De Leeuw, G.A.M., Hilton, D.R., Murton, B.J., Taylor, R.N., 2006: Source mixing, degassing and crustal contamination of basaltic glasses along Reykjanes Ridge, AGU fall meeting, San Francisco:
We report new stepped heating Ne and Ar abundance and isotope data for 36 basaltic glasses erupted along the Reykjanes Ridge between latitudes 57.5° and 63°N. The aim of this study is to integrate the new Ne and Ar data with existing CO2-H2O-He abundance and isotope data obtained on the same samples. We compare two segments on the ridge: 1) north of latitude 61.5°N at eruption depths < 775 m where H2O concentrations are 0.3 – 0.4 wt.%, and 2) between 57.5° and 61.5°N at eruption depths of 620 – 2060 m where H2O contents are consistently ~0.2 wt.%.
Neon south of 61.5°N reaches up to 1.5 × 10-9 cm3 STP/g for the vesicle-sited gas (released at 800°C) whereas the maximum [Ne]ves north of 61.5°N is 8.8 × 10-10 cm3 STP/g: however, in both cases, [Ne]ves >> [Ne]dis (dis = dissolved and released at 1200°C). The 20Ne/22Ne ratio ranges between air-like values (9.8) and 10.7 for both vesicle- and dissolved-sited gas with no systematic variation along the ridge. The 21Ne/22Ne ratio ranges between air-like values (0.029) and 0.040 for both vesicle- and dissolved-sited gas with an increase towards the south. In three-isotope neon space, the samples north of 61.5°N mostly lie along the mass fractionation line, whereas samples erupted to the south fall between the solar-neon and MORB trajectories. Argon north of 61.5°N reaches up to 1.3 × 10-7 cm3 STP/g for the vesicle-sited gas whereas to the south the maximum [Ar]ves is 3.2 × 10-6 cm3 STP/g. Again, in both cases, [Ar]ves >> [Ar]dis. The 40Ar/36Ar ratios for both vesicle- and dissolved-sited gas are near atmospheric values (295 – 360) north of ~61.5°N but distinctly higher (up to 1500) further south.
The lower [Ne] and [Ar] and atmospheric-like isotope ratios north of 61.5°N are consistent with enhanced magmatic degassing/atmospheric contamination closer to Iceland. To the south, there is a positive relationship between both CO2/40Ar* and 4He/40Ar* versus CO2/3He. This correlation is in contrast to the expected (negative) degassing trend for tholeitic magmas. In addition, the CO2/40Ar* ratios are two orders of magnitude higher (2.2 × 107) than typical MORB-like values (40,000) implying admixture with a CO2-rich component. We interpret our data south of 61.5°N as mixing between a low CO2/3He (~109) endmember distal to Iceland with a higher CO2/3He (>1010) endmember increasingly prominent closer to Iceland. We propose that the high CO2/3He endmember represents the influence of altered oceanic crust, thicker towards Iceland, superimposed on a degassed magmatic signature.