From basalts to boninites: The geodynamics of volcanic expression during induced subduction initiation (original) (raw)
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Fore-arc basalts and subduction initiation in the Izu-Bonin-Mariana system
Geochemistry, Geophysics, Geosystems, 2010
1] Recent diving with the JAMSTEC Shinkai 6500 manned submersible in the Mariana fore arc southeast of Guam has discovered that MORB-like tholeiitic basalts crop out over large areas. These "fore-arc basalts" (FAB) underlie boninites and overlie diabasic and gabbroic rocks. Potential origins include eruption at a spreading center before subduction began or eruption during near-trench spreading after subduction began. FAB trace element patterns are similar to those of MORB and most Izu-Bonin-Mariana (IBM) back-arc lavas. However, Ti/V and Yb/V ratios are lower in FAB reflecting a stronger prior depletion of their mantle source compared to the source of basalts from mid-ocean ridges and back-arc basins. Some FAB also have higher concentrations of fluid-soluble elements than do spreading center lavas. Thus, the most likely origin of FAB is that they were the first lavas to erupt when the Pacific Plate began sinking beneath the Philippine Plate at about 51 Ma. The magmas were generated by mantle decompression during near-trench spreading with little or no mass transfer from the subducting plate. Boninites were generated later when the residual, highly depleted mantle melted at shallow levels after fluxing by a water-rich fluid derived from the sinking Pacific Plate. This magmatic stratigraphy of FAB overlain by transitional lavas and boninites is similar to that found in many ophiolites, suggesting that ophiolitic assemblages might commonly originate from near-trench volcanism caused by subduction initiation. Indeed, the widely dispersed Jurassic and Cretaceous Tethyan ophiolites could represent two such significant subduction initiation events.
The arc arises: The links between volcanic output, arc evolution and melt composition
Subduction initiation is a key process for global plate tectonics. Individual lithologies developed during subduction initiation and arc inception have been identified in the trench wall of the Izu–Bonin–Mariana (IBM) island arc but a continuous record of this process has not previously been described. Here, we present results from International Ocean Discovery Program Expedition 351 that drilled a single site west of the Kyushu–Palau Ridge (KPR), a chain of extinct stratovolcanoes that represents the proto-IBM island arc, active for ∼25 Ma following subduction initiation. Site U1438 recovered 150 m of oceanic igneous basement and ∼1450 m of overlying sediments. The lower 1300 m of these sediments comprise volcaniclastic gravity-flow deposits shed from the evolving KPR arc front. We separated fresh magmatic minerals from Site U1438 sediments, and analyzed 304 glass (formerly melt) inclusions, hosted by clinopyroxene and plagioclase. Compositions of glass inclusions preserve a temporal magmatic record of the juvenile island arc, complementary to the predominant mid-Miocene to recent activity determined from tephra layers recovered by drilling in the IBM forearc. The glass inclusions record the progressive transition of melt compositions dominated by an early 'calc-alkalic', high-Mg andesitic stage to a younger tholeiitic stage over a time period of 11 Ma. High-precision trace element analytical data record a simultaneously increasing influence of a deep subduction component (e.g., increase in Th vs. Nb, light rare earth element enrichment) and a more fertile mantle source (reflected in increased high field strength element abundances). This compositional change is accompanied by increased deposition rates of volcaniclastic sediments reflecting magmatic output and maturity of the arc. We conclude the 'calc-alkalic' stage of arc evolution may endure as long as mantle wedge sources are not mostly advected away from the zones of arc magma generation, or the rate of wedge replenishment by corner flow does not overwhelm the rate of magma extraction.
