Date of Original Version
Heavy noble gases (Ne, Ar, Xe) can record long-lasting heterogeneities in the mantle because of the production of isotopes from extant (238U, 40K) and extinct (129I and 244Pu)13 radionuclides. However, the presence of ubiquitous atmospheric contamination, particularly for ocean island basalts (OIBs) that sample the Earth’s deep mantle, have largely hampered precise characterization of the mantle source compositions. Here we present new high-precision noble gas data from gas-rich basalts erupted along the Rochambeau Rift in the northwestern corner of the Lau Basin. The strong influence of a deep mantle plume in the Rochambeau source is apparent from low 4He/3He ratios down to 25,600 (3He/4He of 28.1 RA). We find that the Rochambeau source is characterized by low ratios of radiogenic to non-radiogenic nuclides of Ne, Ar, and Xe (i.e., low 21Ne/22Ne, 40Ar/36Ar, and 129Xe/130Xe) compared to the mantle source of mid-ocean ridge basalts (MORBs). Additionally, we observe differences in elemental abundance patterns between the Rochambeau source and the mantle source of MORBs as characterized by the gas-rich popping rock from the Mid-Atlantic Ridge. However, the 3He/22Ne ratio of the Rochambeau plume source is significantly higher than the Iceland and Galapagos plume sources, while the 3He/36Ar and 3He/130Xe ratios appear to be similar. The difference in 3He/22Ne between Rochambeau and the Galapagos and Iceland plume sources could reflect long lasting accretional heterogeneities in the deep mantle or some characteristic of the back-arc mantle source. High-precision xenon isotopic measurements indicate that the lower 129Xe/130Xe ratios in the Rochambeau source cannot be explained solely by mixing atmospheric xenon with MORB31 type xenon; nor can fission-produced Xe be added to MORB Xe to produce the compositions seen in the Rochambeau basalts. Deconvolution of fissiogenic xenon isotopes demonstrate a higher proportion of Pu-derived fission Xe in the Rochambeau 33 source compared to the MORB source. Therefore, both I/Xe and Pu/Xe ratios are different between OIB and MORB mantle sources. Our observations require heterogeneous volatile accretion and a lower degree of processing for the mantle plume source compared to the MORB source. Since differences in 129Xe/130Xe ratios have to be produced while 129I is still alive, OIB and MORB sources were degassed at different rates for the first 100 Ma of Solar System history, and subsequent to this period, the two reservoirs have not been homogenized. In combination with recent results from the Iceland plume, our observations require the preservation of less-degassed, early-formed heterogeneities in the Earth’s deep mantle throughout Earth’s history.
Heterogeneities from the first 100 million years recorded in deep mantle noble gases from the Northern Lau Back-arc Basin. Petö, M., S. Mukhopadhyay, and K. Kelley, Earth and Planetary Science Letters 369-370, 13-23, doi:10.1016/j.epsl.2013.02.012, 2013.
Available at: http://dx.doi.org/10.1016/j.epsl.2013.02.012