Slab dehydration beneath forearcs: Insights from the southern Mariana and Matthew-Hunter rifts
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Keywords: southern Mariana forearc rifts, near-trench spreading, Matthew-Hunter convergent margin, slab dehydration, subduction channel
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Figure 1 Geochemical maps. (a) Location map. The red boxes show the expanded area of (b) for the Mariana and of (c) for Matthew-Hunter convergent margins. (b) Rb/Th geochemical map in the southern Mariana convergent margin, designed using www.geomapapp.org. The Fina-Nagu volcanic arc: FNVC, Santa Rosa Bank fault: SRBF. (c) Ba/Th geochemical map of the Matthew-Hunter intra-oceanic arc. |
Figure 2 Geochemical features of the near trench magmas. (a) MgO vs. TiO2 diagram. The southern Mariana olivine-hosted melt inclusions are primitive basalts with a boninitic fingerprint. (b) Picture of an olivine hosting a melt inclusion taken under a binocular microscope. (c) Cs/Th and (d) Ba/Th vs. Rb/Th diagrams showing a stronger enrichment in water-rich slab fluids (as shown by the higher Cs/Th, Ba/Th and Rb/Th) in the near trench basalts, as compared to arc and back-arc basalts. (e) Rb/Th vs. 206Pb/204Pb diagram tracking the origin of the water-rich slab fluids. Serp.: serpentinised mantle, sed.: sediments. Composition of the end-members is reported in Table S-1. |
Figure 3 Slab dehydration in the forearc and P-T conditions of mantle melt equilibrium. (a) H2O/Ce vs. Rb/Th diagram showing the stronger enrichment in water-rich slab fluids of the near trench magmas. The rough correlation between H2O/Ce and Rb/Th indicates that H2O/Ce also tracks the aqueous slab fluids. The different slope between the MH and SEMFR basalts likely reflect different slab lithology. (b) P-T conditions of the last primitive basaltic melt in equilibrium with the mantle (Lee et al., 2009). Basalts were filtered for MgO ≥ 6 wt. %. Fe3+/FeT = 0.25 was used for the arc basalts, and Fe3+/FeT = 0.17 for the back-arc basalts and the near trench basalts (Kelley and Cottrell, 2009). An averaged water content of 1.90 ± 0.61 wt. % was used for the MH basalts, and of 2.53 ± 0.55 wt. % for the MH boninites. Error bars are 1σ standard deviation. |
Figure 4 Enrichment in slab-derived water of the near trench magmas. (a) H2O content vs. slab depth (km). The averaged composition of the worldwide arc and back-arc magmas are from Ribeiro et al. (2015). (b, c) Sketches illustrating the enrichment in water-rich slab fluids in the near trench magmas in MH and the southern Marianas (b), and the contribution of the subduction channel to the water supplied beneath the volcanic arc front in long lived subduction zones (c). |
| Figure 1 | Figure 2 | Figure 3 | Figure 4 |
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Introduction
Subduction zones have efficiently cycled seawater between the Earth’s surface and its interior to maintain the ocean masses over geological times. Seawater is removed from the oceans during alteration of the oceanic plate, and it is released back to the surface during dehydration of the sinking plate, which ultimately generates subduction zone magmas (Schmidt and Poli, 1998
Schmidt, M., Poli, S. (1998) Experimentally based water budgets for dehydrating slabs and consequences for magma generation. Earth and Planetary Science Letters 163, 361–379.
). Dehydration of the subducted plate is believed to be modulated by its thermal state (van Keken et al., 2011van Keken, P.E., Hacker, B.R., Syracuse, E.M., Abers, G.A. (2011) Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide. Journal of Geophysical Research: Solid Earth 116, B01401. doi: 10.1029/2010JB007922.
). The slab thermal model predicts that cold slabs should release most of their subducted water beneath the volcanic arc front (∼70 % slab dehydration), while warmer slabs should mostly dehydrate beneath the forearc (∼90 % slab dehydration). Following this view, arc lavas from cold subduction zones should record greater involvement of water-rich slab fluids, while hot subduction zone arc magmas should be drier (Shaw et al., 2008Shaw, A.M., Hauri, E.H., Fischer, T.P., Hilton, D.R., Kelley, K.A. (2008) Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle. Earth and Planetary Science Letters 275, 138–145.
; Walowski et al., 2015Walowski, K.J., Wallace, P.J., Hauri, E.H., Wada, I., Clynne, M.A. (2015) Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite. Nature Geoscience 8, 404–408.
). Yet, slab dehydration beneath forearcs has largely remained theoretical due to the difficulties in direct observations, which are often obscured by mantle serpentinisation (Hyndman and Peacock, 2003Hyndman, R.D., Peacock, S.M. (2003) Serpentinization of the forearc mantle. Earth and Planetary Science Letters 212, 417–432.
). The forearc mantle is usually too cold to melt, and occurrence of forearc lavas that have captured the fluids released from the shallow part of the subducted slab is rare. However, knowledge of slab dehydration beneath forearcs is essential to understand whether subducted seawater can be returned into the lower mantle.Stretching in the southern Mariana and in the Matthew-Hunter convergent margins within 100 km from the trench provides a unique window into the subduction processes that occurred above the shallow part of the subducted plate. Using chemical markers, the composition of the near trench magmas is investigated to better comprehend slab dehydration in the forearc of two modern subduction zones.