Geochemistry, Geophysics, Geosystems, 2012
1] A wide variety of different rock types were dredged from the Tonga fore arc and trench between 8000 and 3000 m water depths by the 1996 Boomerang voyage. 40 Ar-39 Ar whole rock and U-Pb zircon dating suggest that these fore arc rocks were erupted episodically from the Cretaceous to the Pliocene (102 to 2 Ma). The geochemistry suggests that MOR-type basalts and dolerites were erupted in the Cretaceous, that island arc tholeiites were erupted in the Eocene and that back arc basin and island arc tholeiite and boninite were erupted episodically after this time. The ages generally become younger northward suggesting that fore arc crust was created in the south at around 48-52 Ma and was extended northward between 35 and 28 Ma, between 9 and 15 Ma and continuing to the present-day. The episodic formation of the fore arc crust suggested by this data is very different to existing models for fore arc formation based on the Bonin-Marianas arc. The Bonin-Marianas based models postulate that the basaltic fore arc rocks were created between 52 and 49 Ma at the beginning of subduction above a rapidly foundering west-dipping slab. Instead a model where the 52 Ma basalts that are presently in a fore arc position were created in the arc-back arc transition behind the 57-35 Ma Loyalty-Three Kings arc and placed into a fore arc setting after arc reversal following the start of collision with New Caledonia is proposed for the oldest rocks in Tonga. This is followed by growth of the fore arc northward with continued eruption of back arc and boninitic magmas after that time.
Earth and Planetary Science Letters, 2003
We perform thermo-mechanical laboratory experiments designed to explore the behaviour of the volcanic arc during intra-oceanic arc^continent collision following oceanic subduction and subsequent back-arc opening. The overriding oceanic lithosphere is made of two layers representing the oceanic crust and the lithospheric mantle. This lithosphere carries a volcanic arc and is thinned and weakened beneath both the arc and the back-arc basin. The subducting plate contains three parts: one-layer oceanic and two-layer (crust and mantle) continental lithosphere with a continental margin between them. When the continental margin reaches the trench and starts subducting, the overriding plate undergoes growing horizontal compression and finally fails in the vicinity of the back-arc spreading centre, which is the weakest part of this plate. The failure can result in subduction of the whole arc plate comprised between the trench and the back-arc spreading centre. During subduction of the arc plate, the mantle part of this plate subducts completely, while the behaviour of the arc crust depends on its thickness and strength, which is a function of composition and temperature. We tested four cases with different arc crust thicknesses and composition (rheology), with total lithosphere thickness in the arc being constant. Three types of tectonic evolution have been obtained: complete arc subduction, complete arc accretion, and partial arc subduction/accretion. The result is largely controlled by the crustal thickness of the arc. A thin arc (thickness equivalent to V16 km in nature) made of the same strong material as the oceanic crust subducts completely without leaving any trace at the surface. On the contrary, a thick arc (equivalent to V26 km in nature) made of the same material is scraped off and accreted to the overriding plate. The lower crust of such an arc is hotter, therefore its strength at 'Moho' depth and coupling between crust and mantle are small. In addition, the thick arc has a high isostatic relief and hence a greater mechanical resistance to subduction. Therefore, the arc is scraped off. If the arc is made of a weaker 'continental-like' material or contains a weak layer/ low friction interface, it is completely or partially scraped off even if it is small. When there is no back-arc opening before collision (no thin and weak lithosphere in the rear of the arc), the overriding plate fails in the arc area, which may result in a complete fore-arc block subduction, with the volcanic arc remaining at the surface. The obtained models are compared with mountain belts with nearly no trace of arc activity (Oman), with accreted arc (Kohistan), and with small remnants of subducted arc (southern Tibet). ß
Magmatic processes under arcs and formation of the volcanic front
Journal of Geophysical Research: Solid Earth, 1993
Temperature structure and stress under arcs are simulated in a two-dimensional cross section taking into account the flow induced by the subducting slab in the mantle wedge. Results of the calculations show three important features with respect to magmatic processes under arcs: (1) Temperature structure in the crust and the mantle wedge under arcs is insensitive to the angle and velocity of slab subduction, the temperature structure of the slab, and that of the back-arc region. This indicates that physical conditions such as temperature and pressure are similar under various arcs. It is thus inferred that primary magmas generated under various arcs should have similar chemical compositions, if chemical composition and the flux rate of fluid from the slab are similar and the chemical compositions of mantle wedge materials are the same. (2) Calculated deviatoric stress magnitude is relatively large (more than a few tens of megapascals) in the partially molten mantle. Cracks may open under