Heavy noble gas signatures of the North Atlantic Popping Rock 2ΠD43: Implications for mantle noble gas heterogeneity

Abstract

Heavy noble gas (Ne, Ar, Xe) isotopic compositions provide powerful constraints on the nature of mantle heterogeneities. The North Atlantic popping rock sample 2ΠD43 is unusually gas-rich and has long formed the basis of our understanding of the noble gas composition of the mid-ocean ridge basalt (MORB) mantle source. Here we present new high-precision He, Ne, Ar and Xe isotopic compositions as well as He, Ne, Ar and Xe abundances measured in a split of the 2ΠD43 sample. After correcting the measured values for syn- to post-eruptive atmospheric contamination, we estimate a mantle source 21Ne/22Ne of 0.0590 ± 0.0002, 40Ar/36Ar of 23,900 ± 100 and 129Xe/130Xe of 7.41 ± 0.02 for 2ΠD43. Significant regassing of atmospheric Xe into the North Atlantic popping rock mantle source is evident, though differential regassing cannot explain systematic variations between MORB and plume-derived signatures. Rather, the fissiogenic Xe isotope signature of the 2ΠD43 mantle source indicates a greater extent of long-term degassing of the upper mantle relative to the plume mantle to explain heavy noble gas isotopic variations. The popping rock data also add to growing evidence that the MORB mantle differentiated from the reservoir supplying noble gases to plume sources within the first 100 Myr of Earth history, and that the two reservoirs have not been homogenized though 4.45 Ga of mantle convection.