Alfred G. Hochstaedter

Trace element and SrNdPb isotopic constraints on a three-component model of Kamchatka Arc petrogenesis

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.

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with KAr 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_2O 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_2O_3 depletion and delays the initiation of Fe_2O_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_2O_3_(tot) and TiO_2 enrichment. Thus, major element trends are more similar to those of MORB than arcs. H_2O, CO_2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H_2O-rich mixed gas phase. High H_2O/S, high δD, and low (MORB-like) δ^(34)S ratios are considered primary and distinctive of the back-arc basin setting.

Volcanism in the Sumisu rift. II, Subduction and non-subduction related components

Abstract A bimodal suite of volcanic rocks collected from the Sumisu Rift by alvin provide present day examples of the first magmatic products of arc rifting during the initiation of back-arc spreading. The trace element and isotopic composition of these rocks, which are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, differ from those of arc rocks and N-MORB in their relative incorporation of both subduction-related and non-subduction-related components. Subduction-related components, i.e., those that distinguish volcanic arc basalts from N-MORB, are less pronounced in rift lavas than in arc lavas. Alkali and alkaline earth to high field strength element and REE ratios as well as 87 Sr/ 86 Sr are intermediate between those of N-MORB and Izu arc lavas and indicate that Sumisu Rift basalts are similar to BABB erupted in other, more mature back-arc basins. These results show that back-arc basins may begin their magmatic evolution with BABB rather than more arc-like lavas. Evidence of non-subduction related components remains after the effects of subduction related components are removed or accounted for. Compared to the arc, higher HFSE and REE concentrations, contrasting REE patterns, and ⩽e Nd in the rift reflect derivation of rift lavas from more enriched components. Although SR basalt resembles E-MORB in many trace element ratios, it is referred to as BABB because low concentrations of Nb are similar to those in volcanic arcs andH 2 O/REE andH 2 O/K 2 O exceed those of E-MORB. Differences in HREE pattern ande Nd require that the E-MORB characteristics result from source heterogeneities and not lower degrees of melting. Enriched mantle beneath the rift may reflect enriched blobs entrained in a more depleted matrix, or injection of new, more enriched mantle. High 208 Pb/ 204 Pb and moderate 207 Pb/ 204 Pb ratios with respect to Pacific MORB also reflect ancient mantle enrichment. Trends on Pb isotopic diagrams and ⩽e Nd in the rift than in the arc indicate that recent sediment recycling is not an important process in the generation of these back-arc lavas.

Across‐arc geochemical trends in the Izu‐Bonin arc: Constraints on source composition and mantle melting

Across-arc geochemical trends can constrain subduction zone dynamics by providing clues to the source composition and melting systematics of subduction-related magmas over a wide portion of the mantle wedge. The Izu-Bonin arc contains an active volcanic front, an active extensional zone, and a series of 3–9 Ma southwest trending acrossarc seamount chains. The volcanic front (VF) contains one of the most depleted suites of any volcanic arc, with basalt containing 0.2–0.7 ppm Nb, 25–50 ppm Zr, Nb/Zr < 0.015, and Zr/Y < 2.5. Ratios and concentrations of mantle-derived elements change significantly across the Izu-Bonin arc. The westernmost portions of the across-arc seamount chains (WS) contain much higher incompatible element concentrations and associated ratios: 1–8 ppm Nb, 50–130 ppm Zr, Zr/Y = 2–7, and Nb/Zr = 0.02–0.1. The extensional zone contains intermediate concentrations and ratios of these elements. Trace element modeling shows that VF and WS compositions cannot be produced by different degrees of melting of a homogeneous source. Instead, heterogeneous sources are required, implying that enriched source material must exist in the back arc regions of the Izu-Bonin arc. Melt extraction of fractional melts from the WS source may produce a residual, depleted source capable of generating VF magma. Age dating studies show that the VF and WS suites have retained similar compositions over the last 15 million years, implying that steady state processes have continuously produced these diverse suites of magmas.

Petrology and geochemistry of lavas from the Sumisu and Torishima backarc rifts

Thirteen dredge hauls from the active Sumisu and Torishima rift grabens west of the Izu-Bonin arc at about 30°–31°N, 140°E, recovered a suite of tholeiitic basalts to sodic rhyolites. Volcanism occurs along tensional faults within and bounding the rift grabens and along the transfer zones between adjoining rift segments. The Sumisu and Torishima rift lavas differ significantly from the lavas of the adjacent arc volcanic centers in having lowerAl2O3/Na2O,Ba/Zr,V/Ti, andBa/Ce and higher abundances of the rare earth elements. The rift lavas also have characteristics of backarc basin basalts, in that they are enriched in Al2O3, and depleted in total iron and TiO2 relative to mid-ocean ridge basalt, characteristics which are consistent with a higher water content in the source. Thus, the model of a progressive change in backarc basin basalt composition from arc-like to mid-ocean ridge-like, as a function of evolution of the basin, as has been suggested from many backarc regions, is not generally applicable. The comparison of the Sumisu and Torishima rift lavas with Mariana backarc basin lavas indicates that backarc basin basalts differ in composition from one basin to another. The comparison of these backarc basin suites with mid-ocean ridge suites from similar axial depths indicates that the overall control over the spectrum of backarc basin basalt compositions may be different extents of melting of the mantle. The Sumisu and Torishima rift lavas formed by a slightly higher extent of melting than the Mariana backarc basin basalts, a phenomenon which is related to the depth of the ridge segments. Furthermore, the data suggest a systematically higher extent of melting in the arc lavas than in the backarc lavas for both of these arc/backarc systems, consistent with a greater flux of water beneath the arc than beneath the backarc region.

Insights into the volcanic arc mantle wedge from magnesian lavas from the Kamchatka arc

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.

Volcanism in the earliest stage of back-arc rifting in the Izu-Bonin arc revealed by laser-heating 40Ar/39Ar dating

Abstract The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.

Alvin-SeaBeam studies of the Sumisu Rift, Izu-Bonin arc

Abstract Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31°N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080° ± 10°) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50–700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven alvin heat flow measurements at 30°48.5′N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as Sumisu and South Sumisu volcanoes.

On the Tectonic Significance of Arc Volcanism in Northern Kamchatka

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 Vyvenka 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 ~15 Ma Golovin and 6-8 Ma Valovayam volcanic rocks, two andésite suites within the Vyvenka volcanic field. The Golovin suite consists of medium- to high-K andesites with strong arc-like trace-element 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.