Mariana Trough Back-arc Basin Research Articles

Diffuse spreading, a newly recognized mode of crustal accretion in the southern Mariana Trough backarc basin

Abstract South of the latitude of Guam, the Mariana Trough exhibits both trench-parallel and trench-normal extension. In this study, we examined the locus of trench-normal extension separating the Philippine Sea plate from the broadly deforming Mariana platelet. Along this boundary, we identified three distinct modes of extension and described their distinguishing characteristics using deep- and shallow-towed side-scan sonar and ship multibeam data along with regional geophysical, geochemical, and seismicity data. In the west, the Southwest Mariana Rift is an active tectonic rift exhibiting abundant strong earthquakes up to mb 6.7 and limited evidence of volcanism. In the east, the Malaguana-Gadao Ridge is a seafloor spreading center producing few and weak earthquakes less than mb 5. Between these zones, there is an ~20–40-km-wide and ~120-km-long area of high acoustic backscatter characterized by closely spaced volcano-tectonic ridges and small volcanic cones with distributed intermediate-strength seismicity up to mb 5.7. Fresh-looking volcanic rocks with high water contents and strong arc chemical affinities have been recovered from the high-backscatter zone. We interpret this morphologically and geophysically distinct zone as undergoing diffuse spreading, a distributed form of magmatic crustal accretion where new crust forms within a broad zone tens of kilometers across rather than along a narrow spreading axis. Diffuse spreading appears to be a rheological threshold effect enabled by slow opening rates and a high slab-fluid flux that facilitate the formation of a broad zone of weak hydrous lithosphere, within which new crust is accreted. Our findings describe a poorly understood process in plate tectonics, and observations of similar terrains in other backarc basins suggest that this process is not unique to the Mariana Trough.

Geochronology and geochemistry of early Carboniferous basalts from Baogutu Formation in West Junggar, Northwest China: evidence for a back-arc extension

ABSTRACT Early Carboniferous magmatism of the southern West Junggar (SWJ) region is essential for understanding the accretionary processes and crustal growth in the southwestern segment of the Central Asian Orogenic Belt. A comprehensive study of geochronological, geochemical, and Sr-Nd-Pb-Hf isotopic compositions was carried out in this study for basalts from the Baogutu Formation in Sulushuoke region, aiming to provide constraints on the Early Carboniferous tectonic evolution of the SWJ, NW China. LA-ICP-MS zircon U-Pb dating of four basalt samples yielded crystallization ages ranging from 339 Ma to 347 Ma, indicating the presence of early Carboniferous volcanic rocks in West Junggar. The studied tholeiitic basalts are geochemically characterized by nearly flat REE patterns, enriched large iron lithosphere elements (e.g. K) and depleted high field strength elements (e.g. Nb, Ta, Zr, Hf) relative to those in N-MORB, resembling those of modern Mariana Trough back-arc basin basalts. In addition, they have high positive εNd(t) values (+5.86 to +6.59) with initial 87Sr/86Sr ratios in the range of 0.704783–0.705315 and variable εHf(t) values (+3.99 to +15.6). The rocks cover restricted ranges in initial Pb isotope ratios with (206Pb/204Pb)i from 17.529 to 18.289, (207Pb/204Pb)i from 15.509 to 15.627, and (208Pb/204Pb)i from 37.628 to 38.397. These signatures indicate that the tholeiitic basalts were derived from 10% to 20% partial melting of a spinel-lherzolitic N-MORB-like depleted mantle source modified by fluids released from the subducted lithosphere in a back-arc extensional setting. From our new data presented in this study and in conjunction with those from previous works, we, therefore, suggest that a back-arc basin related to the northwestward subduction of the Junggar oceanic crust was developed in the Hatu-Baogutu region during the early Carboniferous time.

