V52A-01 INVITED
Back-Arc Crustal Accretion: Interplay between Spreading Center Position and a Chemically Variable Mantle Wedge
At distances greater than about 150 km from the arc, back-arc ridges are similar to their open-ocean counterparts. Closer to the arc the crustal thickness of back-arc ridges decreases then increases independently of spreading rate. Co-varying lava compositions indicate that these changes are controlled by mantle wedge chemistry. Two compositional variables that can affect crustal thickness are water content (increasing it) and chemical depletion (decreasing it). Both these variables generally increase toward the arc but observed crustal thickness variations suggest that depletion effects may extend farther from the arc before rapidly increasing water contents dominate. These inferences suggest that crustal thickness variations in back-arc basins reflect the position of the spreading axes relative to depletion and hydration gradients in the mantle wedge at the time the crust formed. Thus basin-wide geophysical mapping and tectonic reconstructions can reveal the patterns of these interactions in space and time. Two examples are the Mariana Trough and Lau Basin. At the Mariana Trough the spreading axes are closest to the arc at the northern and southern ends of the basin. Shallow crust there dominates the basin width suggesting that these spreading axes have remained close to the volcanic front during the entire opening history of the basin. The Lau basin has a complex western terrain with variable seafloor depths. This area has been interpreted as a rifted remnant arc but new seafloor mapping reveals morphologies similar to the actively spreading arc-proximal axes. This suggests that rather than tectonically rifted crust, the western terrain may reflect magmatic processes like those presently active on the arc-proximal axes, but which were abandoned when the these axes propagated southward. This model has implications for subduction material fluxes as arc-proximal spreading axes accrete thick arc-like crust with strong slab chemical signatures, but at rates that can be an order of magnitude or more than at the arc volcanic front itself. Thus the volume of this arc-like crust accreted during the opening history of a basin may represent a significant fraction of the total subduction output.
V52A-02
Seismological constraints on mantle upwelling in Japan: Implications for the genesis of arc magmas
The Japan subduction zone is one of the most-studied areas in the world by seismological approaches. In particular, the mantle-wedge structure beneath northeastern (NE) Japan has been investigated and an inclined low-velocity zone is clearly imaged in the mantle wedge, sub-parallel to the down-dip direction of the slab. The zone is interpreted to be an upwelling flow (return flow) mechanically induced by the subduction of the Pacific slab [Zhao et al., 1992; Nakajima et al., 2001; Hasegawa and Nakajima, 2004]. A quantitative analysis of the inclined low-velocity zone was done in terms of thermal anomaly and fluid content [Nakajima et al., 2005]. The results show a systematic change in melt-filled pore shapes with depth, suggesting the existence of 3-6 vol% melts as grain boundary tubules at a depth of 90 km, 0.04-0.05 vol% melts as thin cracks or dikes with aspect ratio of ~0.001 at a depth of 65 km, and 1-2 vol% melts as cracks or dikes with aspect ratio of 0.02-0.04 at a depth of 40 km. Based on these observations Hasegawa and Nakajima (2004) proposed a conceptual model for fluid- transportation path and magmatism in NE Japan. The objectives of this study are (1) to estimate detailed 3D seismic velocity structures beneath Tohoku and construct a more accurate model of arc magmatism and (2) to confirm the existence of mantle upwelling beneath Hokkaido and Kyushu where a clear volcanic front is well developed. In this study, we apply the tomography method by Zhao et al. [1992] to high-quality arrival-time data obtained by a nation-wide seismograph network in Japan. In Tohoku, we collected 626,463 P and 382,796 S-wave arrivals from 10,623 earthquakes that occurred from October 1997 to August 2006. A number of seismograph stations used in this study is 879. The number of arrival- times is thee-times larger than those used in our previous work (Nakajima et al., 2001). Grid nets with horizontal and vertical spacing of 0.15-0.25 degrees and 10-15 km, respectively, were set up in the model space. The obtained velocity structure shows a narrower low-velocity zone in the mantle wedge compared to that by Nakajima et al. (2001). The thickness of the zone is ~ 20 km. This observation suggests that a mantle upwelling is localized in the core of the mantle wedge. In case of Kyushu, arrival-time data of 238,340 for P wave and 182,495 for S waves were collected from 5635 earthquakes that occurred around the Kyushu district. Grid spacing is 0.2 degrees in the horizontal and 10-20 km in the vertical directions. The preliminary results show that an inclined low-velocity zone exits in the mantle wedge even in Kyushu.
