Helium Isotopic Compositions of Antarctic High-Mg Rocks Related to the Karoo Continental Flood Basalts: Evidence for a Depleted Upper Mantle Source?

Abstract:

The isotopic composition of helium is often considered to be one of the key elements in resolving deep mantle plume vs. upper mantle origin of hotspot-related volcanic rocks. High ³He/⁴He values, greater than 10 times atmospheric (R_a), are generally thought to indicate plume-related sources in the lower mantle. The use of helium isotopes in continental flood basalt (CFB) provinces has been limited by the lack of fresh rock material, poor exposures, time-integrated ingrowth of radiogenic ⁴He, and strong lithospheric overprinting.

Vestfjella mountain range at western Dronning Maud Land, Antarctica, is comprised of lava flows and intrusive rocks that belong to the Jurassic (~180 Ma) Karoo continental flood basalt province, the bulk of which is exposed in southern Africa. The Karoo CFBs and related rocks show strong lithospheric influence in their geochemistry in general, but some high-Mg dikes from Vestfjella show geochemical evidence of derivation from sublithospheric sources.

In an attempt to determine the first estimate for the helium isotopic composition of the Karoo mantle sources, we performed He isotopic measurements on six primitive Vestfjella dike samples collected from variably exposed nunataks. Helium was extracted by in-vacuo stepwise crushing and melting of separated and carefully hand-picked olivine phenocrysts (Ø = 0.25–1 mm; ~10 000 grains in total; abraded and unabraded fractions).

The results of coupled crushing and melting measurements show evidence of both cosmogenic and radiogenic helium contributions within the olivines (i.e. by having high He contents and anomalously low or high ³He/⁴He released by melting), which complicates interpretation of the data. As a best estimate for the mantle isotopic composition, we use the sample with the highest amount of He released (> 50%) during the first crushing step of an abraded coarse fraction, which gave ³He/⁴He of 7.03 ± 0.23 (2σ) R_a. This value is indistinguishable from those measured from Southwest Indian Ridge MORBs (R_a = 6.3‒7.3), the modern successor of the Africa-Antarctica rift zone. These new helium data are compatible with previous trace element and Sr, Nd, Pb, and Os isotopic analyses that indicate the involvement of a depleted upper mantle source in Karoo CFB petrogenesis.