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Geological background
The southern Marianas represent the southern end of the Izu-Bonin-Mariana (IBM) convergent margin, which has long been recognised as a typical example of a cold subduction system (slab age ∼150 Ma) (Müller et al., 2008
Müller, R.D., Sdrolias, M., Gaina, C., Roest, W.R. (2008) Age, spreading rates, and spreading asymmetry of the world’s ocean crust. Geochemistry, Geophysics, Geosystems 9, Q04006. doi: 10.1029/2007GC001743.
; Syracuse et al., 2010Syracuse, E.M., van Keken, P.E., Abers, G.A. (2010) The global range of subduction zone thermal models. Physics of the Earth and Planetary Interiors 183, 73–90.
). To the south, the Eocene proto-arc crust has been recently stretched (<5 Ma) to accommodate the opening of the Mariana Trough above the shallow part of the subducting Pacific plate (<100 km depth to the slab). The SE Mariana forearc rift (SEMFR) is now floored with basaltic pillow lavas and lava flows (SiO2 < 59 wt. %, K2O ≤ 1 wt. %), which erupted within ∼80 km from the trench (Fig. 1a) (Ribeiro et al., 2013Ribeiro, J.M., Stern, R.J., Martinez, F., Ishizuka, O., Merle, S.G., Kelley, K.A., Anthony, E.Y., Ren, M., Ohara, Y., Reagan, M., Girard, G., Bloomer, S.H. (2013) Geodynamic evolution of a forearc rift in the southernmost Mariana Arc. Island Arc 22, 453–476.
). The SEMFR basalts host some olivine mantle xenocrysts (Fo90–92), which can enclose fresh melt inclusions with a boninitic fingerprint (Fig. 2a, b) (Ribeiro et al., 2015Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
).
Figure 1 Geochemical maps. (a) Location map. The red boxes show the expanded area of (b) for the Mariana and of (c) for Matthew-Hunter convergent margins. (b) Rb/Th geochemical map in the southern Mariana convergent margin, designed using www.geomapapp.org. The Fina-Nagu volcanic arc: FNVC, Santa Rosa Bank fault: SRBF. (c) Ba/Th geochemical map of the Matthew-Hunter intra-oceanic arc.
Figure 2 Geochemical features of the near trench magmas. (a) MgO vs. TiO2 diagram. The southern Mariana olivine-hosted melt inclusions are primitive basalts with a boninitic fingerprint. (b) Picture of an olivine hosting a melt inclusion taken under a binocular microscope. (c) Cs/Th and (d) Ba/Th vs. Rb/Th diagrams showing a stronger enrichment in water-rich slab fluids (as shown by the higher Cs/Th, Ba/Th and Rb/Th) in the near trench basalts, as compared to arc and back-arc basalts. (e) Rb/Th vs. 206Pb/204Pb diagram tracking the origin of the water-rich slab fluids. Serp.: serpentinised mantle, sed.: sediments. Composition of the end-members is reported in Table S-1.
The Matthew-Hunter (MH) intra-oceanic arc has been proposed to represent a juvenile subduction zone, which initiated at ∼1.8 Ma as a result of the collision of the Loyalty Ridge with the southern termination of the New Hebrides Trench (Patriat et al., 2015
Patriat, M., Collot, J., Danyushevsky, L., Fabre, M., Meffre, S., Falloon, T., Rouillard, P., Pelletier, B., Roach, M., Fournier, M. (2015) Propagation of back-arc extension into the arc lithosphere in the southern New Hebrides volcanic arc. Geochemistry, Geophysics, Geosystems 16, 3142–3159.
). The young Australian slab (<34 Ma) (Davey, 1982Davey, F.J. (1982) The structure of the South Fiji basin. Tectonophysics 87, 185–241.
) is now subducting along the MH Trench, so its slab would possess a warm to intermediate thermal structure (Syracuse et al., 2010Syracuse, E.M., van Keken, P.E., Abers, G.A. (2010) The global range of subduction zone thermal models. Physics of the Earth and Planetary Interiors 183, 73–90.
). Near trench rifting is accommodated by en échelon rifts and grabens, and transient spreading at ∼90 km from the trench (Fig. 1b). A wide compositional range of volcanic rocks has been recovered within the rifts and in front of the trench, which include low- to medium-K tholeiitic basalts, rhyolites, adakites and boninites (Figs. 2a, S-1) (Patriat et al., 2019Patriat, M., Falloon, T.J., Danyushevsky, L., Collot, J., Jean, M.M., Hoernle, K., Hauff, F., Maas, R., Woodhead, J.D., Feig, S.T. (2019) Subduction initiation terranes exposed at the front of a 2 Ma volcanically-active subduction zone. Earth and Planetary Science Letters 508, 30–40.
). Occurrence of adakites, which are believed to represent melts of the subducted crust (Defant and Drummond, 1990Defant, M.J., Drummond, M.S. (1990) Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665.