The magma plumbing system in the Mariana Trough back-arc basin at 18° N

Abstract Mafic magmas are common in back-arc basin, once stalled in the crust, these magmas may undergo different evolution. In this paper, compositional and textural variations of plagioclase as well as mineral-melt geothermobarometry are presented for basalts erupted from the central Mariana Trough (CMT). These data reveal crystallization conditions and we attempt a reconstruction of the magma plumbing system of the CMT. Plagioclase megacrysts, phenocrysts, microphenocrysts, microlites, olivine, spinel, and clinopyroxene have been recognized in basalt samples, using BSE images and compositional features. The last three minerals are homogeneous as microphenocrysts. Mineral-melt barometry indicates that plagioclase crystals crystallized and eventually grew into phenocrysts and megacrysts in mush zone with depth of 5–9 km, in which the normal zoning plagioclases crystallized in the interval of various batches of basic magma recharging. Plagioclase megacrysts and phenocrysts were dissolved and/or resorbed, when new basic magmas injected into the mush zone near Moho depth. It is inferred that magma extracted from the mush zone, and adiabatically ascended via different pathways. Some basaltic magmas underwent plagioclase and clinopyroxene microphenocrysts crystallization in low-pressure before eruption. Plagioclase microlites and outermost rims probably crystallized after eruption.

Chemical composition of hydrothermal fluids in the central and southern Mariana Trough backarc basin

We present a geochemical data set from hydrothermal fluids collected from the central and southern Mariana Trough during dive programs conducted in the 1990s. The fluid samples were collected from two hydrothermal fields: (1) the Alice Springs field (18°13′N, 144°42′E, water depth: 3600m) on the axial ridge crest of the central Marana Trough; and (2) the Forecast field (13°24′N, 143°55′E, water depth: 1470m) on Peak B seamount, one of a cluster of small volcanic seamounts which represents a unique tectonic setting in the southern Mariana Trough. Gas geochemistry of the Alice Springs fluid showed He and C isotope ratios in the range of MORB, whereas that of the Forecast fluid showed a higher CO2/3He ratio and 13C-enriched isotope ratio, both of which are commonly recognized in hydrothermal fluids from intraoceanic arc volcanoes. The major and minor element composition of the Alice Springs fluid is characterized by enrichment in mobile alkali elements such as K, Rb and Cs. This geochemical signature is in accordance with the well-known characteristics of basaltic lava collected from the same area, which show substantial enrichment in incompatible elements. As interpreted in previous petrological studies, this signature is considered to be an arc geochemical signal reflecting contribution from the subduction component. The B concentration of the Alice Springs fluid is also high and is another arc geochemical signal. The Forecast fluid, by contrast, did not show enrichment in the mobile alkali elements, although its B concentration is high. The Forecast fluid is characterized instead by enrichment in Ca and Sr coupled with depletion in Na and Li. As discussed in previous studies, this signature is attributed to hydrothermal reactions at rather low temperature due to the shallow water depth. We recognize two different arc geochemical signals in these two hydrothermal fluids: mobile alkali elements in the Alice Spring fluid and gaseous species in the Forecast fluid. This difference could reflect different behaviors of these species during magmatic processes in the mantle wedge beneath the Mariana Trough backarc spreading center.

Basaltic volcaniclastics from the Challenger Deep forearc segment, Mariana convergent margin: Implications for tectonics and magmatism of the southernmost Izu–Bonin–Mariana arc

