Helium Isotopic Compositions of Antarctic High-Mg Rocks Related to the Karoo Continental
Flood Basalts: Evidence for a Depleted Upper Mantle Source?
JUSSI S. HEINONEN
Finnish Museum of Natural History, University of Helsinki, Finland
MARK D. KURZ
Woods Hole Oceanographic Institution, MA, United States
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 (Ra), 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
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He, 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) Ra. This value is indistinguishable from those measured from
Southwest Indian Ridge MORBs (Ra = 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.