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First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths.

Liu Y, He D, Gao C, Foley S, Gao S, Hu Z, Zong K, Chen H - Sci Rep (2015)

Bottom Line: They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions.Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement.The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China.

ABSTRACT
Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ(18)OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

No MeSH data available.


Related in: MedlinePlus

Occurrence of the diamond in carbonatitic xenolith.(a) Polished section of sample DLH1114. (b) Microphotograph, (c) SEM image, and (d) close-up SEM image of the diamond.
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f3: Occurrence of the diamond in carbonatitic xenolith.(a) Polished section of sample DLH1114. (b) Microphotograph, (c) SEM image, and (d) close-up SEM image of the diamond.

Mentions: The carbonatitic xenoliths contain Opx, Cpx and calcite phenocrysts cemented by a fine-grained to cryptocrystalline carbonate matrix (Fig. 1d–f). Abundant irregular blebs, cavities or veins occur on the scale of micrometres to millimetres (Fig. 1c), which are generally surrounded or filled by calcite phenocrysts (Fig. 1d). The proportion of silicate minerals ranges from 1.5 to 15 vol.%, and the ratio of Opx/Cpx is 2 to 5. Crystals of feldspar (~20 μm), garnet (50–300 μm) and quartz (5–50 μm) were also identified. Rare tiny olivines (Fo = 91) were found in a few samples. Mg-Fe-silicate minerals were generally “resorbed” by carbonate as demonstrated by recrystallized pseudomorphs of calcite after pyroxene (Figs 1e,f). The carbonatitic xenoliths also contain small grains of diamond (approximately 20 μm), graphite (10–150 μm), moissanite (10–200 μm), titanium carbide (TiC; as inclusions in corundum), corundum (~50 μm), native metals (Si, Fe, Cu, Ni, Pt and Au), alloy phases (2–100 μm), pyrite and FeO (Figs 2 and 3). Graphite, moissanite, native metals and alloy phases were widely observed in the carbonatitic xenoliths.


First direct evidence of sedimentary carbonate recycling in subduction-related xenoliths.

Liu Y, He D, Gao C, Foley S, Gao S, Hu Z, Zong K, Chen H - Sci Rep (2015)

Occurrence of the diamond in carbonatitic xenolith.(a) Polished section of sample DLH1114. (b) Microphotograph, (c) SEM image, and (d) close-up SEM image of the diamond.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4477412&req=5

f3: Occurrence of the diamond in carbonatitic xenolith.(a) Polished section of sample DLH1114. (b) Microphotograph, (c) SEM image, and (d) close-up SEM image of the diamond.
Mentions: The carbonatitic xenoliths contain Opx, Cpx and calcite phenocrysts cemented by a fine-grained to cryptocrystalline carbonate matrix (Fig. 1d–f). Abundant irregular blebs, cavities or veins occur on the scale of micrometres to millimetres (Fig. 1c), which are generally surrounded or filled by calcite phenocrysts (Fig. 1d). The proportion of silicate minerals ranges from 1.5 to 15 vol.%, and the ratio of Opx/Cpx is 2 to 5. Crystals of feldspar (~20 μm), garnet (50–300 μm) and quartz (5–50 μm) were also identified. Rare tiny olivines (Fo = 91) were found in a few samples. Mg-Fe-silicate minerals were generally “resorbed” by carbonate as demonstrated by recrystallized pseudomorphs of calcite after pyroxene (Figs 1e,f). The carbonatitic xenoliths also contain small grains of diamond (approximately 20 μm), graphite (10–150 μm), moissanite (10–200 μm), titanium carbide (TiC; as inclusions in corundum), corundum (~50 μm), native metals (Si, Fe, Cu, Ni, Pt and Au), alloy phases (2–100 μm), pyrite and FeO (Figs 2 and 3). Graphite, moissanite, native metals and alloy phases were widely observed in the carbonatitic xenoliths.

Bottom Line: They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions.Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement.The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

View Article: PubMed Central - PubMed

Affiliation: State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Sciences, China University of Geosciences, Wuhan, 430074, China.

ABSTRACT
Carbon in rocks and its rate of exchange with the exosphere is the least understood part of the carbon cycle. The amount of carbonate subducted as sediments and ocean crust is poorly known, but essential to mass balance the cycle. We describe carbonatite melt pockets in mantle peridotite xenoliths from Dalihu (northern China), which provide firsthand evidence for the recycling of carbonate sediments within the subduction system. These pockets retain the low trace element contents and δ(18)OSMOW = 21.1 ± 0.3 of argillaceous carbonate sediments, representing wholesale melting of carbonates instead of filtered recycling of carbon by redox freezing and melting. They also contain microscopic diamonds, partly transformed to graphite, indicating that depths >120 km were reached, as well as a bizarre mixture of carbides and metal alloys indicative of extremely reducing conditions. Subducted carbonates form diapirs that move rapidly upwards through the mantle wedge, reacting with peridotite, assimilating silicate minerals and releasing CO2, thus promoting their rapid emplacement. The assimilation process produces very local disequilibrium and divergent redox conditions that result in carbides and metal alloys, which help to interpret other occurrences of rock exhumed from ultra-deep conditions.

No MeSH data available.


Related in: MedlinePlus