high differential stress, and magma can easily segregate and accumulate through interconnected cracks while the buoyancy driven compaction of partially molten mantle proceeds. (3) The deviatoric stress values in the region over the partially molten mantle are relatively large, and the direction of the principal stress changes horizontally; the direction of the maximum compressional stress is nearly vertical under the volcanic zone and is nearly horizontal under the fore-arc region. It is considered that magma segregated in partially molten mantle migrates upward through the brittle mantle and crust by the magrna fracturing mechanism. The propagation direction of magma-filled fissures is controlled by the stress field in the crust and mantle and is parallel to the maximum principal stress. The calculated stress is highly compressive horizontally on the trench side, while it becomes tensile on the back-arc side. The location of this stress transition coincides with that of the volcanic front. The location of this transition indicates that the volcanic front marks a change in the ease of upward migration of the magma-filled cracks under relatively high differential stress field. Introduction Island arcs are regions of active volcanism and seismicity. One of the most important features of arc volcanism is the formation of the volcanic front, the trenchward boundary of Copyright 1993 by the American Geophysical Union. Paper number 93JB00350. 0148-0227/93/93JB-00350505.00 the volcanic arc. It is well-known that the depth of seismic slab under the volcanic front is nearly constant at about 100 km [Gill, 1981; Tatsumi, 1986]. This is an important John Wiley, New York, 1982. Weertman, J., Theory of water-filled crevasses in glaciers applied to vertical magma transport beneath oceanic ridges,
Geochemistry, Geophysics, Geosystems, 2018
The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore-arc basalt (FAB). FABs have mid-ocean ridge basalt (MORB)-like compositions, however, FAB are consistently lower in the high-field strength elements (TiO 2 , P 2 O 5 , Zr) and Ni compared to MORB, with Na 2 O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off-axis magmas return to more primitive compositions. Melt models require a two-stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400°C-1,480°C) at relatively low pressures (1-2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle. Plain Language Summary This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu-Bonin subduction zone to recover samples from its earliest history, before formation of Izu-Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid-ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it SHERVAIS ET AL.
Geochemistry, Geophysics, Geosystems
The Izu-Bonin-Mariana (IBM) fore arc preserves igneous rock assemblages that formed during subduction initiation circa 52 Ma. International Ocean Discovery Program (IODP) Expedition 352 cored four sites in the fore arc near the Ogasawara Plateau in order to document the magmatic response to subduction initiation and the physical, petrologic, and chemical stratigraphy of a nascent subduction zone. Two of these sites (U1440 and U1441) are underlain by fore-arc basalt (FAB). FABs have mid-ocean ridge basalt (MORB)-like compositions, however, FAB are consistently lower in the high-field strength elements (TiO 2 , P 2 O 5 , Zr) and Ni compared to MORB, with Na 2 O at the low end of the MORB field and FeO* at the high end. Almost all FABs are light rare earth element depleted, with low total REE, and have low ratios of highly incompatible to less incompatible elements (Ti/V, Zr/Y, Ce/Yb, and Zr/Sm) relative to MORB. Chemostratigraphic trends in Hole U1440B are consistent with the uppermost lavas forming off axis, whereas the lower lavas formed beneath a spreading center axis. Axial magma of U1440B becomes more fractionated upsection; overlying off-axis magmas return to more primitive compositions. Melt models require a two-stage process, with early garnet field melts extracted prior to later spinel field melts, with up to 23% melting to form the most depleted compositions. Mantle equilibration temperatures are higher than normal MORB (1,400°C-1,480°C) at relatively low pressures (1-2 GPa), which may reflect an influence of the Manus plume during subduction initiation. Our data support previous models of FAB origin by decompression melting but imply a source more depleted than normal MORB source mantle. Plain Language Summary This projects looks at how subduction zones form and evolve before island arc volcanism becomes established. Subduction zones are important because they are the primary sites for recycling chemically enriched crustal materials and because they form some of Earth's most important ore deposits. We drilled two deep core holes on the inner trench wall of the Izu-Bonin subduction zone to recover samples from its earliest history, before formation of Izu-Bonin island arc volcanoes. Samples from these cores were analyzed chemically to establish how lava compositions varied through time and to understand the processes that control their chemistry. We found a diverse set of lavas, with chemical compositions that are low in elements that are normally enriched in arc lavas. Calculations also show that these lavas were hotter than normal mid-ocean ridge lavas. We found that the first lavas in a new subduction zone form by the upwelling of hot material from deeper in the Earth, which partially melts as it SHERVAIS ET AL.