V52A-03
Two contrasting magmatic types coexist after the cessation of back-arc spreading
A characteristic of the Izu-Bonin arc is its voluminous back-arc volcanism. In the central part of the arc this volcanism is represented by a series of seamount chains extending across the arc. These back-arc seamount chains were active between 17 and 3 Ma, which is the period between the cessation of spreading in the Shikoku Basin and the initiation of currently active back-arc rifting just behind the Quaternary volcanic front. Enrichment in fluid-mobile elements and depletion in HFSE relative to MORB indicates that these seamount chains have a significant contribution of slab-derived material. In this context these volcanoes can be regarded as a manifestation of arc magmatism. A number of unexplored seamounts of various sizes (East Shikoku Basin seamounts) are also found in the region between the western end of the back-arc seamount chains and the Kinan Seamount Chain (KSC), which traces along the extinct spreading center of the Shikoku Basin. We have recovered relatively fresh volcanic rocks from these unsampled seamounts using a Benthic multi-coring system. Preliminary data indicate that these volcanoes, like the seamount chains, show variable enrichment in LILE and depletion in HFSE, and are distinct from the MORB-like crust of the Shikoku Basin. Ar-Ar ages range from 15 to 12 Ma, indicating eruption after Shikoku Basin spreading. These observations imply that arc magmatism occurred as far as 270km west of the current volcanic front immediately after the Shikoku Basin ceased its spreading. Although contemporaneous and in a similar area, the KSC have strikingly distinct chemical characteristics to the East Shikoku Basin seamounts. The KSC lavas are alkaline basalt and are enriched not only in LILE, but also in HFSE and other highly incompatible elements. In terms of their isotope composition, they have slightly enriched Nd but comparable Sr and Pb to Philippine Sea MORB. A remarkable feature of the KSC lavas is a clear temporal variation of incompatible element concentrations such as K2O. Their K2O content increases with time and reaches a maximum at ca. 7 Ma when the chain ceased its activity. These observations imply that KSC magmas were produced by a small degree of partial melting of Philippine Sea MORB source. The degree of melting became smaller with time, probably reflecting decreasing temperature and/or upwelling rate of mantle beneath the dying spreading center. Unlike the other seamounts, these lavas show no influence from the on- going subduction to the East. The Izu-Bonin arc to back-arc system gives us the chance to examine magmatism in the period following back- arc spreading but with on-going subduction. This stage in arc evolution is characterised by widespread across arc volcanism and the occurrence of two contrasting and contemporaneous magmas.
V52A-04
Melts, Minerals and Inclusions: Unpacking Magmatism in the Sulu Ranges, New Britain Arc.
The New Britain Arc, lying off the eastern coast of mainland Papua, New Guinea, offers a unique insight into the 3D chemical structure of an island arc. In addition to typical arc-front and limited rear-arc ranges observed in other convergent margin settings, sub-aerial volcanism in New Britain extends to 500km above the subducting slab. Previous studies of this unique record of Quaternary volcanism have revealed striking chemical trends related to the distance to the Benioff zone; however, these data are restricted to whole-rock analyses of a small "representative" sample set taken from across the whole arc. To further investigate these remarkable across arc trends, a detailed study of individual volcanoes in the New Britain Arc is required. The Sulu Ranges comprise a group of arc front volcanoes on the central north coast of New Britain. The suite of rocks analysed spans the compositional range from basalt through to rhyolite and samples are highly phyric (30- 40 percent phenocrysts by volume). Clinopyroxene and plagioclase feldspar phenocrysts are ubiquitous whereas olivine and orthopyroxene are observed in the most and least mafic samples, respectively. The Sulu Ranges therefore provide an unparalleled opportunity to investigate the effects of differentiation and accumulation on whole-rock chemistry in the New Britain Arc. Olivine-hosted melt inclusions, as well as olivine, pyroxene and plagioclase feldspar phenocrysts from the most primitive Sulu Ranges sample were analysed for major and trace element contents. Large variations in olivine, clinopyroxene and plagioclase feldspar compositions together with dissolution textures present throughout indicate that the observed phases cannot represent an equilibrium assemblage. The data arrays defined by bulk rock compositions cannot be explained by liquid lines of descent resulting from simple fractional crystallisation. Estimates of groundmass compositions are used to test the extent to which whole-rock compositions may be used as proxies for melt compositions, and how melt inclusions are related to macro-scale magmatic reservoirs.
