V43D-1626
Mantle Wedge Processes from Primitive Lavas in the Western Aleutians
The 2005 Western Aleutian Volcano Expedition mapped more than 19,000 sq km of Aleutian seafloor and dredged >4000 kg of fresh lava from volcanic cones and other young, constructional features along more than 1600 km of arc length. The most interesting feature is the field of active seafloor volcanism discovered in the Ingenstrem Depression - a rectangular basin, approximately 60 km in length by 10-15 km wide along the Aleutian Ridge west of Buldir island, the western-most emergent Aleutian volcano. We found more than 90 volcanic cones and associated lava flows within and on the margins of this basin, which is bounded by WNW-oriented strike-slip faults. The largest cones are 2-4 km in diameter and 300-500 m high. Some appear to be offset by strike-slip faulting, but most are un-deformed, constructional features. A striking feature of Ingenstrem dredge samples is that they include an unusually large proportion of primitive lavas with whole-rock Mg#>0.60, including basalt, andesite and dacite. Basalts and basaltic andesites have moderately enriched trace element patterns (e.g., La/Yb 4-8, Sr/Y<30) and relatively radiogenic Sr (87/86Sr=0.7031-0.7034), typical of lavas from the Aleutians and other island arcs. In contrast, Ingenstrem primitive andesites and dacites have high Sr (700-2300 ppm) and strongly fractionated trace element patterns (Sr/Y>50), with low Y (<12 ppm) and HREE. Among these, andesites and dacites with high Sr/Y also have systematically higher SiO2, lower Y, and higher Mg#, Cr and Ni. Strontium isotope ratios in Ingenstrem lavas are inversely correlated with Sr/Y and SiO2, so the most felsic samples (66-67% SiO2) have the most fractionated trace element patterns (Sr/Y>120) and the least radiogenic Sr (87Sr/86Sr<0.7029). Interestingly, samples that span the full range of geochemical variability are found in close spatial association within the Ingenstrem Depression. In one case, primitive lavas within an individual dredge span almost the entire range of SiO2 and Sr/Y. The small length scale of compositional variation in primitive lavas implies significantly different melt temperatures (basalts ~1250C vs dacites ~1000C) and source compositions (87Sr/86Sr=0.7027-0.7034), over short distances at sub-Moho depths. This, in turn, requires advective transport of cold material through a hot mantle wedge `cold plumes': e.g., Ringwood, JGSL 74; Marsh AJS 76; Kelemen et al. ToG 03; Gerya & Yuen EPSL 03) or advective transport of hot material through a cold shallow mantle conductive layer (`hot fingers': e.g., Tatsumi et al. JGR 03; Tamura et al EPSL 02). In the first case, primitive dacites with high Sr/Y could be formed by reaction between low T melts of subducting eclogite and hotter residual peridotite in the wedge. In the second case, primitive dacites could be formed by reaction between fractionating basalt in sub-crustal magma chambers and surrounding, colder residual mantle in the lithosphere.
V43D-1627
Back-arc Seamount Distribution Along the Western Aleutian Volcanic Arc
Seamounts and seafloor volcanoes provide important insights into magma production, fluid flow in the crust, and habitat oases for marine life. In 2005, we obtained the first modern multibeam bathymetric maps, using the 30 kHz Simrad EM300 on the RV Thompson, for large areas of the Aleutian back-arc from west of Attu to Unalaska. Hundreds of previously unknown small seamounts were discovered, and many of the larger previously known seamounts were bathymetrically mapped completely for the first time. Equally important, the high-resolution bathymetry was able to confidently establish areas where no seamounts exist. Our mapping concentrated on several rhombochasms and the general back-arc region. Individual seamounts were identified and characterized using a GIS-based method for detecting local peaks in digital elevation models. Our results indicate that seamounts become progressively smaller but more abundant westward along the arc. Seamounts tend to be organized in linear chains that are subparallel to the local direction of maximum compressive stress indicated by scarps bounding their rhombochasm. Finally, no seamount was found in the Kresta Trough west of Attu, suggesting that this marks the westward limit of (normal?) volcanic production along the Aleutian Arc. These results are correlated to the obliquity of subduction of Pacific Plate beneath the arc.
V43D-1628
High-Mg# andesites and basalts from the Kamchatka-Kurile subduction system: Implications for primitive arc magma genesis and mantle wedge processes
Primitive arc magmatism and mantle wedge processes are investigated through a petrologic and geochemical study of high Mg# (Mg/Mg+Fe>0.65) basalts and andesites from the Kurile-Kamchatka subduction system. The primitive andesites are from the Shisheisky complex (Portnyagin et al., AGU Monograph 172, 2007), a field of Quaternary-age, monogenetic cones located in the Aleutian-Kamchatka junction, north of Shiveluch Volcano, the northernmost active composite cone in Kamchatka. The Shisheisky lavas are similar to primitive andesites from Mt. Shasta, Piip Volcano, and Setouchi, Japan. They have Mg# of 0.66-0.73 at intermediate SiO2 (54-58 wt%), low CaO/Al2O3 (<0.54), and high Ni (184-243 ppm) and Cr (418-880 ppm). Olivine phenocryst core compositions of ~FO90 appear to be in equilibrium with whole-rock `melts', consistent with the aphyric to sparsely phyric nature of these lavas. Compared to the Shishiesky andesites, primitive basalts from the region (Alaid, Tolbachik, Kharchinsky) have higher CaO/Al2O3 (0.69-0.86), and lower whole-rock Ni (105-182 ppm), Cr (395-531 ppm), and Ni/MgO (10-17) at similar Mg# (0.66-.70). Olivine phenocrysts in the basalts have similarly higher CaO, lower Ni, and lower Ni/MgO at ~FO88 compared to the andesites. The absence of plagioclase phenocrysts from the primitive andesites strongly contrasts petrographic observations of the plagioclase-phyric basalts, indicating relatively high pre-eruptive water contents for the andesites compared to the basalts. Petrographic and mineral composition data suggest that the Shisheisky primitive andesites were liquids in equilibrium with mantle peridotite, and were not produced by mixing between primitive basalts and evolved felsic magmas or from contamination by xenocrystic olivine. The key features of the Shisheisky primitive andesites (e.g., low CaO/Al2O3 and high Ni/MgO at high Mg#) appear to have been acquired at sub-moho depths, by processes and under physical conditions in the mantle wedge (lower temperatures, higher water contents), which were distinct from those that produced primitive basalts in the region.
