Various types of records of ancient hydrothermal activity were found on the acrossarc seamount chains of the Izu-Ogasawara arc. They are mainly hydrothermal Mn-oxides and nontronite. Low temperature hydrothermal activity is supposed to have widely occurred associated with the volcanism on the across-arc seamount chains during Miocene to Pliocene.

The igneous geochemistry of lavas and breccias from the basement of Sites 790 and 791, and pumice clasts from the Pliocene-Pleistocene sedimentary section of Sites 788, 790, 791, and 793 were studied. Arc volcanism became silicic about 1.5 m.y. before the inception of rifting in the Sumisu Rift at 2 Ma, but eruption of these silicic magmas reflects changes in stress regime, especially during the last 130,000 yr, rather than crustal anatexis. Arc magmas have had a larger proportion of slab-derived components since the inception of rifting than before, but are otherwise similar. Rift basalts and rhyolites are derived from a different source than are arc andesites to rhyolites. The rift source has less slab-derived material and is an E-MORB-like source, in contrast to an N-MORB-type source overprinted with more slab-derived material beneath the arc. Rift magma types, in the form of rare pumice and lithic clasts, preceded the rift, and the earliest magmas that erupted in the rift already differed from those of the arc. The earliest large rift eruption produced an exotic explosion breccia ("mousse") despite eruption at >1800 mbsl. Although this rock type is attributed primarily to high magmatic water content, the clasts are more MORB-like in trace element and isotopic composition than are modern Mariana Trough basalts. After rifting began, arc volcanism continued to be predominantly silicic, with individual pumice deposits containing clasts that vary in composition by about 5 wt% SiO2, or about as much as in historical eruptions of submarine Izu Arc volcanoes. The overall variations in magma composition with time during the inception of arc rifting are broadly similar in the Sumisu Rift and Lau Basin, though newly tapped OIB-type mantle seems to be present earlier during basin formation in the Sumisu than Lau case.

We propose that across‐arc differences in the geochemistry of Izu‐Bonin arc magmas are controlled by the addition of fertile‐slab fluids to depleted mantle at the volcanic front, and residual‐slab fluids to fertile mantle in the back arc without slab melting or contemporaneous back arc spreading. The arc consists of a volcanic front, an extensional zone, and seamount chains (the Western Seamounts) that trend into the Shikoku Basin. Each province produces a distinct suite of arc‐like volcanic rocks that have relative Nb depletions and high ratios of fluid‐mobile elements to high field strength elements. The volcanic front has the lowest concentrations of incompatible elements and the strongest relative enrichments of fluid‐mobile elements (high U/Nb, Ba/Nb, Pb/Zr, Th/Nb, 206Pb/204Pb, ɛNd, and 87Sr/86Sr). A fluid derived from both sediment and altered oceanic crust explains most of the slab‐related characteristics of the volcanic front. The Western Seamounts and some of the extensional zone rocks have lower ɛNd, 87Sr/86Sr, 206Pb/204Pb, Ba/Th, and U/Th; moderate Ba/Nb and U/Nb; and similar or higher Th/Nb and Th/Nd. Although the lower ɛNd and higher Th/Nd tempt a sediment melt explanation, a lack of correlation between the strongest sediment proxies, such as ɛNd, Th/Nb, and Ce/Ce*, precludes sediment melts. The subduction component for the Western Seamounts is probably a fluid dehydrated from a residual slab that was depleted in fluid‐mobile elements beneath (as well as trenchward of) the volcanic front. This depleted fluid is added to elementally and isotopically more enriched mantle beneath the Western Seamounts.

The Kamchatka arc (Russia) is located in the northwestern Pacific Ocean and is divided into three segments by major sub-latitudinal fault zones (crustal discontinuities). The southern (SS) and central (CS) segments are associated with the subduction of old Pacific lithosphere, whereas the northern, inactive segment (NS) was formed during westward subduction of young (< 15 Ma) Komandorsky Basin oceanic crust. Further segmentation of the arc is outlined by the development of the Central Kamchatka Depression (CKD) intra-arc rift, which is oriented parallel to the arc and is splitting the CS into the active Eastern Volcanic Front (EVF) and the largely inactive, rear-arc Sredinny Range. The NS volcanics (15-5 Ma) include calc-alkaline lavas, shoshonites, adakites, and Nb-enriched arc basalts. Isotopically all magma types share high 143Nd/144Nd ratios of 0.512976-0.513173 coupled with variable 87Sr/86Sr (0.702610-0.70356). NS lavas plot within or slightly above the Pacific MORB field on the Pb isotopic diagrams. The EVF volcanoes have more radiogenic 143Nd/144Nd (0.51282-0.513139) and 208Pb/204Pb (38.011–38.1310) than the NS lavas. CKD lavas display MORB-like Nd isotope ratios at slightly elevated 87Sr/86Sr values accompanied by a slightly less radiogenic Pb composition. Kamchatka lavas are thought to be derived from a MORB-like depleted source modified by slab-derived siliceous melts (adakites) and fluids (NS), or fluids alone (CS and SS). The NS and EVF lavas may have been contaminated by small fractions of a sedimentary component that isotopically resembles North Pacific sediment. Petrogenesis in the Kamchatka arc is best explained by a three-component model with depleted mantle wedge component modified by two slab components. Slab-derived hydrous melts produced incompatible element characteristics associated with northern segment lavas, while hydrous slab fluids caused melting in the depleted mantle below the southern and central segments of the Kamchatka arc. Trace element characteristics of Kamchatka lavas appear to be controlled by slab fluids or melts, while radiogenic isotope ratios which are uniform throughout the arc reflect depleted composition of sub-arc mantle wedge.