), also suggests a warmer pressure-temperature (P-T) slab path in MH.top
Characteristics of the near trench magmas
Using a compiled dataset (Table S-1), the composition of the SEMFR and MH magmas was examined (see Supplementary Information for details). Magmas were filtered for basaltic (≤56 wt. %) and boninitic composition, as well as minimally degassed volatile contents (S > 500 ppm or μg/g and CO2 > 50 ppm or μg/g) to ensure that they reliably tracked subduction processes. Because most magmas degas upon ascent, their water contents likely represent minimum estimates.
Basalts, boninites, and associated olivine-hosted melt inclusions from the SEMFR and MH have ∼2 wt. % H2O on average (Danyushevsky et al., 1993
Danyushevsky, L.V., Falloon, T.J., Sobolev, A.V., Crawford, A.J., Carroll, M., Price, R.C. (1993) The H2O content of basalt glasses from Southwest Pacific back-arc basins. Earth and Planetary Science Letters 117, 347–362.
; Ribeiro et al., 2015Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
). The near trench magmas are strongly enriched in Rb/Th, Cs/Th and H2O/Ce (Rb/Th = 3–141, Cs/Th = 0.04–17.79, H2O/Ce = 436–23,531; Figs. 2b, 3a), as compared to their associated arc magmas (Rb/Th ≤ 68, Cs/Th ≤ 4, H2O/Ce ≤ 9829; Table S-1), while they possess arc-like Ba/Th ratios (Ba/Th = 31–798). P-T conditions of the primary melt in equilibrium with the asthenospheric mantle were also constrained using a water sensitive geobarometer (Lee et al., 2009Lee, C.-T.A., Luffi, P., Plank, T., Dalton, H., Leeman, W.P. (2009) Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters 279, 20–33.
). Near trench basalts recorded shallower P-T conditions of mantle melt equilibrium (averaged T = 1289 ± 26 °C, P = 0.84 ± 0.17 GPa for the SEMFR basalts, and averaged T = 1270 ± 26 °C, P = 1.03 ± 0.17 GPa for the MH basalts) than the back-arc basalts (averaged T = 1278 ± 26 °C, P = 1.05 ± 0.17 GPa for the Mariana Trough, and averaged T = 1358 ± 26 °C, P = 1.44 ± 0.17 GPa for the New Hebrides and north Fiji back-arc basins) and the arc basalts (averaged T = 1308 ± 26 °C, P = 1.58 ± 0.17 GPa for the Mariana arc, and averaged T = 1305 ± 26 °C, P = 1.19 ± 0.17 GPa for the New Hebrides arc) (Fig. 3b).
Figure 3 Slab dehydration in the forearc and P-T conditions of mantle melt equilibrium. (a) H2O/Ce vs. Rb/Th diagram showing the stronger enrichment in water-rich slab fluids of the near trench magmas. The rough correlation between H2O/Ce and Rb/Th indicates that H2O/Ce also tracks the aqueous slab fluids. The different slope between the MH and SEMFR basalts likely reflect different slab lithology. (b) P-T conditions of the last primitive basaltic melt in equilibrium with the mantle (Lee et al., 2009
Lee, C.-T.A., Luffi, P., Plank, T., Dalton, H., Leeman, W.P. (2009) Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters 279, 20–33.
). Basalts were filtered for MgO ≥ 6 wt. %. Fe3+/FeT = 0.25 was used for the arc basalts, and Fe3+/FeT = 0.17 for the back-arc basalts and the near trench basalts (Kelley and Cottrell, 2009Kelley, K.A., Cottrell, E. (2009) Water and the Oxidation State of Subduction Zone Magmas. Science 325, 605–607.
). An averaged water content of 1.90 ± 0.61 wt. % was used for the MH basalts, and of 2.53 ± 0.55 wt. % for the MH boninites. Error bars are 1σ standard deviation.The shallow P-T conditions of mantle melt equilibrium recorded by the near trench basalts suggest that the asthenospheric mantle is melting just above the shallow part of a dehydrating slab. Infiltration of water-rich fluids into the asthenospheric mantle above the shallow subducted slabs allowed the near trench magmas to equilibrate with the mantle at shallower P-T conditions of mantle melt equilibrium than did the arc and the back-arc basalts. High Cs/Th, H2O/Ce, Rb/Th, Cs/Ba, and shallow P-T conditions likely represent diagnostic features of near trench magmas.
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Implications for slab dehydration beneath forearcs
Cold conditions are believed to prevail in forearcs, so that slab dehydration triggers mantle serpentinisation (Hyndman and Peacock, 2003
Hyndman, R.D., Peacock, S.M. (2003) Serpentinization of the forearc mantle. Earth and Planetary Science Letters 212, 417–432.
). However, stretching of the pre-existing crust in the southern Marianas and MH has permitted the asthenospheric mantle to flux in and melt within 90 km from the trench, creating a new oceanic crust in the forearc. Hence, the SEMFR and MH magmas can provide unique insights into the composition of the slab fluids that are usually released to serpentinise the cold forearc mantle.Slab dehydration can be inferred from chemical markers (H2O/Ce, Rb/Th, Cs/Th, Ba/Th), which rely on the differential behaviour of the incompatible elements for the water-rich slab fluids. For instance, Ba, Cs and Rb are easily mobilised with the aqueous fluids and the sediment melts, while Th is only mobilised with the sediment melts (Pearce et al., 2005
Pearce, J.A., Stern, R.J., Bloomer, S.H., Fryer, P. (2005) Geochemical mapping of the Mariana arc-basin system : Implications for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems 6, Q07006. doi: 10.1029/2004GC000895.