AbstractConvergent margin igneous activity is generally limited to 100–200 km from the trench except where spreading ridges are subducted or in association with Subduction‐Transform Edge Propagators (STEP faults). The southernmost Mariana forearc, facing the Challenger Deep, subducts Mesozoic seafloor and is not in a STEP fault setting but includes at least one site where tholeiitic basalts recently erupted close to the trench, the SE Mariana Forearc Rift (SEMFR). We present evidence of young basaltic volcanism from ca. 100 km west of SEMFR. Shinkai 6500 diving during YK13‐08 (Dive 1363) recovered volcaniclastics from 5.5 to 6 km deep in the inner wall of the Mariana Trench, 50 km NE of the Challenger Deep. Glassy fragments are tholeiitic basalts similar to MORB except for much higher contents of magmatic water (approx. 2% H2O vs. <0.2% H2O in MORB) and enrichments in trace elements Rb‐Cs‐Ba, K, Pb, and Sr. Dive 1363 glasses are similar to basalts from SEMFR erupted near the trench and to Mariana Trough backarc basin basalts. Basalt fragments and palagonitized matrix dominate the studied samples, but small xenocrysts and xenoliths derived from mantle peridotite and Neogene volcanics are also present, probably torn from the vent walls. Dive 1363 hyaloclastites erupted at 3–6 km water depth accompanied by vigorous degassing of volatiles, most likely CO2. These results provide further evidence that the forearc adjacent to the Challenger Deep has been invaded by asthenospheric mantle and derivative hydrous melts. Extension, hydration, and melt invasion combine to further weaken Challenger Deep forearc lithosphere. Combined effects of: (i) absence of strong, cold lithosphere of the overriding plate; (ii) rapid rollback of a narrow, short subducted slab; and (iii) weak coupling between the subducting Pacific plate and the overriding Mariana plate may be responsible for the great depth of the Challenger Deep.

Geodynamic evolution of a forearc rift in the southernmost Mariana Arc

AbstractThe southernmost Mariana forearc stretched to accommodate opening of the Mariana Trough backarc basin in late Neogene time, erupting basalts at 3.7–2.7 Ma that are now exposed in the Southeast Mariana Forearc Rift (SEMFR). Today, SEMFR is a broad zone of extension that formed on hydrated, forearc lithosphere and overlies the shallow subducting slab (slab depth ≤ 30–50 km). It comprises NW–SE trending subparallel deeps, 3–16 km wide, that can be traced ≥ ∼30 km from the trench almost to the backarc spreading center, the Malaguana‐Gadao Ridge (MGR). While forearcs are usually underlain by serpentinized harzburgites too cold to melt, SEMFR crust is mostly composed of Pliocene, low‐K basaltic to basaltic andesite lavas that are compositionally similar to arc lavas and backarc basin (BAB) lavas, and thus defines a forearc region that recently witnessed abundant igneous activity in the form of seafloor spreading. SEMFR igneous rocks have low Na8, Ti8, and Fe8, consistent with extensive melting, at ∼23 ± 6.6 km depth and 1239 ± 40°C, by adiabatic decompression of depleted asthenospheric mantle metasomatized by slab‐derived fluids. Stretching of pre‐existing forearc lithosphere allowed BAB‐like mantle to flow along the SEMFR and melt, forming new oceanic crust. Melts interacted with pre‐existing forearc lithosphere during ascent. The SEMFR is no longer magmatically active and post‐magmatic tectonic activity dominates the rift.

Volcanoes of the Diamante cross-chain: evidence for a mid-crustal felsic magma body beneath the Southern Izu–Bonin–Mariana arc

