The heavy noble gas composition of the depleted MORB mantle (DMM) and its implications for the preservation of heterogeneities in the mantle

摘要

To characterize the heavy noble gas composition of MORBs we present new He, Ne, Ar, and Xe abundances and isotopic compositions from the equatorial Mid-Atlantic Ridge. Both depleted MORBs nominally devoid of plume influence and more enriched MORBs thought to represent the influence of a HIMU mantle plume are present in close geographical proximity in this region. Ne-Ar-Xe isotopic compositions in individual step-crushes are correlated, which, along with significant radiogenic excesses, allows correction for shallow-level air contamination. Based on the relationship between the noble gases and the lithophile isotopes (Sr, Nd and Pb), the depleted MORB mantle has a ~(21)Ne/~(22)Ne between 0.0617 and 0.0646, ~(40)Ar/~(36)Ar ratio of 41,500±9000 and ~(129)Xe/~(130)Xe ratio of 7.77±0.06. On the other hand, the HIMU-type MORBs are characterized by far less radiogenic Ne, Ar and Xe isotopic compositions with mantle source 21Ne/22Ne between 0.0544 and 0.0610, and ~(40)Ar/~(36)Ar and ~(129)Xe/~(130)Xe ratios of 18,100±600 and 7.21±0.06, respectively. The observation of less nucleogenic ~(21)Ne/~(22)Ne in HIMU-type MORBs is similar to observations from HIMU ocean islands and requires the HIMU plume to be comprised of both recycled and primitive material. Within the depleted MORBs we observe He and Ne to be negatively correlated. The observation suggests that along the equatorial Atlantic the most depleted MORBs are related to normal MORBs through the addition of a small proportion of a HIMU plume component.Our new Xe isotopic measurements demonstrate distinct ~(129)Xe/~(136)Xe ratios in the mantle sources of depleted MORBs, HIMU-type MORBs and the Iceland plume. While substantial injection of atmospheric Xe into these mantle sources is implied, the differences in Xe isotopic composition cannot result solely from recycling of air. Rather, they require that mantle plumes sample a reservoir less degassed than the depleted MORB mantle. This conclusion is consistent with a higher proportion of Pu- to U-derived fission Xe in Iceland and HIMU-type MORBs compared to the depleted MORBs. Overall, the Xe isotopic compositions imply that mantle plumes tap a reservoir that separated from the MORB source within the first 100 million years of Earth's history and that the two reservoirs have had limited direct mixing since then.