V43D-1629
U-Th Disequilibria in Island Arc Lavas from the Western Aleutians
Th-excesses observed in MORB and OIB, where (230Th/238U) > 1, are commonly interpreted to result from melting in the presence of garnet. In contrast, U-excesses typical of island arc lavas, where (230Th/238U) < 1, are widely believed to originate when hydrous fluids from the subducting plate preferentially mobilize U. Analyses of island arc lavas from four sites in the western Aleutians reveal along-arc trends in trace element composition, including decreasing Y, increasing Sr/Y, and (230Th/238U) disequilibria, and suggest a possible westward increasing role for residual garnet in generating some of these subduction related melts. The lavas at Korovin Volcano, a subaerial volcano and the easternmost site in this study (174.1 °W), include basalts, andesites, and dacites with trace element characteristics typical of island arc lavas, including Sr/Y = 6- 30. Y increases with increasing SiO2, from 17 - 51 ppm. Three analyses of (230Th/238U) in two basalts and one dacite show ~6-17% U-excesses. At Little Sitkin Volcano (178.5 °E), basalts, andesites and dacites have Sr/Y = 13-25, and Y between 17-30 throughout the SiO2 range. U-excesses range from 2-8%. The Ingenstrem Depression is a pull-apart basin at the summit of the Aleutian ridge at 175 °E, within which are more than 90 submarine volcanic features. The basalts from the Ingenstrem Depression have high Mg# (0.64 - 0.72), Sr/Y <35, and low Y (15-20 ppm). Three analyses of basalts show both U- and Th-excesses. Three distinct andesite compositions are observed: one group of Mg# ~0.45 and Sr/Y = 80, another with Mg# ~0.6 and Sr/Y = 130, and a third with Mg# ~0.63 and Sr/Y = 32- 50. A single andesite analysis shows (230Th/238U) near secular equilibrium. The Ingenstrem dacites show a positive correlation between Mg# and Sr/Y, with Mg# from 55-64 and elevated Sr/Y between 84-166. One dacite has Th-excess, while another is close to secular equilibrium. Lavas from submarine Piip volcano (167.3 °E) range from the most primitive andesites in this study, with Mg# up to 0.72, to dacites and rhyolites with Mg# down to 0.44. Consistently low Sr and Y (400 and 18 ppm) give Sr/Y from 20-30. In five analyses, four Piip lavas have Th-excesses of 5-17%, and one has U-excess of 2%. The westward increase in Sr/Y, decrease in Y, and transition from exclusively U-excesses to Th-excesses observed in the western Aleutians suggest a westward emerging contribution of melts from a garnet bearing source. Eastward from this study area, where plate convergence is more nearly orthogonal, U-excesses are predominant (George et al., JGR, 2003). The oblique subduction environment of the western Aleutians may be favorable to the generation, transport, and eruption of these magmas with their unique geochemical features intact.
V43D-1630
Imprints of an "Arc" Signature onto Subduction Zone Eclogites from Central Guatemala
High-pressure, low-temperature (HP-LT) rocks associated with the Motagua fault zone in central Guatemala occur as tectonic blocks in serpentinite mélange. Dismembered jadeitite and albitite veins within the melange are crystallization products of subduction fluids at <400° C and 0.4-1.4 GPa. Lawsonite eclogites represent the deepest, coldest rocks, with peak metamorphic conditions of approx. 2.6 GPa and 480°C. They contain a subduction fluid overprint acquired during retrogression to blue- and green-schist-facies conditions, seen mostly as hydrous phases (e.g. phengite, glaucophane) in veins and overgrowths. The low temperatures recorded in these rocks indicate they have only seen an aqueous fluid, not a melt, and therefore, could provide a window into the acquisition of an arc signature at a cold margin. Trace-element patterns for both eclogite and jadeitite resemble arc lavas, with large enrichments in the most fluid mobile elements (e.g. Cs, Tl, Ba, Pb), moderate enrichments in U, Th, Be and LREE and generally little to no enrichment in HFSE and HREE, although enriched Nb in jadeitite indicates some HFSE mobility. Trace-element patterns also have similarities to average subducting sediment (GLOSS), with enrichments in Th, Be, Ba and Li that suggest a sediment contribution. Nd versus Sr isotopes lie to the right of the mantle array, indicating a hydrous fluid contribution from altered ocean crust or sediment. Overall, Guatemalan eclogites resemble counterparts from the Franciscan Complex (CA) and the Dominican Republic. Guatemalan and Franciscan eclogites are interpreted to have had a MORB protolith despite the arc trace element signature because of: 1) similarities in major elements to MORB; 2) HREE and HFSE abundances similar to MORB; and 3) high 143Nd/144Nd that overlap MORB values. The modifications that transformed these eclogites from a MORB trace element pattern to an arc one can be attributed to an aqueous subduction fluid at moderate depths (<75km). This transformation may be due to the increased solubilities of some minerals (e.g., jadeite, albite, clays, sulfates) at high pressure, high water/rock ratios from dehydration reactions, and an abundance of alkali-aluminosilicate components in subduction fluids. Together these may act to dissolve and transport trace elements (including elements considered insoluble like Nb) out of the slab and into the mantle wedge. The Guatemala data thus indicate that the arc geochemical fingerprint may be achieved at cold margins without the need for melting.