AbstractThree submarine Diamante cross-chain volcanoes in the southern Mariana arc mark a magma-healed zone of along-arc (north–south) extension that allows either mafic mantle-derived basalts or felsic magmas from the middle of thickened arc crust to erupt. The largest volcano is East Diamante, with a well-developed (5×10 km) caldera that formed via violent felsic submarine eruptions beginning nearly 0.5 Ma. One or more of these eruptions also formed a giant submarine dune field extending 30 km to the NW of the volcano. Felsic igneous activity continues at least as recently as c. 20 000 years ago, with emplacement of resurgent dacite domes, some hot enough to power the only black smoker hydrothermal system known in the Mariana arc. In contrast, felsic eruptions do not occur on the two volcanoes to the west, implying that the mid-crustal felsic zone does not underlie the thinner crust of the Mariana Trough back-arc basin. Diamante cross-chain lavas define a medium K suite; mafic lava phenocryst assemblages show arc-like associations of anorthite-rich plagioclase with Fe-rich olivine. Magmatic temperatures for a basaltic andesite and three dacites are c. 1100 °C and c. 800 °C, respectively, typical for cool, wet, subduction-related felsic magmas. Felsic magmas formed under low-P crustal conditions. The Diamante cross-chain is the southernmost of at least seven and perhaps eight Mariana arc volcanoes in a c. 115 km long arc segment characterized by felsic eruptions. This is the ‘Anatahan Felsic Province’, which may have formed above a mid-crustal tonalite body that formed by fractionation or was re-melted when heated by c. 1200 °C mafic, mantle-derived magmas. Across- and along-arc variations suggest that felsic eruptions and dome emplacement occurred when midcrustal tonalite was remobilized by intrusions of mafic magma, while north–south extension facilitated the development of conduits to the surface.Supplementary material:Detailed Hyperdolphin ROV dive tracks, Cook 7 dredge locations, 40Ar/39Ar analytical data, analytical methods, major and selected trace element analyses of whole rock samples, and compositional data for minerals are available at http://www.geolsoc.org.uk/SUP18611

Hf-Nd isotope variation in Mariana Trough basalts: The importance of “ambient mantle” in the interpretation of subduction zone magmas

In the study of geochemical mass balances at subduction zones, the composition of the mantle wedge prior to additions from the slab is a critically important yet poorly constrained parameter. Deconvolving the influence of ancient versus modern enrichments is particularly difficult, especially when considering elements that are highly mobile. Here we provide an alternative approach, using less mobile elements, and a filter to remove the effects of recent slab additions. We provide new Hf isotope data for 30 Mariana Trough (MT) backarc basin lavas. Once filtered, Hf and Nd isotope ratios are highly correlated, of Indian mid-oceanic ridge basalt character, and display variations similar to ocean ridges of comparable lengths. The isotopic variability observed in this “ambient mantle” provides a new paradigm for the interpretation of the varied volcanic products of the arc. Thus, shoshonites associated with the northern termination of the backarc basin rift axis reflect the interaction of a subducted sediment melt with an isotopically enriched mantle source. In contrast, the large volcanoes of the Central Island province have a consistent offset in Nd isotope compositions from the MT array resulting from fluid addition. Existing data for smaller edifices in the submarine portion of the arc have larger variations resulting from fluid addition on a more local scale. We suggest that the similar characterization of ambient mantle elsewhere may help to resolve many conflicting geochemical observations in arc lavas worldwide.

Mantle Melting as a Function of Water Content beneath the Mariana Arc

Subduction zone magmas are characterized by high concentrations of pre-eruptive H_2O, presumably as a result of an H_2Oflux originating from the dehydrating, subducting slab. The extent of mantle melting increases as a function of increasing water content beneath back-arc basins and is predicted to increase in a similar manner beneath arc volcanoes. Here, we present new data for olivine-hosted, basaltic melt inclusions from the Mariana arc that reveal pre-eruptive H_2O contents of ~1•5-6•0 wt %, which are up to three times higher than concentrations reported for the Mariana Trough back-arc basin. Major element systematics of arc and back-arc basin basalts indicate that the back-arc basin melting regime does not simply mix with wet, arc-derived melts to produce the observed range of back-arc magmatic H_2O concentrations. Simple melting models reveal that the trend of increasing extents of melting with increasing H_2O concentrations of the mantle source identified in the Mariana Trough generally extends beneath the Mariana volcanic front to higher mantle water contents and higher extents of melting. In detail, however, each Mariana volcano may define a distinct relationship between extent of melting and the H_2O content of the mantle source. We develop a revised parameterization of hydrous melting, incorporating terms for variable pressure and mantle fertility, to describe the distinct relationships shown by each arc volcano. This model is used in combination with thermobarometry constraints to show that hydrous melts equilibrate at greater depths (34-87 km) and temperatures (>1300°C) beneath the Mariana arc than beneath the back-arc basin (21-37 km), although both magma types can form from a mantle of similar potential temperature (~1350°C).The difference lies in where the melts form and equilibrate. Arc melts are dominated by those that equilibrate within the hot core of the mantle wedge, whereas back-arc melts are dominated by those that equilibrate within the shallow zone of decompression melting beneath the spreading center. Despite higher absolute melting temperatures (>1300°C), Mariana arc melts reflect lower melt productivity as a result of wet melting conditions and a more refractory mantle source.

