Thermochemical evolution of the sub‐arc mantle due to back‐arc spreading

Abstract

[1] We present the results of a series of numerical geodynamic experiments designed to characterize the thermal and compositional evolution of the sub-arc mantle in response to spreading in the back-arc. We find large changes in both the temperature and composition of the sub-arc mantle with time as the BASC migrates away from the arc. In particular, the sub-arc mantle becomes increasingly more depleted with time following the onset of spreading, as mantle that has experienced decompression melting and melt extraction beneath the BASC is gradually drawn beneath the arc plate by slab-induced corner flow. The rate at which this depletion increases during the $2 Myr immediately following the onset of spreading is controlled by the spreading rate at the BASC, with faster spreading leading to a more rapid increase in depletion. Following this initial period, depletion within the sub-arc mantle continues to increase at a somewhat slower pace. During this phase, the rate at which depletion increases is chiefly dictated by the subduction rate, with faster subduction leading to a more rapid increase in depletion beneath the arc. Depletion within the sub-arc mantle is also found to increase with increasing mantle potential temperature, decreasing age of the overriding plate, and decreasing distance between the initial location of the BASC and the arc. Predicted changes in the depletion of the sub-arc mantle with time are shown to be consistent with observations of systematic along-strike geochemical variations within a portion of the Tonga Arc adjacent to the Eastern Lau Spreading Center.Citation: Hall, P. S., L. B. Cooper, and T. Plank (2012), Thermochemical evolution of the sub-arc mantle due to back-arc spreading,

Evolution of Magmatism in the New Hebrides Island Arc and in Initial Back‐Arc Rifting, SW Pacific

“…The high convergence rate of 120 mm/year at the South NHIA (Baillard et al, 2018) probably causes a fast mantle flow that may transport the depleted mantle into the melting region beneath the arc within a few 100,000 years. Models suggest that some 5% depletion of the mantle occurs within 1 million years of back-arc magmatic activity (Hall et al, 2012). Consequently, the inflow of heterogeneous North Fiji Basin mantle, and its melting beneath Vate Trough, means that the mantle delivered to beneath the South NHIA arc front is more depleted in incompatible elements than it would be otherwise.…”

Iron isotopic evidence for convective resurfacing of recycled arc-front mantle beneath back-arc basins

“…The resurfacing of depleted/metasomatized wedge material in the Lau Basin implies that wedge mantle recycling is an effective process to preserve depleted mantle sections by rotation in a recycling loop. In addition to depletion by back‐arc melting and convective transportation toward the arc front [ Hall et al ., ], this may explain why some IAB sources are depleted in incompatible elements by multiple phase melt extraction, as e.g., in high‐field strength elements (HFSE), compared to their MORB counterparts, [ Pearce and Peate , ; Woodhead et al ., ]. Even though some arc sources may retain their fertility [ Thirlwall et al ., ], likely depending on rehomogenization during wedge convection or age of the subduction zone, the depletion in HFSE in most arc sources may thus relate to prolonged melt extraction in a maturing and partially recycled local mantle.…”

“…It is long known that most arc melts are sourced from a wedge with a multiple depletion history [Woodhead et al, 1993]. Loci and time scales of prior melting events remain enigmatic but are commonly attributed to preconditioning by melting at back-arcs and subsequent mantle advection [Hall et al, 2012;Kincaid and Hall, 2003;McCulloch and Gamble, 1991]. Depending on slab geometry, subduction speed and local temperature gradients, dehydration of the slab occurs at 100 to up to 400 km surface distance from the trench [van Keken et al, 2011], as evidenced by the distribution of arc magmas.…”

Mantle dynamics beneath the Pacific Northwest and the generation of voluminous back‐arc volcanism

“…Geodynamic models have long been employed to investigate melt production in subduction zones [e.g., Spiegelman and McKenzie , 1987; McCulloch and Gamble , 1991; Hebert et al , 2009]. Decompression melting is related to upwelling rates; mantle upwelling has been shown to result from the initiation of subduction [ Faccenna et al , 2010], differences in the style of slab sinking [ Kincaid and Sacks , 1997; Kincaid and Hall , 2003; Kincaid and Griffiths , 2004], segmentation or breakup of the slab [ Liu and Stegman , 2011, 2012], and extension within the overriding plate [ Ribe , 1989; Conder et al , 2002; Kincaid and Hall , 2003; Hall et al , 2012]. Subduction‐driven upwelling in the PNW has been proposed and modeled to explain specific features of volcanic surface expressions [ Faccenna et al , 2010; Liu and Stegman , 2012].…”