V43D-1631
Oxygen isotope geochemistry of back-arc lavas
Oxygen isotope ratios of arc-related magmas can constrain the amount and nature of slab-derived components contributing to their mantle sources because: (i) most geological solids and fluids contain similar concentration of oxygen; and (ii) possible slab components (fluids, melts or rocks from sedimentary, mafic or ultramafic parts of the lithosphere) differ from one another in δ18O values and these distinctive oxygen-isotope compositions can be preserved in deeply subducted materials. Previous laser fluorination oxygen isotope data on back-arc basin basalts (BABB) show slightly but consistently elevated δ18O values (from 5.5 to 6.1 ‰ [1-3]) with respect to mid-ocean ridge basalts or MORB (δ18O = 5.5 ± 0.2 ‰ [4]). The subtle elevations in δ18O characteristic of BABB are generally associated with ‘enriched' radiogenic isotope compositions (e.g., high 87Sr/86Sr) and high apparent degrees of melting of their peridotitic sources that may reflect fluxed melting of the mantle by fluids and/or hydrous melts released from subducted oceanic lithosphere [1-3]. These trends could be used to precisely define the sources and amounts of slab- derived components delivered to the back arc mantle. However, two issues demand that this problem be better constrained through further studies. First, the subtle difference in δ18O between MORB and BABB involves comparison of data generated in different laboratories that may not share common standardization; this difference must be demonstrated within a single laboratory using a common set of standards. Second, the trends of δ18O vs. other geochemical indices observed in BABB lavas are based on samples from several relatively small and unrelated suites of lavas. These trends must be documented through study of one or more relatively large and diverse suites of closely related lavas. We are performing new laser-fluorination oxygen isotope measurements of a comprehensive suite of fresh back-arc glasses collected along the Eastern Lau Spreading Center (ELSC). This suite displays a continuous trend of increasing subduction-component influence with decreasing distance from the Tonga arc (from north to south). So far, new δ18O values of ELSC lavas are within the range previously described for back-arc samples and are consistent with the global trend of 18O enrichment with increasing indices of slab-derived components. In particular, when compared to the MORB dataset obtained in the same laboratory, ELSC lavas appear to define a continuous "mixing" trend between low-δ18O depleted peridotite and high- δ18O subducted component. We anticipate that further expansion of the ELSC data set will permit detailed tests of hypotheses for the origins of geochemical variability of the Lau basin lavas and help constraining the properties and amounts of slab-derived fluids and/or melts sampled by these lavas and the parts of the slab from which they were derived. [1] Macpherson and Mattey (1998), Chem. Geol., 144, 177-194. [2] Macpherson et al. (2000), EPSL, 176, 171-183. [3] Eiler et al., J. Petr. (2000), 41, 229-256. [4] Eiler (2001), Rev. Min. Geochem., 43, 319-362.
V43D-1632
New constraints from Tonga-Kermadec on the origin of O-Hf-Os isotope signatures in oceanic arc lavas
O, Hf and Os isotope ratios have been measured on selected lavas from the Tonga-Kermadec arc that have previously been analysed for Be isotopes which provides an unambiguous tracer of subducted sediment. O isotope values overlap with those of MORB and indicate minimal interaction with the arc crust. O isotopes do not increase northwards with increasing subduction rate as would be expected if slab derived fluids had high O isotope ratios. The northward decrease in HFSE concentrations reflects prior depletion not increasing extents of melting. Hf isotopes are strongly negatively correlated with Be isotopes. Simple binary mixing of subducted pelagic sediment into the mantle wedge can replicate the Hf-Be isotope correlation, without recourse to fluid mobility of Hf. However, the sediment component must have been stored in the mantle wedge for several Myr, as suggested previously. The same mixing model can replicate those samples with the lowest Os isotopes and one sample has subchondritic Os indicating that some melt depletion of the mantle wedge pre-dates the opening of the Lau Basin. Thus, some slab-derived Os isotope signatures are preserved which requires rapid magma ascent to minimise melt – wall rock interaction. Higher Os isotope ratios in some samples may reflect interaction with the arc crust. There is no overlap between the Hf isotope composition of the north Tonga arc lavas and either the Samoan plume or the subducting Louisville volcaniclastic sediments. Thus, the unusual Pb and Nd isotope signatures in these lavas must derive from the Louisville sediments but be added with minimal mobilisation of Hf indicating conservative behaviour for this element.
V43D-1633
Detailed Distribution of the Helium Isotope Ratios in Northeastern Japan
The geographical distribution of helium isotope ratios (3He/4He ratios) is characterized by high values of 4 to 8RA (where RA is the atmospheric 3He/4He ratio of 1.39×10-6) along the volcanic front and in the back-arc region at Tohoku district, northeastern Japan. In contrast forearc region shows low values less than 1RA. On the other hand, there is no clear contrast of the 3He/4He ratios except at the central region (e.g., Sano and Wakita, 1985). We perform the helium isotope ratio analysis in northeastern Japan, and around the source region of the Niigataken Chuetsu-oki Earthquake in 2007 (M6.8) where 3He/4He ratios data were reported. We have collected 41 samples of gases from hot springs, mineral springs, and deep wells, distributing mainly in the forearc region at Tohoku district. In addition, we also collected 19 samples of gases from hot springs, volcanoes and natural gas fields around the source region of the Niigataken Chuetsu-oki Earthquake in 2007. We measured 3He/4He ratios by noble gas mass spectrometers (Helix and VG5400) of Ocean Research Institute (ORI), the University of Tokyo. The 4He/20Ne were measured by a quadruple mass spectrometer to evaluate air contamination in the samples. δ13C (CO2) values were measured by using a mass spectrometer (DELTA plus XP) of ORI. Main features of our results for Tohoku region are as follows: 1) The 3He/4He ratios in the forearc region are less than 1RA. 2) The 3He/4He ratios vary along the volcanic front. In Miyagi prefecture [38-39N], the ratios range from 2 to 5 RA. On the other hand, the ratios are less than 1RA in and around the southern boundary of Iwate and Akita prefectures [39-39.5N]. The distribution of 3He/4He ratios in Niigata plans to be discussed by comparing with the well-studied seismotectonics and the structure of the crust and upper mantle.