Migrating shoshonitic magmatism tracks Izu–Bonin–Mariana intra-oceanic arc rift propagation

The southernmost Izu–Bonin arc and northernmost Mariana arc are characterized by K-rich and shoshonitic lavas, referred to as the alkalic volcano province (AVP). These compositions are unusual for intra-oceanic arcs and the interpretation of the AVP is controversial. Rifting to form the Mariana Trough back-arc basin occurs just south of the AVP although back-arc seafloor spreading has not begun. Here we report the results of dredge sampling of the West Mariana Ridge (WMR) in the region of rift propagation; this recovered exclusively medium K to shoshonitic basalts that show clear arc-like geochemical signatures. Ar–Ar ages of WMR shoshonitics systematically young northward. Age of c. 6 Ma was obtained at 21.5°N, c. 3 Ma at 23–23.5°N, and zero-age shoshonites occur on Io-to Island (formerly Iwo Jima) at 24.8°N. Shoshonitic magmatism migrated northward at 4.3 cm/year, in advance of northward-propagating Mariana Trough rifting. This implies that AVP shoshonitic magmatism manifests processes and sources that are uniquely associated with earliest back-arc basin rifting. High-precision Pb isotopic analyses reveal that WMR lavas form a single trend between 2 components, one with lower 206Pb/ 204Pb and high Δ7/4 (arc-like), and another with high 206Pb/ 204Pb as well as low Δ 7/4 and 8/4 (HIMU-like). These components could correspond respectively to subducted pelagic sediment and subducted seamounts and volcaniclastics with HIMU isotopic signature. These slab-derived components alone, however, cannot fully explain chemical characteristics of WMR shoshonitic lavas. These lavas require a component with high Δ7/4 and high Ce/Pb, which is not likely to be either pelagic sediment or seamount volcanics. This component is only expressed when rifting begins, suggesting that it resides in enriched lithosphere or uppermost asthenosphere, which is easily melted due to decompression caused by rifting, when the lithosphere is first ruptured. This component might be linked to slow Vs anomalies in the mantle wedge beneath the AVP.

A seismic stratigraphic analysis of Mariana forearc basin evolution

New seismic data collected in the 14.5°–18.5°N Mariana segment of the Izu‐Bonin‐Mariana island arc system image six seismic stratigraphic sequences that can be mapped throughout the inner forearc. These sediments were most likely deposited from 35 Ma to the present. The oldest stratigraphic Units 1, 2, and 3 are syn‐rift volcaniclastic deposits. Unit 4 deposits accumulated during a period of mild structural inversion, which resulted in several isolated reverse‐faulted anticlines within the forearc sedimentary basin. A late period of extensional deformation began near the end of Unit 5 deposition and continued through Unit 6 sedimentation to the present. Seismic lines show that the basement of the forearc is composed of large rotated fault blocks and half grabens with NE, NW, and NNE trends. Fault offset calculations show that basement faults with dips between 45° and 50° account for only ∼4% total extension in the forearc. South of 16.3°N, normal growth faults initiated during basement extension offset the frontal arc high from a deep forearc basin. From correlations with the known geologic history, we hypothesize that extension during deposition of Units 1 through 3 corresponds to rifting of the Eo‐Oligocene Arc, between ∼35 Ma and 29 Ma, older deposits being too thin to be seismically resolvable. Localized compression during Unit 4 accumulation occurred some time after Eo‐Oligocene rifting in the early Miocene. Late‐stage normal faulting near the end of deposition of Unit 5 and throughout Unit 6 accumulation may be associated with the opening of the Mariana Trough backarc basin from ∼8 Ma years to the present. There is a higher density of these later faults in the inner forearc between 15.5° and 17°N and in the outer forearc between 14°N and 18°N. Recent extension is at least partially accommodated by reactivation of older basement faults with the same NE, NW, and NNE‐ trends. Stratigraphic relationships indicate that the inner forearc south of 16.3°N has differentially subsided and tilted trenchward, possibly as a result of a recent change in subducting slab geometry or subducted relief under the forearc.

