The fate of carbon subducted to mantle depths remains uncertain, yet strongly influences the distribution of terrestrial carbon on geologic timescales. Carbon fluxes into subduction zones are exceptionally high where downgoing plates contain thick sedimentary fans. This study uses volcano geochemistry to assess sedimentary carbon recycling in the high-flux Makran subduction zone, where the Arabian plate subducts northward beneath Eurasia. On the basis of strontium isotope geochemistry and 40Ar–39Ar geochronology, I show that a portion of the submarine Indus Fan entered the Makran Trench, melted and ascended as magmas that erupted in southern Afghanistan. The resulting volcano, composed primarily of carbonate minerals, formed at approximately 3.8 million years ago. The 87Sr/86Sr ratios of the lavas indicate that their magmatic precursors were derived from marine sediments deposited at 28.9 ± 1.4 Ma. This implies that sedimentary carbon was subducted to and returned from mantle depths in less than 27 million years, indicating that magmas can efficiently recycle sedimentary carbon from subducting slabs to the overlying plate.
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The Ar–Ar geochronology and strontium isotope results are publicly available via EarthChem (https://doi.org/10.26022/IEDA/111960).
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R. Seal at the US Geological Survey granted access to the samples, which were collected by the US Geological Survey and the US Department of Defense Task Force for Business and Stability Operations personnel led by R. Tucker and E. King. The Afghanistan Geological Survey provided access to Soviet reports about the Khanneshin volcano that were essential for the fieldwork. The Afghanistan Ministry of Mines and Petroleum granted permission for sample collection. Combined Forces Special Operations Command - Afghanistan provided logistics and security support. At the time of sample collection, F.H. was an employee of TFBSO. J. Blusztajn and D. Miggins carried out the strontium isotope analyses and Ar–Ar geochronology, respectively. Discussions with S. Nielsen and G. Gaetani improved this paper. This project was supported by the Woods Hole Oceanographic Institution Independent Research & Development Program and a National Science Foundation grant (EAR number 1911699) awarded to F.H.
The author declares no competing interests.
Peer review information Nature Geoscience thanks Elsa Amsellem, Kwan-Nang Pang and Sabin Zahirovic for their contribution to the peer review of this work. Primary Handling Editor: Rebecca Neely.
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The core of the central vent consists mostly of sövite with abundant fenite xenoliths. Outer portions of the central vent are composed mostly of ankerite-barite carbonatite, inferred to be younger than the sövite because they host sövite xenoliths. Samples (white circles) were collected from two drainages that exit the massif to the northeast. This figure is a modified version of the geologic map published by ref. 27, which is based on ref. 57 and ref. 23.
a, Sawed surface image. The sedimentary clasts are dominantly K-feldspar. Bedding is defined by alternating sandstone and siltstone layers, of which the latter appear darker because they have more interstitial Fe-oxide. The rock is brecciated and crosscut by calcium carbonate veins. B, Closer inspection of a carbonate vein reveals anastomosing networks of smaller veins extending into the sandstone. Beneath the large vein is a porous metasomatized region where cavities are partially filled by carbonate minerals, including minor amounts of REE-carbonates. C, Thin section photomicrograph. Along calcium carbonate vein margins are secondary Fe-Mn-oxides (black).
a, KHAN-1, hand sample image. Medium- to coarse-grained calcite carbonatite contains fine-grained biotite. Three small fenite xenoliths are visible in the upper left. b, KHAN-1, PPL. Small fenite xenoliths, like the one imaged here, are composed of biotite. They share sharp and diffuse boundaries with the surrounding calcite matrix. c, KHAN-2, sawed surface image. Larger fenite xenoliths are highly brecciated and crosscut by calcite veins. d, KHAN-2, XPL. Large twinned calcite crystal with Fe-oxide and biotite inclusions. Contact twinning separates the upper and lower portions of the crystal, both of which have subtle lamellae twins. e, KHAN-2, sawed surface image. Large fenite xenoliths have brecciated textures and are crosscut by multiple generations of veins. The white veins are calcite and the green vein, bound by dashed lines, is mostly apatite. f, KHAN-2, XPL. Apatite also forms clusters with biotite in the calcite matrix of sövite samples. g, KHAN-2, XPL. Large brecciated fenite xenoliths have biotite-rich zones adjacent to calcite veins and fine-grained interiors composed of biotite and K-feldspar. h, RT-10K-09, hand sample image. This sövite contains large (>1 cm) phlogopite books intergrown with coarse calcium carbonate. i, RT-10K-09, XPL. Calcite twin lamellae are visible in this sample. Mineral abbreviations: ap = apatite, bio = biotite, cc = calcium carbonate, kfs = K-feldspar. Hand sample and sawed surface images were taken on a stereomicroscope. Thin section images were taken on a polarizing microscope under plain polarized light (PPL) or cross polarized light (XPL).