V43D-1634
Back-arc compositional variability in the Mariana Trough as a potential function of subduction zone geometry
Compositional variability of back-arc magmas may be attributed in part to differing amounts of added slab component. Such variability would presumably correlate with distance to the magmatic front and height above the subducting slab. However, compositions of glasses and melt inclusions sampled from the southern Mariana Trough back-arc (13.3° - 14° N) do not show any linear correlations with distance to the magmatic front (MF). Instead, notable increases of trace element signatures for an added subduction component only occur in magmas located within 41 km of the arc. At these locations, Ba/La > 20 and Ba/Nb >13, while elsewhere along the back-arc Ba/La< 5 and Ba/Nb < 10. K2O contents along the back-arc range from 0.09 to 0.26 wt%, but reach 0.37 wt% in glass sampled 33 km from the MF. However, the latter melts are more evolved than most in the Mariana Trough, with 54-56 wt% SiO2. Cl concentrations range from 150 to 300 ppm along the most of the back-arc, however glasses sampled within 41 km of MF have Cl contents up to 1318 ppm. Although water contents vary from 0.6-1.34 wt% along the back-arc, there are no strong linkages with distance to the arc or added subduction component. These data suggest that compositional variability in the Mariana Trough is not strongly controlled by subduction processes except in portions of the back-arc spreading center located within ~40 km of the magmatic front. However, the geometry of the subducting slab and proximity to the end of the back-arc spreading center must also be considered. The MF of the Marianas is very arcuate in comparison to the more linear back-arc and as a result the southern end of MF curves towards, and potentially intersects, the Mariana Trough. The compositional variation described here may thus reflect the spatial characteristics of the Mariana subduction zone.
V43D-1635
Seismic Imaging of the Mantle Under the Western-Pacific arc and Back-arc Regions
We have used multi-scale seismic tomography to determine the detailed 3-D structure of the crust and mantle under the Western-Pacific arc and back-arc regions. The subducting Pacific and Philippine Sea (PHS) slabs are imaged clearly from their entering the mantle at the oceanic trenches to their reaching the mantle transition zone and finally to the core-mantle boundary (CMB). Recent local tomography of Northeast Japan has imaged the shallow portion of the slab from the Japan Trench down to about 200 km depth under Japan Sea. The 3-D Vp and Vs structures of the forearc region under the Pacific Ocean are constrained by accurately located hypocenters with sP depth phases. Strong lateral heterogeneity is revealed along the main thrust zone under the forearc region, and there is a good correlation between the heterogeneity and the distribution of large thrust earthquakes. A similar feature is found in the forearc region of the Kyushu subduction zone. A joint inversion of local and teleseismic data imaged the subducting Pacific slab down to 670 km depth under the Japan Islands and the Japan Sea. The PHS slab is detected down to 500 km depth under SW Japan. A mantle upwelling is found under SW Japan that rises from about 400 km depth right above the Pacific slab up to the PHS slab. Continuous slow anomalies are revealed under the volcanic front in the Ryukyu arc. The Eurasian lithosphere is found to subduct down to about 300 km depth under South Taiwan. Regional and global tomography revealed the Pacific slab that is stagnant in the mantle transition zone under Eastern China. The stagnant slab may have played an important role in the formation of the intraplate volcanoes in NE Asia. Global tomography shows pieces of fast anomalies in the middle and lower mantle as well as in the D" layer above the CMB, suggesting that the stagnant slab finally collapses down to the lower mantle and CMB as a result of very large gravitational instability from phase transitions. Prominent slow anomalies are also revealed in the mantle under the subducting slabs, which may represent either mantle plumes or upwelling flows associated with the deep subduction of the slabs. http://www.aob.geophys.tohoku.ac.jp
V43D-1636
Regional variation in shear-wave splitting in Japan
Nakajima and Hasegawa [2004] and Nakajima et al. [2006] investigated shear-wave splitting in northeastern Japan and found a striking rotation of fast direction across the arc, suggesting the different nature of anisotropy between the fore-arc and back-arc sides. Trench-parallel fast directions are observed in the fore-arc side, while fast directions observed in the back-arc side show approximately E-W or ESE-WNW in the NE Japan arc and N-S in the southwestern Kurile arc, which are characterized by the local dip direction of the subducted Pacific plate. In this study, we investigate shear-wave splitting beneath SW Japan using local S phases to constrain a spatial pattern of mantle flow generated by the subduction of the Pacific and Philippine Sea plates. The subduction of the two oceanic plates probably has resulted in a complicated upper mantle structure. Therefore this region is an excellent natural laboratory for studying the effect of local slab geometry and the subduction of two slabs on mantle corner flow. We applied the cross-correlation method [Ando et al., 1983] to S-wave arrivals of local events to constrain fast polarization directions and delay time between fast- and slow-shear waves for each event and station pair. In the cross-correlation method, seismograms are rotated at angles ranging from 0 to 175 degrees in steps of 5 degrees. One of the horizontal components is shifted by a time lag ranging from 0 to 1 sec with an interval of 0.01 sec. The length of the time window used to calculate the correlation coefficient was set to nearly equal to one cycle of the wave. When the value of cross-correlation coefficient reaches its maximum, the direction of rotation is regarded as the fast direction, and the amount of time lag as the delay time. If the maximum cross-correlation coefficient is less than 0.8, the data is rejected. All of the observed seismograms were filtered with band-passed ranges of 2-8 Hz. Waveforms of 557 intermediate-depth earthquakes recorded at 457 seismic stations were used, and 2062 splitting parameters, the leading shear-wave polarization direction (fast direction) and delay time between two split waves, were observed. The obtained results show that most fast directions observed in SW Japan are nearly E-W, which is roughly consistent with the direction of the maximum dip direction of the Pacific plate. We also found that fast directions polarized in N-S are locally observed around the Hida Mountains, central Japan, which has been pointed out by the previous studies [e.g., Ando et al., 1983; Hiramatsu et al., 1998]. This direction is sub-parallel to the maximum-dip direction of the subducted Philippine Sea plate and hence this anisotropy is inferred to be related to a mantle return flow generated by the subduction of the Philippine Sea plate.