). Similarly, H2O is easily mobilised with the aqueous fluids, while Ce remains relatively immobile (Dixon et al., 2002Dixon, J.E., Leist, L., Langmuir, C.H., Schilling, J.G. (2002) Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt. Nature 420, 385–389.
). Hence, their ratios can track the aqueous slab fluids. Because Cs is more easily mobilised with the water-rich fluids released during deserpentinisation, the Cs/Ba ratio has the potential to track the water-rich fluids released from the subducted lithospheric mantle. Using elemental ratios has the main advantage to minimise the effects of melting and fractionation, as the selected elements behave similarly during such processes. The higher proxies in arc magmas imply that they captured greater extents of slab-derived water than did the back-arc lavas (Figs. 2, 3), which has been interpreted as a dehydration peak beneath the arc (Ruscitto et al., 2012Ruscitto, D.M., Wallace, P.J., Cooper, L.B., Plank, T. (2012) Global variations in H2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes. Geochemistry, Geophysics, Geosystems 13, Q03025. doi: 10.1029/2011GC003902.
) (Fig. 4a). The SEMFR and MH magmas recorded the highest markers of water-rich slab fluids (Cs, Th, Rb/Th, H2O/Ce) yet observed in subduction zone magmas (worldwide arc magmas display Rb/Th ≤ 111, Cs/Th ≤ 4, H2O/Ce ≤ 10,612) (Figs. 2, 3a) (Ribeiro et al., 2015Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
), implying that most of the intra-slab water could be released beneath these forearcs (i.e. <100 km depth to the slab) (Fig. 4). Geochemical mapping further suggests that slab dehydration could peak at ∼70 ± 5 km from the southern Mariana Trench, while it may peak within 10 km of the MH Trench (Fig. 1). The aqueous fluids were likely released from dehydrating a slab composed of 0–70 % serpentinised mantle, 10–100 % altered oceanic crust (AOC), and 0–90 % sediments. The SEMFR and MH magmas captured up to 60–80 % of this water-rich slab fluid (Fig. 2e). These results imply that dehydration of the subducted mantle likely triggered dehydration of the oceanic crust and subducted sediments in both settings. The warmer slab subducting underneath MH likely dehydrated earlier, and hence faster (i.e. within 10 km of the trench), than did the Pacific plate subducting underneath the southern Marianas. Mineral phases in subducted slabs with a cooler P-T path may thus retain a certain fraction of their bound water to break down deeper.
Figure 4 Enrichment in slab-derived water of the near trench magmas. (a) H2O content vs. slab depth (km). The averaged composition of the worldwide arc and back-arc magmas are from Ribeiro et al. (2015)
Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
. (b, c) Sketches illustrating the enrichment in water-rich slab fluids in the near trench magmas in MH and the southern Marianas (b), and the contribution of the subduction channel to the water supplied beneath the volcanic arc front in long lived subduction zones (c).These observations further imply that both cold and warm to intermediate subducted slabs could efficiently dehydrate before reaching the volcanic arc front (Fig. 4a). Although the SEMFR and MH represent two modern examples of near trench spreading, where the inflow of asthenospheric mantle underneath the forearc facilitates shallow slab dehydration, the possibility that large fluxes of water could be released in the forearcs of modern subduction zones thus exists. Additionally, the metamorphic rock records suggest that most subduction zones could have a warmer thermal structure than previously estimated (Penniston-Dorland et al., 2015
Penniston-Dorland, S.C., Kohn, M.J., Manning, C.E. (2015) The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models. Earth and Planetary Science Letters 428, 243–254.
), implying that both hot and cold subducted slabs could dehydrate efficiently beneath forearcs. Extensive slab dehydration beneath forearcs (≥70 %) is also supported by high pressure experiments (Schmidt and Poli, 1998Schmidt, M., Poli, S. (1998) Experimentally based water budgets for dehydrating slabs and consequences for magma generation. Earth and Planetary Science Letters 163, 361–379.
), as well as by estimates of fluid fluxes released during shallow slab dehydration (Hyndman and Peacock, 2003Hyndman, R.D., Peacock, S.M. (2003) Serpentinization of the forearc mantle. Earth and Planetary Science Letters 212, 417–432.
; Savov et al., 2007Savov, I.P., Ryan, J.G., D’Antonio, M., Fryer, P. (2007) Shallow slab fluid release across and along the Mariana arc-basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc. Journal of Geophysical Research: Solid Earth 112, B09205. doi: 10.1029/2006JB004749.
). But the extent of slab dehydration beneath forearcs requires additional investigations. If both hot and cold slabs mostly dehydrate beneath forearcs, slab dehydration alone might not suffice to sustain the water delivered into subduction zone magmas, and additional water reservoirs could be necessary. One possibility is that the subduction channel could contribute to the water delivered to subduction zone magmas, in addition to intra-slab water (Savov et al., 2007Savov, I.P., Ryan, J.G., D’Antonio, M., Fryer, P. (2007) Shallow slab fluid release across and along the Mariana arc-basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc. Journal of Geophysical Research: Solid Earth 112, B09205. doi: 10.1029/2006JB004749.
