Plumes of hot rock surging upward from the Earth’s mantle at volcanic hotspots contain evidence that the Earth’s formative years may have been even more chaotic than previously thought, according to new work from a team of Carnegie and Smithsonian scientists published in Nature.
Cartoon illustrating our model for the co-generation and preservation of Xe and W anomalies. a, Earth experiences multiple large impacts; I and other highly volatile elements are present. FeO-rich silicate liquids are produced via high-P, high-T metal–silicate equilibration. The high density of these FeO-rich liquids makes them prone to long-term preservation as distinct geochemical reservoirs that experienced core extraction under distinct P–T–X conditions. We suggest that an earlier impact results in a high-W anomaly, and vice versa, because of the timing offsets depicted in Fig. 3. b, Potential modern expression of early-forming reservoirs as components within large low-shear-velocity provinces (LLSVPs) and ultralow-velocity zones (ULVZs), which contain materials with relatively low 129Xe*/136Xe*Pu and variable 182W/184W.
The decay of short-lived iodine (I) and plutonium (Pu) results in xenon (Xe) isotopic anomalies in the mantle that record Earth’s earliest stages of formation. Xe isotopic anomalies have been linked to degassing during accretion, but degassing alone cannot account for the co-occurrence of Xe and tungsten (W) isotopic heterogeneity in plume-derived basalts and their long-term preservation in the mantle. Here a team led by Carnegie’s Yingwei Fei and Carnegie and the Smithsonian’s Colin Jackson describes measurements of I partitioning between liquid Fe alloys and liquid silicates at high pressure and temperature and propose that Xe isotopic anomalies found in modern plume rocks (that is, rocks with elevated 3He/4He ratios) result from I/Pu fractionations during early, high-pressure episodes of core formation. Their measurements demonstrate that I becomes progressively more siderophile as pressure increases, so that portions of mantle that experienced high-pressure core formation will have large I/Pu depletions not related to volatility. These portions of mantle could be the source of Xe and W anomalies observed in modern plume-derived basalts. Portions of mantle involved in early high-pressure core formation would also be rich in FeO, and hence denser than ambient mantle. This would aid the long-term preservation of these mantle portions, and potentially points to their modern manifestation within seismically slow, deep mantle reservoirs with high 3He/4He ratios.
Reference:
Colin R. M. Jackson, Neil R. Bennett, Zhixue Du, Elizabeth Cottrell & Yingwei Fei. Early episodes of high-pressure core formation preserved in plume mantle. Nature, 2018
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