De Leeuw, G.A.M., Hilton, D.R., Shaw, A.M., Murton, B.J., Taylor, R.N., 2002: Testing binary mixing models for lavas erupted along the Reykjanes Ridge; insights from C-He relationships, AGU fall meeting, San Francisco:
We report new CO2 abundance and isotope data for 36 basalt glasses erupted along the Reykjanes Ridge between latitudes 57.5 and 63°N. Lavas can be divided into (a) water-rich samples (~0.4 wt.%), erupted at depths < 775m north of 61.5°N, and (b) samples with water ~0.2 wt.%, erupted at depths of 620 – 2060 m and located between 57.5 and 61.5°N. Based upon He-Pb isotope systematics [Hilton et al. (2000)], deeper samples (category b) lie along binary mixing trajectories between plume-like (3He/4He ~ 30RA; 206Pb/204Pb ~ 18.7) and MORB-like endmembers (3He/4He ~ 8RA; 206Pb/204Pb ~ 18.0). Shallow samples (category a) do not fall on mixing trajectories: consistent with volatile loss followed by addition of a crustal contaminant, resulting in lower 3He/4He ratios. The aim of this study is to test whether binary mixing trends are observed using C-He relationships.
All samples were analyzed using incremental heating techniques which allows for resolution of vesicle-sited CO2 from CO2 dissolved within the glass matrix. Results show that samples north of 61.5°N (category a) have low CO2 contents in both the vesicle (2 - 37 ppm) and dissolved (15 - 61 ppm) phases. The isotopic composition of the CO2 varies between –8 and –34‰ (vesicle) and –6 and –10‰ (glass). The combined effect of low CO2 concentrations and low δ13C values are consistent with extensive gas loss ± contamination of volatile-poor magmas with an isotopically-light C component. In contrast, samples in category b have significantly higher CO2 abundances (vesicles: 7 - 318 ppm; glass: 9 - 200 ppm) and higher and less variable δ13C values (vesicles: –5 to –26‰; glass: –4 to –11‰). This suggests that category b samples have not been subjected to the same degree of degassing and/or contamination as samples in category a.
By combining the vesicle-sited CO2 abundances with He-contents determined by crushing [Hilton (op. cit)], CO2/3He ratios for the vesicle phase can be derived. We observe high ratios (3 × 109 to 2 × 1010) in the more degassed category a samples. Category b CO2/3He ratios show a trend from low CO2/3He values (3 × 108) and MORB-like 206Pb/204Pb to high CO2/3He values (up to 2 × 1010) and more radiogenic 206Pb/204Pb. There are two possible explanations for the observed trends: 1) degassing followed by contamination with a high CO2/3He crustal component. This process controls C-He relationships in low concentration (highly degassed) samples close to Iceland. 2) mixing between a MORB-like source (CO2/3He ~ 2 × 109) and an enriched source with a higher initial CO2/3He value. This process controls samples in category b.