V52A-05 INVITED
The Composition of Water-Rich Components in the Sources of Back Arc and Arc Magmas
Arc and back-arc basin basalts are distinguished by geochemical signatures that are a complex function of the composition of crustal inputs form the subducting plate and the internal "factory" processes that extract materials from the slab and distribute them across the mantle wedge. Here, we use a global data set of basaltic melts from back arcs and arcs to examine relationships between four elements thought to be primary constituents of slab- derived materials: H2O, Na2O, K2O, and Cl. We use the methods of Kelley et al. (2006), coupled with recent constraints on mantle/melt DH2O and DCl (Hauri et al., 2006) to constrain concentrations of these elements in the mantle source, which compare directly with compositional models of the mantle and the slab-derived component beneath the Mariana trough (Stolper & Newman, 1994). The Mariana trough samples define linear trends in Na2O/H2O (1), K2O/H2O (0.2), and Cl/H2O (0.03) consistent with mixing between these two components. Four other back-arc basins (Sumisu, Central Lau, Manus, and E. Scotia) also fall along this trend in Na2O/H2O and K2O/H2O, suggesting that slab additions to most back-arc mantle sources have common major element characteristics. The Valu Fa Ridge segment of the southern Lau basin, however, trends away from other back arcs towards lower Na2O/H2O (0.1) and K2O/H2O (0.08). Such lower ratios are generally characteristic of arc sources in the Marianas and other arcs (e.g., Na2O/H2O=0.5-0.05), but also appear to be distinct for specific arc volcanoes. In back arcs, Cl may also be affected by late-stage magmatic assimilation of seawater, which drives up Cl/H2O and obscures primary trends, although some central Lau basin samples do point towards a mixing array coincident with the Mariana trough. Mariana arc sources have higher H2O and Cl concentrations than the Mariana trough, and indicate overall lower, but volcano-specific, Cl/H2O (0.02-0.01). In terms of these major-element constituents, arc volcanoes sample a broad diversity of slab-derived materials, whereas global back arc sources are infiltrated by a comparatively homogeneous H2O-rich component from the slab.
V52A-06
Subduction in high fluid fluxing environment and the origin of high-δ 18O and high- δ 7Li lavas in Mt. Shasta, Cascade arc, California.
This study presents analyses of O-isotopes in olivine and orthopyroxene phenocrysts from representative samples of Mt. Shasta's lavas and correlates them with Li isotopes and trace elemental ratios. The measured δ 18OOl-values range from 5.31‰ to 6.08‰, is up to 1‰ higher than the mantle value. Added to the presence of primitive magnesian andesite and high primary water content (up to 12wt%), the high-δ 18O measured makes the Mt. Shasta exceptional. The composition of the analyzed olivine phenocrysts shows that these crystals are in equilibrium with their host whole rocks, and were derived by melting of a peridotitic source based on trace element ratios in olivine. Minor (<5%) contamination makes these rocks a good assessment to mantle-derived magma. The origin of the high-δ 18O mantle-derived signature results from the interaction with unusually high fluid fluxes from high-δ 18O slab and subducted sediments. A model based on the flux-melting process is proposed in order to explain why in a general case the subduction fluid signature is lost during the magma genesis and why in exceptional cases, such as Mt Shasta, it is partially preserved. Unusual features such as a fracture zone or volcanic chain subduction lead to a fluid flux that is high enough to maintain a high-δ 18O after reacts the mantle wedge and after dilution by mantle-melt when crossing the hydrated mantle solidus. Consequently, by melt re-equilibration with the fluid, high-δ 18O magmas are generated. The comparison between the Li- and O-isotopes compositions of Mt. Shasta and neighboring volcanoes from the rear-arc (i.e., Medicine Lake), show that both stable isotope values decrease from front to the rear arc. This decrease is explained by the slab dehydration process, due to the Rayleigh loss of heavy isotope with subduction progress and the fact that deeper and deeper portions of the slab are dehydrated. The "adakitic" signature (Sr/Y up to 160) of the Mt. Shasta's lavas appears to result from high fluid fluxing. According to geochemical and petrological characteristics of the Mt. Shasta's high-Mg andesites, it appears that these siliceous, hydrous, and high-δ 18O magmas worldwide are generated in specific, high-fluid-flux environments and are not ubiquitous as general primary magma types in arc petrogenesis.