V43D-1637
Coupled Petrological and Geodynamic Models of Mantle Flow in Subduction Zones; the Importance of Chlorite in the Emergence of a Low-Viscosity Channel
Seismic velocity and attenuation studies have shown that 5-20 km thick low velocity layers exist above seismically fast slabs and are associated with broad zones of high attenuation in many subduction zones. These observations are generally interpreted as formation of hydrous phases by dehydration of the slab, although the impact of water in nominally anhydrous minerals (NAM) on seismic wave propagation is largely unknown. Recent petrological experiments on hydrous peridotite at subduction zone conditions suggest that chlorite will be stable adjacent to the subducting slab in sufficient quantities to be a significant water sink. We use a scheme that couples a petrological model (pHMELTS) with a 2-D thermal and variable viscosity flow model (ConMan) to model energy and mass transfer within a subduction zone. By varying input parameters including the convergence rate and slab dip we have developed models for cases in the Costa-Rica and Izu- Bonin-Marianas arc systems and are able to predict major and trace element compositions of primary melts, as well as geophysical observables, such as the topography and geoid. We find that the emergence of a slab- adjacent low-viscosity channel (LVC) is a natural consequence of the thermal and chemical controls on mantle dynamics and feedback between them. In our earlier models, as the LVC is dragged downwards by the subducting slab, hornblende breaks down at about 2.5 GPa and other hydrous phases such as serpentine are secondary in importance to the NAM water reservoir. The spatial limit of the LVC is the water-saturated solidus of the hydrated peridotite; the LVC thickens as the peridotite is progressively depleted by melting and the solidus migrates into the warmer wedge, despite water replenishment at depth. pHMELTS is a hybrid of the pMELTS model of Ghiorso and co-workers and includes amphiboles, serpentines and micas. Chlorite was lacking but we have recently rectified this omission. Following De Capitani and co- workers, we have chosen the tri-octahedral FeMg-chlorite model of Hunziker [2003, PhD thesis, University of Basel] for its relative simplicity, suitable P-T-X experimental calibration for modeling igneous systems and internal consistency with the Berman thermodynamic dataset. For incorporation into the MELTS family of algorithms the four end-member speciation model must be reformulated as a three end-member solution with an ordering parameter, appropriate interaction parameters and analytical partial derivatives up to third order. When chlorite is included as a potential phase in the coupled scheme a persistent layer of the mineral forms extending from the location of the shallowest water source to at least 5 GPa. Preliminary results suggest that the LVC no longer thickens with depth as described above and melting is restricted to shallower depths. These effects have implications for the chemistry of extracted arc melts, predicted topography and geoid signal, migration of water towards the back-arc basin and the fate of hydrated material as subduction proceeds. Chlorite is the dominant hydrous phase, forming up to 25 wt% of the hydrous assemblage, but the NAM account for 10% or more of the water budget within the LVC. At pressures greater than 5 GPa, chlorite should become unstable and the NAM are likely to be important in transport of water towards the transition zone.
V43D-1638
Magma Generation and Transport in Subduction Zones: Numerical Simulations of Chemical, Thermal and Mechanical Coupling During Magma Ascent by Porous Flow
Most subduction zones are characterized by significant magmatic activity responsible for building trench-parallel volcanic arcs above descending slabs. High magma production rates observed within the arcs result from infiltration of water-rich fluids released by slab dehydration. The released water triggers hydrous melting of hot mantle wedges located above the cold slabs. However, the process of magma transport from the melt generation region located above the hydrated slab surface at 100-300 km depth to the magma extraction zone at the volcanic arc surface, and its influence on mantle wedge deformation, are not well known. In particular, during basaltic liquid ascent through the mantle wedge, decreasing pressure and temperature changes are likely to induce significant compositional variations, especially in terms of dissolved water content. Relationships between melt transport and mantle wedge deformation are also not clearly understood. We present a numerical model of magma generation and transport in subduction zones, that simulates chemical, thermal, and mechanical interactions between fluids and solid rocks along the magma ascent pathway. Magma migration is modelled by a porous flow across a constant permeability matrix, while the solid downward current associated with subduction in the mantle wedge, is included. The heat advected by the percolating liquid phase as well as latent heat effect associated with melting will be included. Water exchanges between the molten rock and the solid matrix are computed as a function of pressure, temperature, and solubilities laws in melt. We will first present benchmark results to validate the porous flow modelling as well as the ernery equation resolution for a two- phase flow. The aqueous and magmatic fluid repartition within the mantle wedge will then be presented. Magma productivity rates, varying along the magma ascent path way, will be discussed as a function of magma viscosity.
