Geochemistry of glass shards in the volcanic ash layers of the Izu-Bonin Arc

To examine the processes and histories of arc volcanism and of volcanism associated with backarc rifting. 130 samples containing igneous glass shards were taken from the Plioccne-Quatemai^ succession on the rift Hank (Site 788) and the Quaternary fill in the basin fill of the Sumisu Rift (Sites 790 and 791). These samples were subsequently analyzed at the University of Illinois at Chicago and Shizuoka University. The oxides determined by electron probe do not account for the total weight of the material; differences between summed oxides and 100% arise from the water contents, probably augmented by minor losses thai result from alkali vaporization during analysis. Weight losses in colorless glasses are up to 9%; those in brown glasses (dacitcs to basalts) arc no more than 4.5%; shards from the rift-flank (possibly caused by prolonged proximity to ihc scafloor) generally have higher values than those from the rift-basin fill How much of the lost water is magmatic, and how much is hydrated is uncertain; however, although the shards absorb potassium, calcium, and magnesium during hydration in the deep sea, they do so only to a minor extent that does not significantly alter their major element compositions. Therefore, the electron-probe results are useful in evaluating the magmatism recorded by the shards. Pre- and syn-rift Izu-Bonin volcanism were overwhelmingly dominated by rhyolile explosions, demonstrating that island arcs may experience significant silicic volcanism in addition to the extensive basaltic and basaltic andestic activity, documented in many arcs since the 1970s, that occurs in conjunction with the andesitic volcanism formerly thought to be dominant. Andesitic eruptions also occurred before rifting, but the andesitic component in our samples is minor. All the pre- and syn-rift rhyolites and andesites belong to the low-alkali island-arc tholeiitic suite, and contrast markedly with the alkali products of Holocene volcanism on the northernmost Mariana Arc that have been attributed to nascent rifting. The Quaternary dacites and andesites atop the rift flank and in the rift-basin fill are more potassic than those of Pliocene age, as a result of assimilation from the upper arc crust, or from variations in degrees of partial melting of the source magmas, or from metasomatic fluids. All the glass layers from the rift-flank samples belong to low-K arc-tholeiitic suites. Half of those in the Pliocene succession are exclusively rhyolitic: the others contain minor admixtures of dacite and andesite, or andesite and either basaltic andesite or basalt. In Contrast, the Quaternary (syn-rift) volcaniclastics atop the rift-flank lack basalt and basaltic andesite shards. These youngest sediments of the rift flank show close compositional affinities with five thick layers of coarse, rhyolitic pumice deposits in the basin fill, the two oldest more silicic than the younger ones. The coarse layers, and most thin ash layers that occur in hemipelagites below and intercalated between them, are low-K rhyolites and therefore probably came from sources in the arc. However, several thin rhyolitic ash beds in the hemipelagites are abnormally enriched in potassium and must have been provided by more distal sources, most likely to the west in Japan. Remarkably, the Pliocene-Pleistocene geochemistry of the volcanic front does not appear to have been influenced by the syn-rift basaltic volcanism only a few kilometers away. Rare, thin layers of basaltic ash near the bases of the rift-basin successions are not derived from the arc. They deviate strongly from trends that the arc-derived glasses display on oxide-oxide plots, and show close affinities to the basalts empted all over the Sumisu Rift during rifting. These basalts, and the basaltic ashes in the basal rift-basin fill, arc compositionally similar to those erupted from mature backarc basins elsewhere.

(Table 1) Summary of horizon, lithology, and paleomagnetic of ODP Hole 126-791B basaltic mousse samples

A paleomagnetic study was made on the highly vesiculated basaltic tuff breccia (the basaltic mousse) drilled by Ocean Drilling Program Leg 126 from the Izu-Bonin backarc, Sumisu Rift, to estimate the mode of its emplacement. Thirty-four 10-cm**3 minicore samples were collected from almost all the horizons of the basaltic mousse. Stepwise thermal and alternating-field demagnetization experiments show that the natural remanent magnetizations of many samples are mainly composed of a single stable component. Although remanence inclinations are not expected to be disturbed by rotary drilling, the measured inclinations of remanence show a random directional distribution as a whole. The remanence inclinations, however, show directional consistency on a smaller scale. High-density sampling and measurements from a limited interval of drilled cores, and the measurement of small disks cut from a single minicore sample show that there is directional consistency over several centimeters. Strong and stable remanent magnetization, the existence of remanence direction consistency, and the fresh lithology of the samples suggest the thermal origin of remanence. Combining the paleomagnetic results with other geological, petrographical, and paleontological characters, the Hole 791B basaltic mousse can be interpreted as a subaqueous explosion breccia produced by deep-sea pyroclastic fountaining.

Eocene to Quaternary benthic foraminifers from the Izu-Bonin Arc

The benthic foraminifer fauna at Sumisu Rift Sites 790 and 791 indicates that a deep open-ocean (>2300 m) or a basin with open-ocean access existed between 1.1 and 0.7 Ma at the time of the initiation of rifting. The appearance of a low- to medium-oxygen fauna (1600-2300 m) between 0.7 and 0.5 Ma suggests that the open-ocean access may have been terminated at this time because of the development of volcanoes and rift flank uplifts around the basin. The occurrence of low-oxygen faunas at 0.03 Ma suggests a secondary closing of the basin. The lower bathyal benthic faunas from lower Pliocene sediments of rift margin Site 788 suggest about 0.6-1.6 km of total basement uplift. This uplift may have led to the formation of the major hiatus between 2.3 and <0.3 Ma. The faunal changes of benthic foraminifers at Sites 792 and 793 in the forearc basin document a shallowing water depth from below the carbonate compensation depth (CCD) (about 3.5 km) in the late early Oligocene to the present depths of 1800 and 2975 m, respectively. These data suggest about 1 km of total basement uplift in the inner part of the forearc basin (Site 792) and about 0.6 km total basement subsidence in the central part of the forearc basin (Site 793) since about 31 Ma. The former uplift led to a thinner sediment accumulation (800 m) and the latter subsidence to a thicker sediment accumulation (1400 m) at these sites. Faunal changes of benthic foraminifers observed in Sites 782 and 786 sequences drilled at the outer-arc high document a deepening water depth from 1.3 to 2.1 km in late Eocene to the present depth of about 3 km. These data suggest about 1.1-1.9 and 1.3-2.1 km of total basement subsidence at Sites 786 and 782, respectively. These results indicate total basement uplift in the inner part of the Bonin arc-trench system since late Oligocene and total basement subsidence in the outer part of the system since late Eocene. The last occurrence (LO) of Stilostomella spp. and Pleurostomella spp. and the first occurrence (F0) of Bulimina aculeata d'Orbigny occurred consistently at 0.7 Ma at all three arc proximal sites (790,791, and 792). This fact is taken to suggest a change of water mass, from one originating from the central part of the ocean to that originating from ocean-margin areas at that time.

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.