The Island Arc, 1998
The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3±4 Ma, $ 3±4 million years after rifting began. This may re¯ect increased sediment subduction during early rifting and roll-back of the Paci®c lithosphere. Parallel trends are not seen in the enrichment of incompatible high ®eld strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that¯ux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.
Criticism of generalised models for the magmatic evolution of arc-trench systems
Earth and Planetary Science Letters, 1978
Recent geological and petrological results from the Lesser Antilles island arc and Papua New Guinea, and from other regions of arc-trench-type volcanism, provide notable exceptions to the spatial, volumetric, and temporal relationships claimed for generalised arc models. For example, many alkalic and shoshonitic associations do not appear to be developed over the deepest parts of downgoing slabs, and there are now several well-documented exceptions to the K20/SiO2/depth-to-Benioff-zone relationship. Moreover, the temporal sequence of early tholeiitic ~ middle calcalkalic --, late shoshonitic/alkalic is not well substantiated, although shoshonitic rocks do appear to be developed most commonly in regions with a long history of plate interactions. Exceptions to the generalised arc model are symptomatic of the need to look for the unique features of individual island arcs, rather than just similarities between different ones, so that the major factors controlling arc evolution may be determined.
We propose the fundamental topic of "Roots of Arc Volcanoes" (RAV) as a major theme for the MARGINS Successor program. This theme is specifically identified in the MARGINS 2009 Review. We envision this initiative as encompassing the arc volcano system from the slab to the surface, involving a comprehensive suite of geophysical, geochemical, and geological studies of submarine and subaerial arc volcanoes over a broad range of spatial and temporal scales. This initiative embodies some of the elements of the original MARGINS program (magma genesis, fluids, and volcanism), but with a change in focus to a specific theme of how arc volcanoes work, from bottom to top. A broad analogy can be made to the vast suite of multidisciplinary, multi-scale studies of the Hawaiian volcanoes and hotspot, funded by multiple NSF programs, and involving: "passive" seismic studies from the scale of Halema'uma'u crater to the mantle plume track; marine seismic profiling and offshore-onshore imaging; deep drilling; petrology and geochemistry of lavas; gravity, magnetic, and electromagnetic field studies; geodesy; and much more. Thus, there is great potential for synergistic work on this theme across disciplinary boundaries. Studies in Cascadia and Alaska can take advantage of recent ARRA initiatives, future USArray deployments, and cooperation with U.S. volcano observatories. The "roots of arc volcanoes" theme also has direct societal relevance in terms of providing a deeper understanding of volcano behavior and hence volcanic hazards and environmental impacts. The potential components of the RAV initiative cut across many earth science disciplines. Overall, the research components for the RAV initiative would largely mirror those of the Subduction Factory, but with a different focus: experimental and theoretical analyses; bathymetry, swath mapping, and dredging; active-and passive-source seismology; drilling; magnetotellurics; heat flow; geodesy; field studies; petrologic, geochemical and isotopic analyses; and database development. The suite of studies would of course vary for submarine volcanoes versus subaerial volcanoes. Considerable debate exists, especially in Cascadia and Alaska, regarding the role of the subducting slab during magma genesis. Geochemical studies of primitive lavas in these arcs indicate that magmas are generated via fluid-flux melting, adiabatic decompression melting of hot, nearly anhydrous mantle, partial melting of the slab, or some combination of these processes. The recent work by Grove et al. (2009) on the primary control that slab dip has on arc volcano location is an example of the type of fundamental issue that requires cross-cutting research that could be supported by the RAV initiative. In this case, a combination of experimental work on chlorite stability, geodynamic modeling of subduction zone thermal structure, and seismic estimates of slab dip led to the conclusion that the melting zone is controlled by the intersection of zones of chlorite dehydration with the (vapor-saturated) peridotite solidus, which in turn is controlled primarily by slab dip and, to a lesser degree, by convergence rate. An ultimate goal is to understand how slab petrology (Figure 1) is linked to its seismic structure and seismicity (Figure 2). Given the recent advances in locating non-volcanic tremor on plate interfaces and relating tremor to fluids, earthquakes, and aseismic slip, there is