V43D-1639
The geochemical behaviour of W in subduction zones: constraints from high precision isotope dilution measurements
Assessing the behaviour of W during silicate Earth's differentiation is hampered by low abundances of W in terrestrial reservoirs, making sufficiently precise and accurate measurements difficult. Previous results (e.g., Newsom et al. 1996) indicate a lower W/Th of the mantle (ca. 0.19) compared to the Earth's crust, (ca. 0.26), suggesting that W appears to be more incompatible than Th. New data for MORB (Munker et al. 2007), however, demonstrate that W/Th is not significantly fractionated during dry peridotite melting, tentatively suggesting a fractionation of the two elements during crust formation by subduction related processes. We present high precision W and Nb-Ta, Zr-Hf data obtained by isotope dilution, using a mixed 183W-180Ta- 94Zr-180Hf-176Lu tracer and multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). This enables the direct determination of W and HFSE from one sample digestion. For some samples, a "nugget effect" as previously reported for PGE was observed, reflecting sample heterogeneity. Measured Ta and W abundances determined in basaltic glasses and whole rock powders from various island arc settings yield Ta/W ratios of 0.6 to 1.7, significantly lower than the values reported for MORB (4-6). In contrast, Nb/Ta for the samples overlap with MORB values, suggesting that Nb and Ta were not mobile in the magma sources. These systematic differences indicate that W does not behave as other HFSE (Nb-Ta, Zr-Hf) in subduction zones but rather displays a higher mobility in slab components. Intra-oceanic arc suites involving subducted pelagic sediment in their sources generally display higher W/Th compared to magmas without sediment-derived components in their sources, reflecting the higher initial W abundances in subducted pelagic sediments. A fractionation of W/Th during crust formation could consequently be explained by a selective W enrichment relative to Th during subduction processes.
V43D-1640
Island-arc magmatic processes beneath South Pagan Volcano, Northern Mariana Islands
The island-arc volcanoes that make up the Northern Mariana Islands are among the most historically active stratovolcanoes along the Pacific plate, yet they have been poorly studied due to their remote location and difficult accessibility. One of the least studied areas in the Northern Mariana Islands is Pagan Island, located near the center of the Mariana ridge. Pagan Island consists of two Holocene stratovolcanoes, Mount Pagan and South Pagan. Remarkably little is known about South Pagan including its eruptive history, potential volcanic hazards, and geochemical evolution due to a small population of inhabitants, a short and intermittent recorded history, and few geological studies. There is abundant evidence that eruption of South Pagan could pose significant hazards to both residents of the Northern Mariana Islands and to aircraft flying in the western Pacific. For example, following Mount Pagan's most recent explosive eruption (VEI = 4) in 1981, destructive rain-triggered volcanic debris flows buried large tracts of land, including the site of a village that contained a school, dispensary, church, and power generating buildings. Preliminary field studies in May 2006 by the USGS showed that a full spectrum of hazardous phenomena originated from South Pagan in the past, including pyroclastic flows and surges, caldera collapses, and volcanic debris flows. Two previously unrecognized active fumaroles near the summit of South Pagan were discovered suggesting that potential volcanic hazards currently exist in this area. A majority of the new lava samples are vesicular, clinopyroxene-plagioclase basalts with minor plagioclase xenocrysts and gabbroic xenoliths. The purpose of this study is to understand the compositional history of South Pagan and how it relates to the crustal and mantle magmatic processes beneath the central Northern Mariana Islands. Pb, Sr and Nd isotope ratios, major and trace element abundances, and mineral chemistry were determined and will be presented.
V43D-1641
Sr-Nd-Hf-Pb Isotopic Constraints on the Role of South China Sea Sediments in Mantle Wedge Metasomatism Beneath the North Luzon Arc
Recycling sediments into mantle through subduction zones causes mantle heterogeneity and is critical on chemical evolution of the Earth. Because the compositions of subducted sediments vary significantly between subduction zones, there is a need to characterize the sediments from individual subduction zones and evaluate their contributions to mantle wedge metasomatism. This study investigates the role of South China Sea (SCS) sediments in the chemical characteristics of the North Luzon arc (NLA) magmatism. Thirty-five sediment samples (0-35 Ma) recovered by ODP Leg 184 at sites 1148 and 1147 were analyzed for Sr, Nd, Hf, and Pb isotope ratios and trace element abundances. Results were compared to the data of the North Luzon arc lavas to establish models for mantle wedge metasomatism. The NLA lavas deviate from the terrestrial array to higher 176Hf/177Hf values at a given 143Nd/144Nd value, consistent with involving subducted sediments in source regions. Since Hf in the subducted slabs cannot be transported to mantle wedges by hydrous fluids, slab-derived siliceous melts are the most probable metasomatic agents. This algorithm leads to three metasomatism models: (I) addition of bulk sediments to depleted mantle, (II) depleted mantle metasomatized by sediment-derived melts, and (III) depleted mantle metasomatized by melts derived from sediments and altered oceanic crust (AOC). Although not considered in model calculations, the contributions of sediment-derived and AOC-derived fluids are also addressed qualitatively. The first model results in mixing curves overlapping with the mantle array in Sr versus Nd and Hf isotope plots, inconsistent with the distributions of the NLA lavas, which deviate from mantle array to lower 143Nd/144Nd and 176Hf/177Hf values. The involvement of sediment-derived melts (Model II) leads to source compositions with larger deviation from the NLA lavas in Sr versus Nd and Hf isotope plots, because sediment-derived melts have higher Sr/Nd and Sr/Hf ratios than their sources. Model III with variable amounts of residual zircons (0.0016-0.005%) provides the best fit to most NLA lavas. The sediment-derived and AOC-derived melts were generated by 15-20% and 2-4% melting, respectively, and these two metasomatic agents contributed sub-equally to sources of the NLA lavas. In all three models, the addition of AOC-derived fluids can improve the model fits to the NLA data. However, their contribution to trace element budgets in the metasomatized mantle should be far less significant compared to that of the slab-derived melts due to their low trace element abundances. These model calculations show that the SCS sediments can be the proper long-term enriched component in the sources of NLA lavas.