; Marschall and Schumacher, 2012Marschall, H.R., Schumacher, J. (2012) Arc magmas sourced from mélange diapirs in subduction zones. Nature Geoscience 5, 862–867.
). A mélange zone, composed of subducted sediments, serpentinised mantle and a few blocks of mafic crust, is believed to form a viscous layer on top of most slabs (Cloos and Shreve, 1988Cloos, M., Shreve, R.L. (1988) Subduction-channel model of prism accretion, melange formation, sediment subduction, and subduction erosion at convergent plate margins: 1. Background and description. Pure and Applied Geophysics 128, 455–500.
). Serpentine group minerals and chlorite have the potential to carry large amount of water (8–13 wt. %) to at least ∼150 km depth (Ulmer and Trommsdorff, 1995Ulmer, P., Trommsdorff, V. (1995) Serpentine Stability to Mantle Depths and Subduction-Related Magmatism. Science 268, 858–861.
; Marschall and Schumacher, 2012Marschall, H.R., Schumacher, J. (2012) Arc magmas sourced from mélange diapirs in subduction zones. Nature Geoscience 5, 862–867.
). If entrained with the subducted plate, this mélange zone could thus transport some additional water and incompatible elements (Ba, Rb, Cs) to sub-arc depths to sustain the water delivery into the arc and the back-arc basin (Savov et al., 2007Savov, I.P., Ryan, J.G., D’Antonio, M., Fryer, P. (2007) Shallow slab fluid release across and along the Mariana arc-basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc. Journal of Geophysical Research: Solid Earth 112, B09205. doi: 10.1029/2006JB004749.
; Scambelluri and Tonarini, 2012Scambelluri, M., Tonarini, S. (2012) Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle. Geology 40, 907–910.
). This process could provide a simple, alternative explanation to the similar average in markers of slab dehydration observed globally in arc magmas (Ribeiro and Lee, 2017Ribeiro, J.M., Lee, C.-T.A. (2017) An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle. Earth and Planetary Science Letters 479, 298–309. doi: 10.1016/j.epsl.2017.09.018.
) (Fig. 4). However, subduction of a mélange zone on top of the slab is not easily reconciled with some geophysical observations (Hyndman and Peacock, 2003Hyndman, R.D., Peacock, S.M. (2003) Serpentinization of the forearc mantle. Earth and Planetary Science Letters 212, 417–432.
). Further examinations of the forearc processes, and better quantifying of slab dehydration at shallow depths are thus essential to comprehend how subducted seawater can bypass subduction zones.top
Acknowledgements
Colin Macpherson, Jeroen van Hunen (both at Durham University), Julian Pearce (Cardiff University) and Martin Patriat (IFREMER) are thanked for thoughtful discussions. Horst Marschall, the editor, Ivan Savov (Leeds University) and one anonymous reviewer are also thanked for insightful comments. I acknowledge a GIG-CAS PIFI fellowship n° 2020VCB000.
Editor: Horst R. Marschall
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References
Cloos, M., Shreve, R.L. (1988) Subduction-channel model of prism accretion, melange formation, sediment subduction, and subduction erosion at convergent plate margins: 1. Background and description. Pure and Applied Geophysics 128, 455–500.
Show in context
A mélange zone, composed of subducted sediments, serpentinised mantle and a few blocks of mafic crust, is believed to form a viscous layer on top of most slabs (Cloos and Shreve, 1988).
View in article
Danyushevsky, L.V., Falloon, T.J., Sobolev, A.V., Crawford, A.J., Carroll, M., Price, R.C. (1993) The H2O content of basalt glasses from Southwest Pacific back-arc basins. Earth and Planetary Science Letters 117, 347–362.
Show in context
Basalts, boninites, and associated olivine-hosted melt inclusions from the SEMFR and MH have ∼2 wt. % H2O on average (Danyushevsky et al., 1993; Ribeiro et al., 2015).
View in article
Davey, F.J. (1982) The structure of the South Fiji basin. Tectonophysics 87, 185–241.
Show in context
The young Australian slab (<34 Ma) (Davey, 1982) is now subducting along the MH Trench, so its slab would possess a warm to intermediate thermal structure (Syracuse et al., 2010).
View in article
Defant, M.J., Drummond, M.S. (1990) Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665.
Show in context
Occurrence of adakites, which are believed to represent melts of the subducted crust (Defant and Drummond, 1990), also suggests a warmer pressure-temperature (P-T) slab path in MH.
View in article
Dixon, J.E., Leist, L., Langmuir, C.H., Schilling, J.G. (2002) Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt. Nature 420, 385–389.
Show in context
Similarly, H2O is easily mobilised with the aqueous fluids, while Ce remains relatively immobile (Dixon et al., 2002).
View in article
Hyndman, R.D., Peacock, S.M. (2003) Serpentinization of the forearc mantle. Earth and Planetary Science Letters 212, 417–432.