V52A-07
Trace element and isotopic constraints on mantle source heterogeneity and subduction- related fluids along the Eastern Lau Spreading Center
The Eastern Lau Spreading Center (ELSC) is located in the Lau Basin behind the Tonga arc. Its distance from the arc front changes progressively from 40 km at the Valu Fa Ridge in the south to 100km in the north. We report >65 new Pb-Sr-Nd isotope analyses and over 130 trace element analyses of axial lavas. The excellent correlations between the trace element and isotope compositions allow the determination of the subduction component and the background (unaffected by subduction) mantle compositions, as well the changes occurring along the arc (N-S), and with distance from the arc (E-W). We see clear N-S changes in "background" mantle, with stronger Indian (Dupal) signatures toward the north. In the southern Valu Fa the isotope ratios intercept the Tonga arc array. 206Pb/204Pb decrease northward (from 18.66 to 18.13), with an increasing offset to high 207Pb/204Pb and 208Pb/204Pb compared to N. Atlantic-Pacific data. With a few exceptions, 87Sr/86 are high but show little variation (0.70323±8), and values are similar to Indian MORB and some Tonga islands. 143Nd/144Nd are even more uniform (0.51306±3). Despite the small variability, the data show that the "background" mantle is less Indian-like in the south. For example, Valu Fa lavas define a trend, distinct from the other segments, toward lower 87Sr/86 for a given 206Pb/204Pb, indicating a more "Pacific-like" background mantle in the southernmost segment. The ODP sites located in the Lau Basin show that the "background" mantle was Pacific- like in earlier stages of volcanism Our on-axis data gives the current compositions of the Lau Basin upper mantle and combined with the ODP data indicate that Pacific mantle has been progressively replaced with Indian mantle from north to south. Latitudinal variations in the compositions of the subduction components can be clearly seen in back-arc lavas. Relationships between Pb isotopes and Nb/U or Ba/Nb reveal that the compositions of subduction components affecting the back arc mantle are the same as those affecting adjacent volcanoes in the main arc (for example Valu Fa is like Ata, and ELSC2 and ELSC3 are Kao and Tofua). Overall, the contributions from subduction components to back-arc basalts decrease with increasing distance from the arc. The southernmost segments display smooth changes of Pb and fluid mobile/immobile element ratios with latitude, whereas further north and farther from the arc, some segments show local subduction signature "spikes". These results show that, close to the arc, the mantle beneath the back-arc is pervasively affected by subduction-related fluids or melts, while far from the arc, the subduction signature is localized into geographically discrete portions of the ridge.
V52A-08
Cross-arc Variations in Lava Chemistry in the Tonga Arc-Lau Back Arc System, 19- 23°S
The Tonga arc system from 19°-23°S consists of the active Tofua arc, the Eastern Lau Spreading Center (ELSC; a back arc spreading center), and numerous seamounts between them. We use the excellent sampling of ELSC and 34 nearby seamounts, along with sparser published analyses of Tofua arc, to examine the spatial relations of chemistry and melting in this subduction system. The spatial constraints can be used to better understand the nature and mechanism of enrichment that is caused by subduction. Geochemistry along the axis of ELSC is related to its distance to the Tofua arc, which decreases continuously from 100 km in the north to 40 km in the south. The subduction influence (e.g., fluid mobile elements) along ELSC increases in several sharp gradients towards the south as ELSC gets closer to the arc. The six different tectonic segments of ELSC display mixing relationships in trace element ratio-ratio diagrams (e.g., Ba/La vs Th/La) in which one end member is a subduction component that is distinctive for each segment (Escrig et al., this meeting). We explore whether the distinctive subduction components of each ELSC segment are reflected by the Tofua arc that is adjacent to that segment, and by the intervening seamounts. Relationships between the arc, back arc and seamounts are different in the north and the south. In the south where the arc-back arc distance is smaller, the Tofua arc volcanic rocks share the distinctive trace element characteristics of their corresponding ELSC segment, and extend the mixing trajectories to higher, more arc-like values. Seamounts that are located between Tofua arc and ELSC also share the distinctive trace element characteristics of the local arc + back-arc, and are intermediate in their trace element ratios. These observations are consistent with the model of Langmuir et al., (2006) in which magmas of back arc spreading centers form from two components: a dry side similar to mid-ocean ridges and a wet (trenchward) side that produces hydrous melts. We suggest that Tofua arc formed completely from the wet side. Seamounts have a small input from the dry side and a greater input from the wet side compared to ELSC, consistent with their location. Our data suggest that the contribution from the wet side varies in composition along the length of the axis, and that the distinctiveness at each latitude is maintained across the arc from Tofua to ELSC. In the north, the relationships between arc, back-arc and seamounts are more complex. Segment NELSC-1 (Bezos et al., in prep and this meeting) displays two distinctive mixing trends toward different subduction components. Neither of the subduction components of NELSC-1 matches the Tofua arc at this latitude: they are both too low in Ba and Pb relative to H2O. This reflects the great distance between Tofua arc and NELSC-1. The seamounts chemistry seems to reflect their proximity to ELSC or Tofua arc. Seamounts that are close to NELSC-1 fall on mixing arrays of NELSC-1. Seamounts that are closer to Tofua arc lie on a mixing arrays between Tofua arc and NELSC-1. A seamount at the northern end of NELSC-1 has more chemical affinity to the ILSC that is offset to the NW from the northern tip of ELSC. It seems that the subduction components associated with Tofua have a limited range of influence toward the back arc.