V43D-1642
Geochemistry and geodynamic setting of Paleogene volcanic rocks from south Ardestan, Iran
South Ardestan Paleogene volcanic rocks comprise a succession of, in order of abundances, rather silica-rich felsic rocks, andesites and rare basaltic andesites. Major and trace element geochemistry of andesites and basaltic andesites were studied in details (Yeganehfar, 2007). The former (e.g., covariation of FeO/MgO-SiO2 and Alkalis-FeO-MgO) indicated a calcalkaline affinity while, the letter (i.e., enrichment in LILE and depletion in HFSE) pointed to a subduction-related signature. The andesites and basaltic andesites are porphyritic in different degrees. Plagioclase, pyroxene, olivine remnants and Fe-Ti oxides are the most abundant phenocrysts in the basic rocks. Normalized trace element abundances and patterns of the andesites and basaltic andesites show similarities to both island arc volcanics and continental margin volcanics. South Ardestan andesites and basaltic andesites are likely produced during northeastward subduction of Neotethyan oceanic slab beneath central Iranian Plate in Paleogene (mainly Eocene). Based on quantitative trace element modeling, south Ardestan basic volcanic rocks are derived from 15 to 30 percent partial melting of a spinel lherzolitic mantle rock. The presence of a basaltic flow with tholeiitic affinity at the base of south Ardestan volcanic succession and the presence of two dykes of tholeiitic affinity that cut the upper part of succession, are noteworthy. The basal basaltic flow is consistent with the chronology of events in island arc, since early stage magmatism is tholeiitic in island arc. Tholeiitic dykes that correspond to the latter stage of magmatism in the study area, would be considered as indications of a change in tectonic regime. Assuming a horizontal component for subduction (oblique subduction) or dominance of a rather stable (relaxation) period is the most probable causes (alternatives). Based on petrological and geochemical investigation of rather basic volcanic rocks, south Ardestan Paleogene volcanic succession would be attributed to an island arc with indications of evolving to an active continental margin. Selected References: Yeganehfar, H., 2007. Geochemistry and petrology of the volcanic rocks from south Ardestan. M.Sc thesis. Tarbiat Modares University, Iran. 115 p.
V43D-1643
Melt Production History and Thermal Structure of Upwelling Mantle Constrained by the Kita- Matsuura Basalt, Southwestern Japan
Temporal change of the amount of melt produced in an upwelling mantle is the most important information relevant to the basalt genesis and the dynamics of melting in the upper mantle. Although this issue has been addressed by experimental and thermodynamic approaches, convincing petrologic constraints from natural samples are still quite few. If an upwelling mantle melts fractionally and quickly separates melt to the Earthfs surface without pondering and aggregation on its way up, basalt magma, particularly less differentiated alkaline basalt, is potentially a good tracer of the melting history. We made systematic geological, petrologic, and geochemical investigations on an intraplate Cenozoic alkaline basaltic volcanism (Kita-Matsuura basalt) in southwestern Japan to reveal the melting history in the upper mantle on the time scale of ~2Myr and the horizontal scale of 35km. The volcanism initiated from mildly alkaline basalt (low- SiO2 group) followed by sub-alkaline basalt (medium to high- SiO2 group) in the western and central sections, while the eastern section produced mildly alkaline basalt (low- SiO2 group) almost all the way up to the uppermost horizon. Each SiO2 group is clearly distinguished by a specific assemblage of fractionated crystals to produce the major element variation, which can be explained neither by crystal fractionation nor crustal assimilation to each other. Average segregation depths of estimated primary melts for each group were estimated by comparing to anhydrous melting experiments; they are 3.1-2.9, 2.8-2.6, and 2.0-1.8 GPa for low-, medium-, and high-SiO2 groups respectively. Major element variations such as K2O, TiO2, Na2O, and Al2O3/TiO2 of primary melts suggest increase of melting degree from low- to high-SiO2 group. A linear relationship between Al2O3/TiO2 and melting degree based on compilation of peridotite melting experiments enables us to estimate differences of melting degree between low- and medium-SiO2 groups and between medium- and high-SiO2 groups equally as ~6±3wt%. The chondrite-normalized REE patterns of primary melts show strong enrichment of LREE with almost linear and variable inclination from LREE to HREE. Trace element variations cannot be explained by batch, fractional, accumulate, or stepwise melting of any depleted MORB mantle or primitive mantle, requiring near fractional melting of an enriched mantle in the garnet to spinel stability field. Combination of melting pressure and relative degree of melting of the three lava groups shows that the melt production rate dF/dP is 1.7 ~ 2.4 %/kbar for low -SiO2 group and 0.7 %/kbar for high-SiO2 group, respectively. Furthermore, the temporal and spatial variations of melting parameters inferred from both the major and trace elements show that a diapirically upwelling mantle with arched isotherms melted progressively to generate this basaltic volcanism. From these results, we conclude that melt production rate decreased during upwelling of the mantle, which is contrasting to increase of production rate for MORB generation as mantle ascends (e.g., Asimow et al., 1997; Yang et al., 1998). The decrease of melt production rate of a diapirically upwelling mantle shown from natural sample for the first time suggests that melting process is different from an ideal steady state adiabatic melting.
V43D-1644
A study on the geophysical characteristics of the summit of the Dokdo volcano in the East Sea (Japan Sea)
Multi-beam, magnetic, sub-bottom profiling, and side scan sonar survey data for the eastern part of the summit area of the Dokdo volcano obtained in 1999, 2004, 2007 were analyzed to investigate the geophysical characteristics of the summit of the Dokdo. The Dokdo volcano is located in the northeastern part of the Ulleung Basin in the East Sea (Japan Sea) and composed of very small islets and a large submerged volcanic edifice. There are two voluminous seamounts (the Simheungtaek and the Isabu Tablemounts) at the east side of the Dokdo. They have submerged guyot summits, occurring at depths of about 200 meters. Bathymetry and topographic data around the Dokdo show uneven seabed and irregular undulations from costal line to 100 m in water depth, indicating the effects of partial erosions and taluses. The stepped slope in the topographic profile is supposed to be a coastal terrace suggesting repetition of transgressions and regressions in the Quaternary. The bathymetry and the side scan sonar data show a small crater, assumed to be formed by the eruption of later volcanism, at depth of 120 m in the northeastern part of the survey area. The sub-bottom profiles and the side scan sonar images propose that, except some areas with shallow sand sedimentary deposits, there are rocky seafloor and lack of sediments in the survey area, dominantly. The rocky seabottom elongated northeastward from the islets of the Dokdo might be the residual part of the eroded and collapsed crater of the Dokdo volcano. The results of the magnetic anomaly, the analytic signal, and the magnetization inversion have a good coherence with above other consequences regarding to the location of the residual crater. The geophysical results of the survey area suggest that the islets of the Dokdo and the rocky seabed elongated northeastward from the islets might be the part of the crater of the Dokdo volcano.
