum clast size and bed thickness of ODP Leg 126 samples

Subaerial debris flows, with water contents ranging from as little as 10 wt% up to no more than about 25 wt% (Pierson, 1986; Pierson and Costa, 1987), are non-Newtonian fluids that move as fairly coherent masses with yield strength (owing to bulk densities and viscosity that are much greater than those of clear water), which enables them to suspend and transport large clasts. Their flow behavior is thought to be predominantly laminar, although the relative importance of laminar and turbulent flow has not been established and is debatable. They leave deposits (debrites) that are characteristically poorly sorted with large clasts in their middle portions and commonly protruding from their tops. Although generally ungraded or normally graded in their upper portions, many have centimeter- to decimeter-thick inversely graded basal zones (Arguden and Rodolfo, 1990, doi:10.1130/0016-7606(1990)102<0865:SADDBH>2.3.CO;2).

(Table 1) Pore-water composition of ODP Leg 126 sites

The Ocean Drilling Program Leg 126 sites may be classified into two categories depending on the presence (Group I: Sites 787, 792, and 793) or absence (Group II: Sites 788, 790, and 791) of steep concentration gradients. Shipboard X-ray diffraction analyses of bulk sediments from Group I sites revealed the presence of a number of diagenetic minerals (some of which are incompatible), but no systematic diagenetic zonation. The results of the chemical analyses of the pore waters from Group I have been used to estimate the activities of dissolved species. Thermodynamic analyses of the composition of the pore waters and the stability of authigenic minerals (gypsum, zeolites, feldspars, smectites, chlorites, and micas) show that the pore waters are close to equilibrium with most of the observed phases. Thus, only a small perturbation of the system (substitution in minerals and fluctuations in pore-water composition, in particular, in pH and SiO2 activity) will cause any of these phases to precipitate. Therefore, one would not expect mineralogical observations to show systematic vertical zonations at these sites. It is suggested that chlorites and high-temperature zeolites are not diagenetic sensu stricto, but were eroded from volcaniclastic highs. The absence of concentration gradients at the Group II sites has been analyzed in terms of reaction kinetics, hydrothermal advection, and temperature distribution. The absence of diagenetic imprints on the pore-water concentration profiles at these sites is probably caused by the slow nucleation of silica phases.

(Table 1) Peak intensity of minerals in ODP Hole 126-793B

Sediments and rocks recovered during Ocean Drilling Program Leg 126 at Sites 792 and 793 in the Izu-Bonin forearc basin are described with a primary focus on clay mineralogy. Evidence for diagenetic hydrothermal alteration processes is present in the upper Oligocene to lower Miocene sediments at these sites. The vitric and pumiceous sand/sandstone and pumiceous gravel contain high concentrations of smectites, zeolites, and gypsum. Microscopic observations show that the volcanic glass and feldspars have been altered to smectites and zeolites. The authigenic mineral assemblages indicate that these minerals resulted from precipitation from circulating fluids, as well as from the alteration of glass and feldspar under temperature conditions that may have reached 200°-300°C. Mineral assemblages in microfractures display thermal gradients that possibly reflect cooling effects.

(Table 1) Sulphur concentration and isotopic composition of ODP Leg 126 igneous rocks

Sulfur isotope ratios have been determined in 19 selected igneous rocks from Leg 126. The d34S of the analyzed rocks ranges from -0.1 â to +19.60 â. The overall variation in sulfur isotope composition of the rocks is caused by varying degrees of seawater alteration. Most of the samples are altered by seawater and only five of them are considered to have maintained their magmatic sulfur isotope composition. These samples are all from the backarc sites and have d34S values varying from +0.2 â to +1.6 â, of which the high d34S values suggest that the earliest magmas in the rift are more arc-like in their sulfur isotope composition than the later magmas. The d34S values from the forearc sites are similar to or heavier than the sulfur isotope composition of the present arc.

(Table 1) Remanence properties of samples from ODP Core 126-792A-7H

A combination of high sedimentation rates and high concentrations of magnetic grains in cores from Ocean Drilling Program Leg 126 resulted in the recovery of detailed direction and intensity records of the Brunhes/Matuyama geomagnetic polarity reversal. Virtual geomagnetic poles (VGPs) computed from azimuthally oriented samples taken from the cores of Hole 792A in the western Izu-Bonin forearc basin reveal that the geomagnetic pole persisted at moderate to high southern latitudes for several thousand years before a rapid migration to northern latitudes. Alternating-field demagnetization behavior, as well as NRM, NRM/ARM, and NRM/IRM intensities for samples from this same interval, and the NRM/IRM intensities derived from unoriented core samples from Holes 790C and 791B, drilled in the ~100-km distant Sumisu Rift, all suggest that the dipole field oscillated widely in intensity before the reversal. The fast polarity change occurred at the low point of an ~1100-yr field intensity cycle. This "reversal cycle" immediately followed earlier intensity cycles whose peaks rivaled or surpassed the normalized intensities of discrete samples from well above and below the reversal interval; furthermore, the troughs indicate a much diminished dipole field at their nadir.