Show in context
Yet, slab dehydration beneath forearcs has largely remained theoretical due to the difficulties in direct observations, which are often obscured by mantle serpentinisation (Hyndman and Peacock, 2003).
View in article
Cold conditions are believed to prevail in forearcs, so that slab dehydration triggers mantle serpentinisation (Hyndman and Peacock, 2003).
View in article
However, subduction of a mélange zone on top of the slab is not easily reconciled with some geophysical observations (Hyndman and Peacock, 2003).
View in article
Extensive slab dehydration beneath forearcs (≥70 %) is also supported by high pressure experiments (Schmidt and Poli, 1998), as well as by estimates of fluid fluxes released during shallow slab dehydration (Hyndman and Peacock, 2003; Savov et al., 2007).
View in article
Kelley, K.A., Cottrell, E. (2009) Water and the Oxidation State of Subduction Zone Magmas. Science 325, 605–607.
Show in context
Fe3+/FeT = 0.25 was used for the arc basalts, and Fe3+/FeT = 0.17 for the back-arc basalts and the near trench basalts (Kelley and Cottrell, 2009).
View in article
Lee, C.-T.A., Luffi, P., Plank, T., Dalton, H., Leeman, W.P. (2009) Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters 279, 20–33.
Show in context
P-T conditions of the primary melt in equilibrium with the asthenospheric mantle were also constrained using a water sensitive geobarometer (Lee et al., 2009).
View in article
(b) P-T conditions of the last primitive basaltic melt in equilibrium with the mantle (Lee et al., 2009).
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Marschall, H.R., Schumacher, J. (2012) Arc magmas sourced from mélange diapirs in subduction zones. Nature Geoscience 5, 862–867.
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One possibility is that the subduction channel could contribute to the water delivered to subduction zone magmas, in addition to intra-slab water (Savov et al., 2007; Marschall and Schumacher, 2012).
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Serpentine group minerals and chlorite have the potential to carry large amount of water (8–13 wt. %) to at least ∼150 km depth (Ulmer and Trommsdorff, 1995; Marschall and Schumacher, 2012).
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Müller, R.D., Sdrolias, M., Gaina, C., Roest, W.R. (2008) Age, spreading rates, and spreading asymmetry of the world’s ocean crust. Geochemistry, Geophysics, Geosystems 9, Q04006. doi: 10.1029/2007GC001743.
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The southern Marianas represent the southern end of the Izu-Bonin-Mariana (IBM) convergent margin, which has long been recognised as a typical example of a cold subduction system (slab age ∼150 Ma) (Müller et al., 2008; Syracuse et al., 2010).
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Patriat, M., Collot, J., Danyushevsky, L., Fabre, M., Meffre, S., Falloon, T., Rouillard, P., Pelletier, B., Roach, M., Fournier, M. (2015) Propagation of back-arc extension into the arc lithosphere in the southern New Hebrides volcanic arc. Geochemistry, Geophysics, Geosystems 16, 3142–3159.
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The Matthew-Hunter (MH) intra-oceanic arc has been proposed to represent a juvenile subduction zone, which initiated at ∼1.8 Ma as a result of the collision of the Loyalty Ridge with the southern termination of the New Hebrides Trench (Patriat et al., 2015).
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Patriat, M., Falloon, T.J., Danyushevsky, L., Collot, J., Jean, M.M., Hoernle, K., Hauff, F., Maas, R., Woodhead, J.D., Feig, S.T. (2019) Subduction initiation terranes exposed at the front of a 2 Ma volcanically-active subduction zone. Earth and Planetary Science Letters 508, 30–40.
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A wide compositional range of volcanic rocks has been recovered within the rifts and in front of the trench, which include low- to medium-K tholeiitic basalts, rhyolites, adakites and boninites (Figs. 2a, S-1) (Patriat et al., 2019).
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Pearce, J.A., Stern, R.J., Bloomer, S.H., Fryer, P. (2005) Geochemical mapping of the Mariana arc-basin system : Implications for the nature and distribution of subduction components. Geochemistry, Geophysics, Geosystems 6, Q07006. doi: 10.1029/2004GC000895.
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For instance, Ba, Cs and Rb are easily mobilised with the aqueous fluids and the sediment melts, while Th is only mobilised with the sediment melts (Pearce et al., 2005).
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Penniston-Dorland, S.C., Kohn, M.J., Manning, C.E. (2015) The global range of subduction zone thermal structures from exhumed blueschists and eclogites: Rocks are hotter than models. Earth and Planetary Science Letters 428, 243–254.
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Additionally, the metamorphic rock records suggest that most subduction zones could have a warmer thermal structure than previously estimated (Penniston-Dorland et al., 2015), implying that both hot and cold subducted slabs could dehydrate efficiently beneath forearcs.
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Ribeiro, J.M., Lee, C.-T.A. (2017) An imbalance in the deep water cycle at subduction zones: The potential importance of the fore-arc mantle. Earth and Planetary Science Letters 479, 298–309. doi: 10.1016/j.epsl.2017.09.018.
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This process could provide a simple, alternative explanation to the similar average in markers of slab dehydration observed globally in arc magmas (Ribeiro and Lee, 2017) (Fig. 4).
