HETEROGENEOUS MANTLE ARGON ISOTOPE COMPOSITION IN THE SUBCONTINENTAL LITHOSPHERIC MANTLE BENEATH THE RED SEA REGION

Abstract

We performed argon measurements on rocks of the subcontinental lithospheric mantle (SCLM) from the Red Sea region applying stepwise crushing extractions, and argon analyses using high-resolution stepwise heating techniques, the latter combined with 40Ar/39Ar dating. Maximum 40Ar/36Ar ratios of individual samples range from 4190 ± 240 to 25,590 ± 620, the latter (sample SA 87-2/12) being the highest measured value of any mantle peridotite or pyroxenite sample so far. For five samples, reasonably defined binary mixing correlations between atmospheric and mantle-derived argon and neon resulted in calculated sample-specific mantle-40Ar/36Ar ratios of 5040 ± 50, 7500 ± 70, 9510 ± 220, 29,600 ± 2700, and 39,000 ± 2500 at an assumed mantle-20Ne/22Ne value of 12.5. These observations indicate a highly heterogeneous argon isotopic composition of the trapped mantle fluids. Furthermore, most samples exhibit mantle-36Ar/22Ne ratios higher than considered to represent an unfractionated mantle source. These characteristics could be explained by mixing of plume-related (low 40Ar/36Ar) and lithospheric (high 40Ar/36Ar) melts or fluids following elemental fractionation between Ar and Ne. Alternatively, we propose the presence of a severely fractionated atmospheric component highly enriched in argon, admixed to the mantle fluids before trapped by the ultramafic rocks. This primary, seriously fractionated atmospheric component cannot be monitored by Ne isotopes. Incorporation of such a primary atmospheric component could occur either during magma ascent at relatively shallow, upper crustal levels, or by mobilization of atmospheric fluids in a subduction-related metasomatized lithospheric mantle during Pan African times (c. 600–800 Ma ago).