Constraints on the noble gas composition of the Icelandic plume source by laser analyses of individual vesicles in the volcanic glass DICE 11

Abstract:

Models of Earth’s volatile acquisition and evolution attempt to reproduce the current noble

gas abundances and isotopic composition of the mantle reservoirs. The volatile composition

of the OIB reservoir - assumed to preserve a higher proportion of primordial noble gases than

the degassed MORB reservoir – is a strong constraint for those models. However, the correct

values of the neon and argon isotopic ratios in OIBs are still a subject of debate, because of

the contamination of the samples by air-derived noble gases.

Although there is no consensus on the origin of this contamination – is it empty vesicles or

cracks in volcanic glasses filled with seawater; air dissolution in the magma at the timing of

magma eruption; assimilation of oceanic crust in the magma chamber?- targeting directly

with a laser the vesicle to analyse in volcanic glasses is an efficient way to reduce this

contamination.

Here we present analyses of individual vesicles of an Icelandic volcanic glass, DICE 11,

that was extensively studied in the past by crushing pieces of the volcanic glass under

vacuum, because it was considered to have a pure plume origin. The mm-sized sample was

imaged tomographically with a 5μm resolution. For opening bubbles, we used a 193nm

Excimer laser to avoid diffusion of noble gases by local heating. CO2 contents were estimated

by pressure measurement in the laser cell using a sensitive manometer. We analysed He and

Ar isotopes, plus 22Ne abundance on a Helix SFT mass-spectrometer. We also present new

He, Ne and Ar compositions obtained by step crushing on similar samples (DICE 10 and

DICE 11).

3He/4He isotopic ratios are homogeneous in all the vesicles and consistent with analyses

by crushing, about 18Ra. Precise 40Ar/36Ar isotopic ratios were obtained on the largest

vesicles only, due to high blank contribution to the smallest vesicles, and are about 9000,

i.e. the highest values obtained by step-crushing. Considering that the Ar and He isotopic

compositions obtained on individual vesicles represent the mantle source values, not

contaminated by air, we can precisely correct the Ne isotopic analyses obtained by crushing

from air contamination; the corrected values are consistent with the value of neon-B, i.e.

favour the hypothesis that the neon in OIB has an irradiated meteorite origin rather than

purely solar.