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Ribeiro, J.M., Stern, R.J., Martinez, F., Ishizuka, O., Merle, S.G., Kelley, K.A., Anthony, E.Y., Ren, M., Ohara, Y., Reagan, M., Girard, G., Bloomer, S.H. (2013) Geodynamic evolution of a forearc rift in the southernmost Mariana Arc. Island Arc 22, 453–476.
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The SE Mariana forearc rift (SEMFR) is now floored with basaltic pillow lavas and lava flows (SiO2 < 59 wt. %, K2O ≤ 1 wt. %), which erupted within ∼80 km from the trench (Fig. 1a) (Ribeiro et al., 2013).
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Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
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The SEMFR basalts host some olivine mantle xenocrysts (Fo90–92), which can enclose fresh melt inclusions with a boninitic fingerprint (Fig. 2a, b) (Ribeiro et al., 2015).
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The SEMFR and MH magmas recorded the highest markers of water-rich slab fluids (Cs, Th, Rb/Th, H2O/Ce) yet observed in subduction zone magmas (worldwide arc magmas display Rb/Th ≤ 111, Cs/Th ≤ 4, H2O/Ce ≤ 10,612) (Figs. 2, 3a) (Ribeiro et al., 2015), implying that most of the intra-slab water could be released beneath these forearcs (i.e. <100 km depth to the slab) (Fig. 4).
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The averaged composition of the worldwide arc and back-arc magmas are from Ribeiro et al. (2015).
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Basalts, boninites, and associated olivine-hosted melt inclusions from the SEMFR and MH have ∼2 wt. % H2O on average (Danyushevsky et al., 1993; Ribeiro et al., 2015).
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Ruscitto, D.M., Wallace, P.J., Cooper, L.B., Plank, T. (2012) Global variations in H2O/Ce: 2. Relationships to arc magma geochemistry and volatile fluxes. Geochemistry, Geophysics, Geosystems 13, Q03025. doi: 10.1029/2011GC003902.
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The higher proxies in arc magmas imply that they captured greater extents of slab-derived water than did the back-arc lavas (Figs. 2, 3), which has been interpreted as a dehydration peak beneath the arc (Ruscitto et al., 2012) (Fig. 4a).
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Savov, I.P., Ryan, J.G., D’Antonio, M., Fryer, P. (2007) Shallow slab fluid release across and along the Mariana arc-basin system: Insights from geochemistry of serpentinized peridotites from the Mariana fore arc. Journal of Geophysical Research: Solid Earth 112, B09205. doi: 10.1029/2006JB004749.
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One possibility is that the subduction channel could contribute to the water delivered to subduction zone magmas, in addition to intra-slab water (Savov et al., 2007; Marschall and Schumacher, 2012).
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If entrained with the subducted plate, this mélange zone could thus transport some additional water and incompatible elements (Ba, Rb, Cs) to sub-arc depths to sustain the water delivery into the arc and the back-arc basin (Savov et al., 2007; Scambelluri and Tonarini, 2012).
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Extensive slab dehydration beneath forearcs (≥70 %) is also supported by high pressure experiments (Schmidt and Poli, 1998), as well as by estimates of fluid fluxes released during shallow slab dehydration (Hyndman and Peacock, 2003; Savov et al., 2007).
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Scambelluri, M., Tonarini, S. (2012) Boron isotope evidence for shallow fluid transfer across subduction zones by serpentinized mantle. Geology 40, 907–910.
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If entrained with the subducted plate, this mélange zone could thus transport some additional water and incompatible elements (Ba, Rb, Cs) to sub-arc depths to sustain the water delivery into the arc and the back-arc basin (Savov et al., 2007; Scambelluri and Tonarini, 2012).
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Schmidt, M., Poli, S. (1998) Experimentally based water budgets for dehydrating slabs and consequences for magma generation. Earth and Planetary Science Letters 163, 361–379.
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Seawater is removed from the oceans during alteration of the oceanic plate, and it is released back to the surface during dehydration of the sinking plate, which ultimately generates subduction zone magmas (Schmidt and Poli, 1998).
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Extensive slab dehydration beneath forearcs (≥70 %) is also supported by high pressure experiments (Schmidt and Poli, 1998), as well as by estimates of fluid fluxes released during shallow slab dehydration (Hyndman and Peacock, 2003; Savov et al., 2007).
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Shaw, A.M., Hauri, E.H., Fischer, T.P., Hilton, D.R., Kelley, K.A. (2008) Hydrogen isotopes in Mariana arc melt inclusions: Implications for subduction dehydration and the deep-Earth water cycle. Earth and Planetary Science Letters 275, 138–145.
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Following this view, arc lavas from cold subduction zones should record greater involvement of water-rich slab fluids, while hot subduction zone arc magmas should be drier (Shaw et al., 2008; Walowski et al., 2015).
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Syracuse, E.M., van Keken, P.E., Abers, G.A. (2010) The global range of subduction zone thermal models. Physics of the Earth and Planetary Interiors 183, 73–90.
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The southern Marianas represent the southern end of the Izu-Bonin-Mariana (IBM) convergent margin, which has long been recognised as a typical example of a cold subduction system (slab age ∼150 Ma) (Müller et al., 2008; Syracuse et al., 2010).