V43D-1645
Ages of Igneous Basement From the Komandorsky Islands, Far Western Aleutian Ridge
Argon-argon ages from four mafic/intermediate rocks from Bering and Medny islands in Komandorsky Islands point to a varied and protracted history for the evolution of the far western Aleutian Ridge, and to the north Pacific as a whole. The oldest sample is low-K tholeiitic basalt with an 40Ar/39Ar isochron age of 46.2 ± 1.5 Ma and some excess argon. It is not significantly altered and compositionally represents immature island arc basalt. This age is identical to the oldest age obtained from dredge samples from the central Aleutians (Jicha et al., 2006, Geology v. 34) and is the oldest reported 40Ar/39Ar age from exposed arc-related igneous rock for the entire Aleutian Ridge. A second sample is similar in composition but contains abundant Na-rich zeolite. It yields a slightly younger age of 42.1 ± 1.8 Ma, perhaps reflecting alteration-related argon loss. Our Eocene basement ages are significantly younger (~10 m.y.) than the paleontologically-based late Paleocene to earliest Eocene age for overlying sedimentary units, which had previously been regarded as representing the oldest ages from the Aleutian Ridge. The most securely dated igneous basement now places the early phase of arc building in the early Eocene at ~47-50 Ma, not Paleocene. The youngest two samples are hornblende- bearing high Mg# andesite ('adakite'), compositionally similar to, but more primitive than, previously-described rocks from the region (Yogodzinski et al.,1995, GSA Bull., v. 107). They have 'classic' adakite trace element characteristics, including high Sr/Y (50-160), low Y (<11 ppm), and high Ni (>80 ppm) and Cr (>100 ppm) and yield ages of 27.4 ± 0.2 and 20.7 ± 0.5 Ma. We interpret these to reflect a transtensional period of deformation in the Komandorsky Islands, which since at least the Oligocene, has resided along the right-lateral transform plate boundary separating the Pacific and North American plates. These Oligocene to Early Miocene ages are significantly older than previously reported Aleutian adakite ages of Middle to Late Miocene and are similar to the initial opening (by spreading) of the Bering Sea's Komandorsky Basin immediately north of the Komandorsky Islands. We consequently suggest that the adakite suite origin is related to formation of a major tear in the subducting Pacific plate beneath the Komandorsky sector.
V43D-1646
A Trace-Element and Radiogenic-Isotopic Pattern of Oceanic Arc Inception, Maturity, Demise, and Rejuvenation: Viti Levu, Fiji
Viti Levu, Fiji and \textquoteleftEua, Tonga have the oldest subaerial exposures of the remnant Vitiaz Arc which formed when subduction initiated (≥45Ma) as the result of a Pacific Plate motion change. We present new high-precision ICP-MS trace element and MC-ICP-MS isotope data for newly collected Fiji rocks and previously analyzed (XRF,TIMS) Viti Levu basalts and \textquoteleftEua gabbro that represent proto- to mature-arc, syn- rifting prior to opening of the South Fiji Basin (SFB), and post-SFB magmatic stages of early Vitiaz arc evolution. Earliest Fiji arc rocks make up the Yavuna formation in southwestern Viti Levu and are mostly basalt to basaltic andesite pillows, dikes, and flows. Most analyzed \textquoteleftEua and Yavuna rocks have chondritic Sm/Hf, but both have populations with negative Hf anomalies. \textquoteleftEua is LREE depleted whereas Yavuna ranges broadly from LREE-depleted to LREE-enriched (LaN/YbN from 0.2 to ~4). Both include boninitic lavas with HREE & HFSE concentrations only about twice those of primitive mantle. Both show similar ranges for 143Nd/144Nd, but Yavuna is slightly more radiogenic and less variable in 176Hf/177Hf. Both are "Pacific" in isotopic character. The demise of the early Yavuna arc (~20-30 Ma) resulted from a shift from arc volcanism to rifting and backarc spreading and, eventually, opening of the SFB. Volcanism associated with rifting is preserved as a bi- modal suite of dacitic (edifice) and basaltic (basin) rocks (Kalaka and Dakadaka formations, respectively) that crop out west and south of Yavuna and adjacent to SFB crust. Edifice rocks are flat to slightly LREE-depleted and basinal rocks are all LREE-enriched. Both are sub-chondritic to Hf-enriched with respect to REE. Post-SFB volcanism (Wainimala Group) is bi-modal basalt plus dacite and included at least one major plutonic event. These rocks comprise most of modern Viti Levu basement. Wainimala rocks have a range of Sm/Hf similar to Yavuna but are generally more LREE-depleted. This is consistent with mantle depletion during SFB formation. Higher 176Hf/177Hf ratios indicate a more depleted source mantle than existed prior to spreading. The early arc was characterized by extreme variations in HFSE, reflecting large variations in percent melting, extending to very high degree melting necessary to generate boninitic trace-element concentrations. Rift stages are indicated by synchronous arc- and basin-like volcanism, and more typical LREE-depleted arc tholeiites appear only after opening of the SFB with rejuvenation of arc volcanism.