Subduction factory processes beneath the Guguan cross-chain, Mariana Arc: no role for sediments, are serpentinites important?

We need to understand chemical recycling at convergent margins and how chemical interactions between subducted slab and the overlying mantle wedge affect mantle evolution and magmagenesis. This requires distinguishing contributions from recycled individual subducted components as well as those contributed by the mantle. We do this by examining magmatic products generated at different depths above a subduction zone, in an intra-oceanic arc setting. The Guguan cross-chain in the intra-oceanic Mariana arc overlies subducted Jurassic Pacific plate lithosphere at depths of ~125--230 km and erupts mostly basalt. Basalts from rear-arc volcanoes are more primitive than those from the magmatic front, in spite of being derived by lower degrees of melting of less-depleted mantle. Rear-arc magmas also show higher temperatures and pressures of equilibration. Coexisting mineral compositions become more MORB- or OIB-like with increasing height above the subduction zone. Trace element and isotopic variations indicate that the subduction component in cross-chain lavas diminishes with increasing depth to the subduction zone, except for water contents. There is little support for the idea that melting beneath the Mariana Trough back-arc basin depleted the source region of arc magmas, but melting to form rear-arc volcanoes may have depleted the source of magmatic front volcanoes. Enrichments in rear-arc lavas were not caused by sediment melting; the data instead favor an OIB-type mantle that has been modestly affected by subduction zone fluids. Our most important conclusion is that sediment fluids or melts are not responsible for the K--h relationship and other cross-chain chemical and isotopic variations. We speculate that an increasing role for supercritical fluids released from serpentinites interacting with modestly enriched mantle might be responsible for cross-chain geochemical and isotopic variations.

Geochemistry and petrogenesis of volcanic rocks of the Neoarchaean Sukumaland Greenstone Belt, northwestern Tanzania

The late Archaean volcanic rocks of the Rwamagaza area in the Sukumaland Greenstone Belt consists of basalts and basaltic andesites associated with volumetrically minor rhyodacites and rhyolites. Most basalts and basaltic andesites yield nearly flat patterns (La/Sm CN = 0.89–1.34) indicating derivation by partial melting of the mantle at relatively low pressure outside the garnet stability field. On primitive mantle normalized trace element diagrams, the basalts and basaltic andesites can be subdivided into two groups. The first group is characterised by moderately negative Nb anomalies (Nb/La pm = 0.51–0.73, mean = 0.61 ± 0.08) with slight enrichment of LREE relative to both Th and HREE. The second group is characterised by nearly flat patterns with no Nb anomalies (Nb/La pm = 0.77 ± 0.39). The observed Nb and Th anomalies in the Rwamagaza basalts and basaltic andesites, cannot be explained by alteration, crustal contamination or melt–solid equilibria. Rather, the anomalies are interpreted, on the basis of Nb–Th–La–Ce systematics, as having formed by partial melting of a heterogeneous mantle consisting of variable mixtures of components derived from two distinct sources. These sources are depleted mantle similar to that generating modern MORB and an LREE-enriched and HFSE-depleted source similar to that feeding volcanism along modern convergent margins. The rhyolites are characterised by high Na 2O/K 2O ratios (>1) and Al 2O 3 (>15 wt.%), low HREE contents (Yb = 0.24–0.68 ppm) leading to highly fractionated REE patterns (La/Yb CN = 18.4–54.7) and large negative Nb anomalies (Nb/La pm = 0.11–0.20), characteristics that are typical of Cenozoic adakites and Archaean TTG which form by partial melting of the hydrated basaltic crust at pressures high enough to stabilize garnet ± amphibole. The Rwamagaza basalts and basaltic andesites are geochemically analogous to the Phanerozoic Mariana Trough Back Arc Basin Basalts and the overall geochemical diversity of Rwamagaza volcanic rocks is interpreted in terms of a geodynamic model involving the interaction of a depleted mantle, a melting subducting oceanic slab in a back arc setting.

