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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

(a) Ternary SiO2-CaO-MgO diagram comparing carbonatitic and peridotite xenoliths to natural carbonatites and sedimentary carbonate rocks. (b) Primitive mantle (PM)-normalized trace element patterns of the carbonatitic xenoliths. Averages of sedimentary limestone2324252627, continental (ConCarb) and oceanic carbonatite (OceCarb)3435 are shown for comparison.
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f4: (a) Ternary SiO2-CaO-MgO diagram comparing carbonatitic and peridotite xenoliths to natural carbonatites and sedimentary carbonate rocks. (b) Primitive mantle (PM)-normalized trace element patterns of the carbonatitic xenoliths. Averages of sedimentary limestone2324252627, continental (ConCarb) and oceanic carbonatite (OceCarb)3435 are shown for comparison.

Mentions: The bulk compositions of the carbonatitic xenoliths show variable CaO/SiO2 ratios, forming a linear array to the SiO2 side of the CaO-peridotite join, similar to impure limestones with a significant argillaceous component. This is more reminiscent of sedimentary limestones than carbonatites (Fig. 4a), except that their compositions may be influenced by reaction with the surrounding peridotites. Trace element patterns are also similar to sedimentary carbonate rocks, with a remarkable positive Sr anomaly but generally low rare earth element (REE), large ion lithophile element (LILE) and high field strength element (HFSE) contents (Fig. 4b). The incompatible trace element contents of most samples are even lower than the average of sedimentary carbonate rocks2324252627 (Fig. 4b). However, they have much higher Cr (51–1159 ppm) and Ni (56–620 ppm) contents than limestone (Cr = 5–37 ppm, Ni = 5–41 ppm2324), and CaO contents show a strong negative correlation with Cr and Ni contents (correlation coefficients are −0.94 for Cr and −0.95 for Ni, respectively; Fig. 5). They have heavy oxygen isotopic compositions with δ18OSMOW = 20.7–21.5.


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)

(a) Ternary SiO2-CaO-MgO diagram comparing carbonatitic and peridotite xenoliths to natural carbonatites and sedimentary carbonate rocks. (b) Primitive mantle (PM)-normalized trace element patterns of the carbonatitic xenoliths. Averages of sedimentary limestone2324252627, continental (ConCarb) and oceanic carbonatite (OceCarb)3435 are shown for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: (a) Ternary SiO2-CaO-MgO diagram comparing carbonatitic and peridotite xenoliths to natural carbonatites and sedimentary carbonate rocks. (b) Primitive mantle (PM)-normalized trace element patterns of the carbonatitic xenoliths. Averages of sedimentary limestone2324252627, continental (ConCarb) and oceanic carbonatite (OceCarb)3435 are shown for comparison.
Mentions: The bulk compositions of the carbonatitic xenoliths show variable CaO/SiO2 ratios, forming a linear array to the SiO2 side of the CaO-peridotite join, similar to impure limestones with a significant argillaceous component. This is more reminiscent of sedimentary limestones than carbonatites (Fig. 4a), except that their compositions may be influenced by reaction with the surrounding peridotites. Trace element patterns are also similar to sedimentary carbonate rocks, with a remarkable positive Sr anomaly but generally low rare earth element (REE), large ion lithophile element (LILE) and high field strength element (HFSE) contents (Fig. 4b). The incompatible trace element contents of most samples are even lower than the average of sedimentary carbonate rocks2324252627 (Fig. 4b). However, they have much higher Cr (51–1159 ppm) and Ni (56–620 ppm) contents than limestone (Cr = 5–37 ppm, Ni = 5–41 ppm2324), and CaO contents show a strong negative correlation with Cr and Ni contents (correlation coefficients are −0.94 for Cr and −0.95 for Ni, respectively; Fig. 5). They have heavy oxygen isotopic compositions with δ18OSMOW = 20.7–21.5.

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