Active volcanism in the Kamchatka arc occurs where the Pacific Plate subducts beneath the Kamchatka peninsula south of its junction with the Aleutian arc. Most volcanism occurs within the Central Kamchatka Depression (CKD), a large graben oriented parallel to the trench, and along the Eastern Volcanic Front (EVF), located south and east of the CKD and closer to the trench. Differentiation trends range from calc‐alkaline to tholeiitic. Fractionation of a mineral assemblage, including olivine, clinopyroxene, and orthopyroxene, produces the tholeiitic trend, whereas separation of amphibole and magnetite, along with possible crustal assimilation, produces the calc‐alkaline trend. A suite of near‐primitive high‐Mg basalts provides geochemical records of mantle sources and processes unobscured by differentiation. Rare earth element (REE) patterns range from slightly depleted ((Ce/Yb)n = 0.8–1.5) to slightly enriched ((Ce/Yb)n = 1.5–3.5). Rocks with the depleted REE patterns occur at the volcanic front in regions where a volcano or volcanic chain exists behind the volcanic front. Lavas with relatively enriched REE patterns occur behind the volcanic front and along portions of the volcanic front where behind‐the‐front volcanism is absent. Modeling of trace element abundances normalized to 10% MgO indicates that the rocks with the depleted REE patterns are derived from a more depleted source, inferred to represent refractory source material remaining after a previous generation of melt extraction within the arc. Mantle source material apparently convects into the mantle wedge from the rear, producing relatively enriched magmas when it melts for the first time. Relatively depleted magmas are produced if a second period of melting ensues as the mantle reaches the volcanic front.

The Vyvenka volcanic field records a period of Neogene, subduction-related volcanism in northern Kamchatka. Most models describing the tectonic evolution of the northwest Pacific do not account for this type of Neogene volcanism because the main locus of Pacific/Kula-North American convergence switched to the Aleutian Ridge during Eocene time. The Vyyenka volcanism, as well as oceanic spreading and crust formation within the Komandorsky Basin, demonstrate that this region remained tectonically and volcanically active in Neogene times. We report petrologic, geochemical, and K-Ar age data for the approximately 15 Ma Golovin and 6-8 Ma Valovayam volcanic rocks, two andesite suites within the Vyyvenka volcanic field. The Golovin suite consists of medium to high-K andesites with strong arc-like trace-elements signatures, while the Valovayam suite consists of medium-K andesites with weaker arc-like trace-element signatures. The Valovayam andesites also contain some trace-element ratios indicative of melting of the subducted oceanic crust. These include high Sr/Y (30-50) and Zr/Sm greater than the chondritic value of 28. The Golovin andesites have overlapping Sr/Y (25-45) and lower Zr/Sm. The compositional differences between the Golovin and Valovayam andesites correlate with Neogene tectonic evolution of the Komandorsky region. In northern Kamchatka, subduction waned as spreading stopped in the Komandorsky Basin and newly generated oceanic crust entered the subduction zone. Thus, the trace-element signals of slab melts in the younger Valovayam rocks indicates melting of the young, hot Komandorsky Basin crust that entered the subduction zone and subsequent metasomatism of the mantle wedge. The weaker subduction signature of the Valovayam suite, which distinguishes it from the Golovin suite, records the decreasing vigor of subduction processes with time.

A bimodal volcanic suite with K Ar ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin . These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures. Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H 2 O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al 2 O 3 depletion and delays the initiation of Fe 2 O 3 (tot) and TiO 2 enrichment. However, unlike arc basalts,Fe 3+/ ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe 2 O 3 (tot) and TiO 2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H 2 O, CO 2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H 2 O-rich mixed gas phase. HighH 2 O/S, highδD, and low (MORB-like)δ 34 S ratios are considered primary and distinctive of the back-arc basin setting.