Peridotites from the Mariana Trough: first look at the mantle beneath an active back-arc basin

Two dives of the DSV Shinkai 6500 in the Mariana Trough back-arc basin in the western Pacific sampled back-arc basin mantle exposures. Reports of peridotite exposures in back-arc basin setting are very limited and the lack of samples has hindered our understanding of this important aspect of lithospheric evolution. The Mariana Trough is a slow-spreading ridge, and ultramafic exposures with associated gabbro dykes or sills are located within a segment boundary. Petrological data suggest that the Mariana Trough peridotites are moderately depleted residues after partial melting of the upper mantle. Although some peridotite samples are affected by small-scale metasomatism, there is no evidence of pervasive post-melting metasomatism or melt-mantle interaction. Spinel compositions plot in the field for abyssal peridotites. Clinopyroxenes show depletions in Ti, Zr, and REE that are intermediate between those documented for peridotites from the Vulcan and Bouvet fracture zones (the American- Antarctic and Southwest Indian ridges, respectively). The open-system melting model indicates that the Ma- riana Trough peridotite compositions roughly corre- spond to theoretical residual compositions after 7% near-fractional melting of a depleted MORB-type upper mantle with only little melt or fluid/mantle interactions. The low degree of melting is consistent with alow magma budget, resulting in ultramafic exposure. We infer that the mantle flow beneath the Mariana Trough Central Graben is episodic, resulting in varying magma supply rate at spreading segments.

Evidence for an 18O-depleted mantle plume from contrasting 18O/ 16O ratios of back-arc lavas from the Manus Basin and Mariana Trough

Back-arc basin glasses from the Mariana Trough and Manus Basin display contrasting oxygen isotope characteristics that require differences in their mantle sources. In both basins, the lavas that are most depleted in high field strength elements possess δ 18O values of around 6.0‰, that are elevated with respect to mid-ocean ridge basalt (MORB). This characteristic is consistent with a mantle source that has been infiltrated by fluids released from subducted oceanic lithosphere. The nature of the more fertile mantle component differs between the two basins. The lowest δ 18O values in the Mariana Trough are similar to MORB and suggest that the ambient upper mantle interacts with a subduction-modified mantle to produce Mariana Trough back-arc basin basalts. Oxygen isotope ratios of basaltic glasses from the Manus Basin display a negative correlation with helium isotope ratios. The subduction-modified component is associated with 3He/ 4He ratios typical of the upper mantle. Glasses with 3He/ 4He ratios greater than average MORB, characteristic of a deep mantle plume, have δ 18O values that are lower than expected for upper mantle melts. This suggests that the Manus Basin plume taps a reservoir that is 18O-depleted relative to the source of MORB. Two mechanisms are identified that might generate this reservoir. Deep recycling of oceanic crust that has been hydrothermally altered at high temperature may provide large 18O-depleted domains in the deep mantle. Alternatively, we propose that interactions between silicate and iron alloy during the segregation of the Earth’s core may have the potential to generate such reservoirs. Resolution between these mechanisms requires further experimental investigation of oxygen partitioning between silicates and iron alloy. Each of these mechanisms has distinct implications for the origins and dynamics of the Manus Basin plume.

