Geochemical evidence for mélange melting in global arcs

Abstract

In subduction zones, sediments and hydrothermally altered oceanic crust, which together form part of the subducting slab, contribute to the chemical composition of lavas erupted at the surface to form volcanic arcs. Transport of this material from the slab to the overlying mantle wedge is thought to involve discreet melts and fluids that are released from various portions of the slab. We use a meta-analysis of geochemical data from eight globally representative arcs to show that melts and fluids from individual slab components cannot be responsible for the formation of arc lavas. Instead, the data are compatible with models that first invoke physical mixing of slab components and the mantle wedge, widely referred to as high-pressure mélange, before arc magmas are generated.

Keywords: Nd isotopes; Sr isotopes; Subduction zone; mantle melting; mass transfer; melange; slab-mantle mixing.

Fig. 1. Illustration of the two different end-member models of slab material transport in subduction zones.

(A) In the conventional model, sediment melts and fluids from AOC, which both display fractionated trace element signatures, are sourced directly beneath the arc volcano, where they percolate rapidly to the region of melting. Here, they mix with ambient mantle melts to form arc magmas. (B) In the mélange model, sediments, AOC, and hydrated mantle physically mix to form hybrid mélange rocks. The mélange subsequently rises as diapirs into the mantle wedge and melts to form arc magmas with fractionated trace element signatures. The critical difference between the two models is that mixing and trace element fractionation for the two models occur in reverse order of each other, which will generate different isotopic mixing relationships. Illustration not to scale.

Fig. 2. Mixing diagrams between sediments and mantle for Tonga, Marianas, and Lesser Antilles arcs.

Plots of Sr isotopes against Nd isotopes (A to C) and Nd/Sr ratio (D to F) for lavas from the Mariana (A and D), Tonga (B and E), and Lesser Antilles (C and F) arcs. Literature data from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/) and only recent subaerial extrusive lavas with <62% SiO₂ have been included to avoid effects from fractional crystallization and assimilation (see the Supplementary Materials for details). Details of the mantle, sediment, and AOC end-member compositions can also be found in the Supplementary Materials. Mixing lines between the mantle and bulk sediment (black bold lines), 1% partial sediment melts (black dashed lines), and 20% partial sediment melts (black dotted lines) show different curvature, because Nd/Sr ratios fractionate strongly during sediment partial melting (8, 14). Additional mixing lines between the mantle and AOC fluids (pink bold lines) and 1% partial sediment melts and 1% by weight AOC fluids (pink dashed lines) are also shown for arcs, where the AOC component is constrained (no AOC data exist for the Tonga arc). Tick marks on individual mixing curves indicate the amount of bulk sediment or sediment melt that is added to the mantle in weight percent. The partition coefficients for Sr and Nd during sediment melting were set to D_Sr = 7.3 and D_Nd = 0.35, respectively, which represent the average values recorded in sediment melting experiments by Hermann and Rubatto (14) over the temperature range of 750° to 900°C. Partition coefficients for AOC fluids were set to D_Sr = 2 and D_Nd = 0.15, which represent the values found for 800°C and 4 GPa (13). Arrows depict the effect on Nd/Sr and/or Nd and Sr isotopes during mélange partial melting (purple graded field), AOC fluid addition (gray arrow), and plagioclase fractional crystallization (brown arrow). Last, we also show a yellow shaded area, which depicts arc lava compositions that cannot be explained via addition of AOC fluids and sediment melts to the mantle wedge. The Nd/Sr ratios of AOC fluids in (D) and (F) are not quantified but conservatively inferred close to 0 to explore the largest possible effect from AOC fluid addition (see text for details).

Fig. 3. Plot of Nd isotopes versus Hf/Nd ratios for lavas from the Tonga arc.

(A) Literature data are from the GEOROC database (http://georoc.mpch-mainz.gwdg.de/georoc/). Mixing lines between the mantle and bulk sediment (bold lines) and between the mantle and sediment melts (dotted lines) are shown as examples. The yellow bar illustrates the range of sediment melts possible for sediment melting down to a degree of as little as 1%. Sediment melts were computed by using relative partition coefficients of Nd and Hf (D_Nd/D_Hf = 0.9 to 4.3) in accordance with the experimental range observed (14). Both Hf and Nd are immobile in AOC fluids (13), which render sediments the most likely source of variation in Hf/Nd. Therefore, no fluid component is plotted in the figure. Schematic illustrations of expected trends when (B) sediment melting is followed by mixing and (C) mélange formation is followed by melting are also shown.