Limits...
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

Field appearance of(a) carbonatitic xenolith with an envelope of metasomatized peridotite, and (b) carbonatitic xenolith exhibiting a sharp boundary with the host basalt. (c) Carbonatitic xenolith containing abundant irregular blebs/cavities on the scale of micrometers to millimetres. (d) Micrometer-sized cavities with an envelope of calcite phenocrysts. (e,f) Resorption of pyroxenes by carbonate demonstrated by recrystallized pseudomorphs of calcite after pyroxene. CP1 = Calcite phenocrysts distributed randomly or around the silicate minerals. CP2 = Radial calcite phenocrysts around the cavity. CP3 = Clean calcite phenocrysts in the veins cross cutting CP2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4477412&req=5

f1: Field appearance of(a) carbonatitic xenolith with an envelope of metasomatized peridotite, and (b) carbonatitic xenolith exhibiting a sharp boundary with the host basalt. (c) Carbonatitic xenolith containing abundant irregular blebs/cavities on the scale of micrometers to millimetres. (d) Micrometer-sized cavities with an envelope of calcite phenocrysts. (e,f) Resorption of pyroxenes by carbonate demonstrated by recrystallized pseudomorphs of calcite after pyroxene. CP1 = Calcite phenocrysts distributed randomly or around the silicate minerals. CP2 = Radial calcite phenocrysts around the cavity. CP3 = Clean calcite phenocrysts in the veins cross cutting CP2.

Mentions: The Dalihu volcanic field, where the lherzolite, pyroxenite and carbonatitic xenoliths were collected, occurs in this southern accretionary zone (SI Fig. 1). The host volcanic rocks have chemical compositions of Mg-rich alkaline basalt (42.5–44.9 wt% SiO2; 9.2–12.3 wt% MgO) with variable and high volatile (LOI = 2.0–6.7 wt.%) and Na2O + K2O contents (1.6–4.3 wt%). The lherzolite xenoliths, typically 2–8 cm in diameter, are composed of medium- to coarse-grained olivine (Ol) + clinopyroxene (Cpx) + orthopyroxene (Opx) + spinel ± calcite. Some xenoliths show features of reaction with a carbonatitic melt, including growth of additional pyroxene. The carbonatite xenoliths are round, 4–30 cm in diameter, and most exhibit a sharp boundary with the host basalt. Some are surrounded by an envelope of metasomatized peridotite too thick to have resulted from solid-state diffusion (Fig. 1a,b), demonstrating the coexistence of carbonatite with peridotite, indicating that the carbonatite existed in the form of quickly solidified melt pockets within the mantle.


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)

Field appearance of(a) carbonatitic xenolith with an envelope of metasomatized peridotite, and (b) carbonatitic xenolith exhibiting a sharp boundary with the host basalt. (c) Carbonatitic xenolith containing abundant irregular blebs/cavities on the scale of micrometers to millimetres. (d) Micrometer-sized cavities with an envelope of calcite phenocrysts. (e,f) Resorption of pyroxenes by carbonate demonstrated by recrystallized pseudomorphs of calcite after pyroxene. CP1 = Calcite phenocrysts distributed randomly or around the silicate minerals. CP2 = Radial calcite phenocrysts around the cavity. CP3 = Clean calcite phenocrysts in the veins cross cutting CP2.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Field appearance of(a) carbonatitic xenolith with an envelope of metasomatized peridotite, and (b) carbonatitic xenolith exhibiting a sharp boundary with the host basalt. (c) Carbonatitic xenolith containing abundant irregular blebs/cavities on the scale of micrometers to millimetres. (d) Micrometer-sized cavities with an envelope of calcite phenocrysts. (e,f) Resorption of pyroxenes by carbonate demonstrated by recrystallized pseudomorphs of calcite after pyroxene. CP1 = Calcite phenocrysts distributed randomly or around the silicate minerals. CP2 = Radial calcite phenocrysts around the cavity. CP3 = Clean calcite phenocrysts in the veins cross cutting CP2.
Mentions: The Dalihu volcanic field, where the lherzolite, pyroxenite and carbonatitic xenoliths were collected, occurs in this southern accretionary zone (SI Fig. 1). The host volcanic rocks have chemical compositions of Mg-rich alkaline basalt (42.5–44.9 wt% SiO2; 9.2–12.3 wt% MgO) with variable and high volatile (LOI = 2.0–6.7 wt.%) and Na2O + K2O contents (1.6–4.3 wt%). The lherzolite xenoliths, typically 2–8 cm in diameter, are composed of medium- to coarse-grained olivine (Ol) + clinopyroxene (Cpx) + orthopyroxene (Opx) + spinel ± calcite. Some xenoliths show features of reaction with a carbonatitic melt, including growth of additional pyroxene. The carbonatite xenoliths are round, 4–30 cm in diameter, and most exhibit a sharp boundary with the host basalt. Some are surrounded by an envelope of metasomatized peridotite too thick to have resulted from solid-state diffusion (Fig. 1a,b), demonstrating the coexistence of carbonatite with peridotite, indicating that the carbonatite existed in the form of quickly solidified melt pockets within the mantle.

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