Extended Data Fig. 4 Mineralogy and petrology of representative ankerite-barite carbonatite samples.
a, FH-10K-08, sawed surface image. Lath-shaped intergrowths of barium, calcium carbonate, and ankerite form the matrix of this carbonatite. Fe-Mn oxides are present and REE-carbonates line the walls of cavities. b, FH-10K-08, PPL. The complex textures of the barium-calcium carbonate intergrowths—perhaps pseudomorphs after witherite—can be observed. Tetraferriphlogopite also exists in this sample as a minor phase. c, RT-10K-03, sawed surface image. Ankerite-barite carbonatites also contain fenite xenoliths, as shown here. REE carbonates appear yellow. d, RT-10K-03, PPL. Ankerite-barite carbonatites exhibit varied textures. Here, barite (outlined with a dashed line) and ankerite are surrounded by a finer-grained matrix of calcium carbonate. e, RT-10K-11, sawed surface image. Intergrowths of ankerite and Fe-oxides often form clusters. f, RT-10K-11, PPL. Tabular barite crystals occur in intimate association with ankerite and Fe-oxide. g, RT-10K-07, sawed surface image. Some samples are relatively homogeneous on the cm scale. Here, ankerite (grey) surrounds intergrowths of barite (white) and calcium carbonate (also white). Yellow regions contain REE minerals hosted in cavities. h, RT-10K-07, PPL. Tetraferriphlogopite is sometime rimmed by REE-carbonate minerals and tends to be associated with barite-calcium carbonate intergrowths. i, RT-10K-03, XPL. Solitary fluorite crystals (isotropic) exist in some ankerite-barite carbonatites and are rimmed by calcium carbonate. Mineral abbreviations: ank = ankerite, ba = barite cc = calcium carbonate, fl = fluorite, tfp = tetraferriphlogopite, REE = rare earth element carbonate minerals.
a, RT-11K-4A0, sawed surface image. Aggregates of yellow REE-carbonates in a matrix of ankerite, barite, and calcite. Clusters of REE-carbonate and strontianite form lighter yellow clusters. b, RT- 11K-4A0, XPL. Contact between REE-carbonate aggregates and ankerite-barite-calcite matrix. c, RT-11K-5B3B, sawed surface image. Ankerite (dark brown patches) plus calcium carbonate (grey) zones alternate with barite, strontianite, and REE-carbonate aggregates. d, RT-11K-5B3B, XPL. Spherulitic acicular strontianite occurs in association with ankerite, calcium carbonate, and barite. e, RT-11K-2B, XPL. In some cases, subhedral domains of barite and ankerite are rimmed by zones of ankerite, strontianite plus REE-carbonates, and apatite. f, RT-11K-5B6C, XPL. Mn oxides occur as veins and clusters. Here, tabular Mn-oxide in a vein contains interstitial calcium carbonate, which grades into intergrowths of strontianite and REE-carbonate minerals. Mineral abbreviations: ank = ankerite, ba = barite, cc = calcium carbonate, REE = rare earth element carbonate minerals, str = strontianite.
a–c, Step heating analyses of RT-10K-09 phlogopite aliquots 1–3, respectively.
a, The trace element concentrations in the Khanneshin carbonatite samples vary by roughly 1–2 orders of magnitude for each element (grey shaded region). In general, sövite samples are less enriched than ankerite-barite carbonatites. Colored lines represent mean compositions for Khanneshin sövites and ankerite-barite carbonatites with (“REE”) and without (“A-B C”) abundant REE minerals. b, The modeled composition of carbonatitic melt derived from subducted Makran sedimentary material—assumed to equal average Indus Fan sediments58—and Khanneshin carbonatite samples (grey shaded region) have similar trace element patterns. The model assumes that 30% melting of subducted sediments produced a carbonated silicate magma that separated into immiscible carbonatitic and silicate magmas during ascent. See Supplementary Information text for details. All values are normalized to primitive mantle59.
Sample descriptions and trace element model.
The complete 40Ar–39Ar geochronology report received from Oregon State University Argon Geochronology Laboratory for RT-10K-09 phlogopite aliquot 1.
The complete 40Ar–39Ar geochronology report received from Oregon State University Argon Geochronology Laboratory for RT-10K-09 phlogopite aliquot 2.
The complete 40Ar–39Ar geochronology report received from Oregon State University Argon Geochronology Laboratory for RT-10K-09 phlogopite aliquot 3.
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Horton, F. Rapid recycling of subducted sedimentary carbon revealed by Afghanistan carbonatite volcano. Nat. Geosci. 14, 508–512 (2021). https://doi.org/10.1038/s41561-021-00764-7
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