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Introduction to the special volume on the Izu-Bonin - Mariana arc system

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gases in pillow basalt glasses from the northern Mariana Trough back-arc basin

Noble gas concentrations and isotopic compositions have been measured in eight samples of pillow basalt glasses collected from seven different localities along 250 km of the Mariana Trough spreading and rifting axis. The samples have uniform and mid-ocean ridge basalt (MORB)-like 3He/4He values of 9–12 × 10–6 (6.4–8.6 times atmospheric) despite large variations in 4He. Concentrations of the noble gases Ne, Ar, Kr, and Xe show much smaller variations between samples, but larger variations in isotopic compositions of Ne, Ar, and Xe. Excess radiogenic 21Ne is observed in some samples. 40Ar/36Ar varies widely (atmospheric to 1880). Kr is atmospheric in composition for all samples. Some samples show a clear excess 129Xe, which is a well-known MORB signature. Isotopic compositions of the heavier noble gases (Ar, Kr, and Xe) in some samples, however, show more atmospheric components. These data reflect the interaction of a MORB-like magma with an atmospheric component such as seawater or of a depleted mantle source with a water-rich component that was probably derived from the subducting slab.

MORB mantle and subduction components interact to generate basalts in the southern Mariana Trough back-arc basin

We report the results of the first geochemical and isotopic survey of basaltic glasses dredged along the spreading ridge of the southern Mariana Trough (SMT; 15–17 °N). This ridge is divided into two segments that have different axial depths, major and trace element compositions, water contents, and isotopic compositions of Sr, Nd, and Pb. Glasses from the shallower, northern segment (N-SMT; 16–17 °N) are OL- and QZ-tholeiites that have compositions consistent with a higher degree of mantle melting relative to that of the OL tholeiites from the southern ridge segment (S-SMT; 15–16 °N). The N-SMT glasses are similar to basalts erupted near 18 °N in the Mariana Trough that have been the focus of previous studies. The more extensive melting inferred for the N-SMT correlates well with higher abundances of water and relative abundances of large ion lithophile and light Rare Earth elements that indicate involvement of a subduction component. The southern ridge segment is deeper and erupts compositions characteristic of lower degrees of melting; this correlates well with a lower proportion of the subduction component, including a suite that is indistinguishable from MORB. The strong correlation between degree of melting, water contents, and LIL elements indicates that hydrous fluxing as well as adiabatic decompression control melting of MORB-like mantle beneath back-arc basins. Details regarding the nature of this hydrous fluxing agent are not known, but it could be water-rich melts related to behind-the-arc volcanoes. These melts may be diverted by the back-arc convective regime, to become entrained in the zone of adiabatic upwelling, where they further stimulate melting.

Beryllium isotope systematics of submarine hydrothermal systems

Both 10Be and 9Be concentrations have been measured in a hydrothermal fluid from the sediment-starved East Pacific Rise at 21°N as well as in fluids and associated sediments from the sediment covered Guaymas Basin and Escanaba Trough hydrothermal systems. The results show that while the 9Be concentrations in fluids from both types of systems are comparable, about three orders of magnitude higher than that of the seawater supplying the systems, their 10Be concentrations are significantly different. The 10Be concentration in the solution emanating from the studied sediment-starved hydrothermal system is at least two times lower than that of the Pacific bottom water. By contrast, the 10Be concentration in hydrothermal solutions from sediment-hosted environments are enriched over ambient seawater and are at least one order of magnitude higher than that measured in the 21°N fluid. The sedimentary cover being the only available source of 10Be in these systems, these data demonstrate that 10Be is an unambiguous tracer of interactions between hydrothermal fluids and recent sediments. As an example, this study examines hydrothermal solutions from the sediment-starved Mariana Trough back-arc basin which have compositions similar in many respects to those from sediment-hosted environments. However, these fluids are depleted in 10Be relative to ambient seawater, which precludes interaction with recent sediment as the cause of their peculiar chemical composition.