C-He systematics in hotspot xenoliths: Implications for mantle carbon contents and carbon recycling

Abstract

We have measured the carbon and helium abundances and isotopic compositions of high-pressure carbon dioxide fluid inclusions in ultramafic xenoliths from oceanic hotspot volcanos to examine the extent to which He and C are separated by igneous processes, and to determine whether or not the ‘undegassed’ isotopic character of hotspot helium extends to carbon. These measurements place limits on upper mantle C contents and the on the fate of carbon recycled at subduction zones. Xenolith fluid inclusions from the Loihi-Hawaii, Re´union and Kerguelen hotspots exhibit carbon isotopic compositions (δ 13C= −1.6to−10.8‰) similar to, although somewhat more variable, than MORB. The greater larger range can be ascribed to physicochemical processes associated with the volcanic systems and there is no evidence that hotspots and ridges tap isotopically distinct carbon sources. CO 2 and He abundances vary by more than three orders of magnitude (1–260 ppm C and 10 −9–10 −6 cc STP He/g) and are strongly correlated. The samples' C/ 3He ratios (2–20 × 10 9) largely overlap with mid-ocean ridge basalt values (1–7 × 10 9). The small overall scatter of C/ 3He ratios argues against residual carbon phases during melting and against large-scale diffusive He transport in the mantle. In contrast to these similarities, most hotspot xenoliths and basalts with high 3He/ 4He ratios have higher C/ 4He ratios (4–40 × 10 4, uncorrelated with C or He abundance) than mid-ocean ridge basalts (0.5–7 × 10 4). Given the similarity in C/ 3He ratios, the lower C/ 4He ratios of MORB with respect to hotspot sources must be produced by radiogenic 4He production. The measured C/ 4He ratios suggest that the upper mantle carbon content must be less than 500 ppm C and is probably in the range 50–250 ppm C. The upper limit relies only on the observed MORB C/ 4He ratios, a maximum mantle U concentration of 26 ppb, and the conclusion from helium isotopic compositions that most upper mantle 4He must be radiogenic rather than trapped primordial 4He. The lower C range relies on the dynamics of the two-reservoir mantle model proposed by Kellog and Wasserburg [1] to explain ridge-hotspot He isotopic differences, and on the hypothesis that the upper (ridge source) and lower (hotspot source) mantle reservoirs had similar initial C/ 4He and 3He/ 4He ratios. Within the framework of an initially homogeneous mantle which has differentiated into two reservoirs, significant carbon recycling to the upper mantle is inconsistent with the similarity of hotspot and ridge C/ 3He ratios. However, recycling of carbon to the lower mantle is consistent with C-He systematics and can account for both the similarity of ridge and hotspot carbon isotopic compositions and the low exospheric C inventory.