Carbon speciation and mantle metasomatism

Steven Creighton, T. Stachel, R. W. Luth

Research output: Contribution to journalMeeting abstractpeer-review

Abstract

Three intensive parameters constrain the speciation of carbon in the Earth’s mantle: temperature, pressure and oxygen fugacity (fO2). The conditions of diamond stability are bounded in pressure and temperature by the diamond–graphite equilibrium curve. The upper fO2 limit for stability of elemental carbon in peridotite is constrained by the reaction: Enstatite + Magnesite = Forsterite + C + O2 (EMOD/G) (Eggler and Baker, 1982). In P-T-fO2 space the intersection of these planar boundaries defines three volumes of mantle carbon speciation—diamond stability at high pressure and low fO2, graphite stability at low pressure and low fO2, carbonate stability at high fO2.

We calculated the fO2of peridotite xenolith samples from the Kimberley Mines, South Africa based on ferric iron concentrations in pyropic garnets obtained by adapting a recently developed electron microprobe technique (Höfer, 2000) to garnets with low total Fe concentrations. The fO2 of these rocks ranges from ∼5 log units below the fayalite-magnetite-quartz reference buffer (FMQ) to greater than FMQ-1. Kimberley peridotite samples are present in each of the three carbon speciation volumes.

The oxidation state of the peridotite samples is correlated with trace element evidence for mantle metasomatism. Three distinct chondrite-normalized rare earth element (REEN) patterns are observed in garnets from the studied Kimberley xenoliths—sinusoidal, humped and normal. The sinusoidal pattern indicates metasomatic re-enrichment by an agent with a high LREEN to HREEN ratio. Humped patterns have a positive slope from LREEN to MREEN with a plateau at ∼10–20 times chondritic abundance from Sm to Gd followed by a negative slope to the HREEN. Normal patterns show a positive slope in the LREEN towards flat MREEN to HREEN at ∼10 times chrondritic abundance. The Gd/Sm ratio is a measure of the degree of sinuosity of REEN patterns. The GdN/SmN ratio is <1 for sinusoidal patterns, ∼1 for humped patterns, and >1 for normal patterns. The most reduced samples have the lowest Gd/Sm ratio and therefore, have garnets with sinuous REEN patterns. The humped and normal patterns are observed in garnets from the more oxidized lherzolites, including those that lie in the carbonate stability volume. This suggests that metasomatic re-enrichment of the lithospheric mantle beneath Kimberley lead to oxidation of the lithosphere and destruction of diamond.
Original languageEnglish
Pages (from-to)A117-A117
Number of pages1
JournalGeochimica et Cosmochimica Acta
Volume70
Issue number18, Supplement
DOIs
Publication statusPublished - 2006
Externally publishedYes
EventGoldschmidt Conference (16th : 2006) - Melbourne, Australia
Duration: 27 Aug 20061 Sep 2006

Cite this