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The young Australian slab (<34 Ma) (Davey, 1982) is now subducting along the MH Trench, so its slab would possess a warm to intermediate thermal structure (Syracuse et al., 2010).
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Ulmer, P., Trommsdorff, V. (1995) Serpentine Stability to Mantle Depths and Subduction-Related Magmatism. Science 268, 858–861.
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Serpentine group minerals and chlorite have the potential to carry large amount of water (8–13 wt. %) to at least ∼150 km depth (Ulmer and Trommsdorff, 1995; Marschall and Schumacher, 2012).
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van Keken, P.E., Hacker, B.R., Syracuse, E.M., Abers, G.A. (2011) Subduction factory: 4. Depth-dependent flux of H2O from subducting slabs worldwide. Journal of Geophysical Research: Solid Earth 116, B01401. doi: 10.1029/2010JB007922.
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Dehydration of the subducted plate is believed to be modulated by its thermal state (van Keken et al., 2011).
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Walowski, K.J., Wallace, P.J., Hauri, E.H., Wada, I., Clynne, M.A. (2015) Slab melting beneath the Cascade Arc driven by dehydration of altered oceanic peridotite. Nature Geoscience 8, 404–408.
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Following this view, arc lavas from cold subduction zones should record greater involvement of water-rich slab fluids, while hot subduction zone arc magmas should be drier (Shaw et al., 2008; Walowski et al., 2015).
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Supplementary Information
The Supplementary Information includes:
- Summary of Supplementary Information
- Sample Location
- Methods and Data Filtering
- End-member Compositions
- Effects of Magma Alteration and Degassing
- Figures S-1 and S-2
- Table S-1
- Supplementary Information References
Download Table S-1 (Excel).
Download the Supplementary Information (PDF).
Figures
Figure 1 Geochemical maps. (a) Location map. The red boxes show the expanded area of (b) for the Mariana and of (c) for Matthew-Hunter convergent margins. (b) Rb/Th geochemical map in the southern Mariana convergent margin, designed using www.geomapapp.org. The Fina-Nagu volcanic arc: FNVC, Santa Rosa Bank fault: SRBF. (c) Ba/Th geochemical map of the Matthew-Hunter intra-oceanic arc.
Figure 2 Geochemical features of the near trench magmas. (a) MgO vs. TiO2 diagram. The southern Mariana olivine-hosted melt inclusions are primitive basalts with a boninitic fingerprint. (b) Picture of an olivine hosting a melt inclusion taken under a binocular microscope. (c) Cs/Th and (d) Ba/Th vs. Rb/Th diagrams showing a stronger enrichment in water-rich slab fluids (as shown by the higher Cs/Th, Ba/Th and Rb/Th) in the near trench basalts, as compared to arc and back-arc basalts. (e) Rb/Th vs. 206Pb/204Pb diagram tracking the origin of the water-rich slab fluids. Serp.: serpentinised mantle, sed.: sediments. Composition of the end-members is reported in Table S-1.
Figure 3 Slab dehydration in the forearc and P-T conditions of mantle melt equilibrium. (a) H2O/Ce vs. Rb/Th diagram showing the stronger enrichment in water-rich slab fluids of the near trench magmas. The rough correlation between H2O/Ce and Rb/Th indicates that H2O/Ce also tracks the aqueous slab fluids. The different slope between the MH and SEMFR basalts likely reflect different slab lithology. (b) P-T conditions of the last primitive basaltic melt in equilibrium with the mantle (Lee et al., 2009
Lee, C.-T.A., Luffi, P., Plank, T., Dalton, H., Leeman, W.P. (2009) Constraints on the depths and temperatures of basaltic magma generation on Earth and other terrestrial planets using new thermobarometers for mafic magmas. Earth and Planetary Science Letters 279, 20–33.
). Basalts were filtered for MgO ≥ 6 wt. %. Fe3+/FeT = 0.25 was used for the arc basalts, and Fe3+/FeT = 0.17 for the back-arc basalts and the near trench basalts (Kelley and Cottrell, 2009Kelley, K.A., Cottrell, E. (2009) Water and the Oxidation State of Subduction Zone Magmas. Science 325, 605–607.
). An averaged water content of 1.90 ± 0.61 wt. % was used for the MH basalts, and of 2.53 ± 0.55 wt. % for the MH boninites. Error bars are 1σ standard deviation.
Figure 4 Enrichment in slab-derived water of the near trench magmas. (a) H2O content vs. slab depth (km). The averaged composition of the worldwide arc and back-arc magmas are from Ribeiro et al. (2015)
Ribeiro, J.M., Stern, R.J., Kelley, K.A., Shaw, A., Martinez, F., Ohara, Y. (2015) Composition of the slab-derived fluids released beneath the Mariana forearc: evidence for shallow dehydration of the subducting plate. Earth and Planetary Science Letters 418, 136–148. doi: 10.1016/j.epsl.2015.02.018.
. (b, c) Sketches illustrating the enrichment in water-rich slab fluids in the near trench magmas in MH and the southern Marianas (b), and the contribution of the subduction channel to the water supplied beneath the volcanic arc front in long lived subduction zones (c).