[SGC49-12] Early evolution of mantle and core-mantle interaction elucidated from 182W isotope
Keywords:chemical evolution of mantle, Hf-W isotope system, 182W isotope, komatiite, core-mantle interaction
The short-lived 182Hf-182W isotopic system (t1/2 = 8.9 Ma; Vockenhuber et al. 2004) provides the evidence in both ancient and modern terrestrial rock record of processes that occurred during the earliest stages of Earth’s accretionary and differentiation history. The utility of the Hf-W isotopic system stems from the fact that hafnium is strongly lithophile element, while tungsten is moderately siderophile. Therefore, Hf is efficiently fractionated from W by metal-silicate equilibration, such as occurrence during planetary core segregation. W isotopic ratios in terrestrial rocks are generally presented as μ182W, where μ182W (ppm) = (182W/184Wsample/182W/184Wstandard − 1) × 106 in parts per million (ppm) relative to the value of standard (present-day mantle). Many investigators have previously reported various μ182W values in terrestrial rock samples; With the exception of a few reports, Hadean to Archean volcanic samples was basically characterized by 182W excesses of +5 to +20 ppm (Willbold et al. 2011, Touboul et al. 2012, Touboul et al. 2014, Willbold et al. 2015, Liu et al. 2016, Rizo et al. 2016, Dale et al. 2017, Mundl et al. 2018, Puchtel et al. 2018, Reimink et al. 2018, Tusch et al. 2019). This 182W isotopic signature was interpreted as the result of 182W heterogeneity in the mantle caused by metal-silicate or silicate-silicate fractionation operating within the first ~60 Myr of Solar System history while 182Hf was extinct. On the other hand, modern ocean island basalts have negative μ182W anomalies (−25 to 0; e.g., Mundl et al. 2017, Mundl-Petermeier et al. 2019, Rizo et al. 2019, Mundl-Petermeier et al. 2020), interpreting that the contribution of material that has inherited its negative μ182W signature through core-mantle equilibration.
To identify the small variations in the μ182W values of natural samples, an extremely precise method is required. More recently, our analytical improvements in multi-collector inductively coupled plasma mass spectrometry have enable the resolution of 182W/184W measurements on the order of 6~9 ppm (2SD) with small amount of sample (~0.4 g) (Takamasa et al., under review). By using the newly developed analytical technique, we determined the W isotopic compositions from plume-related volcanic rocks from the Ontong Java Plateau (OJP), 250 Ma Emeishan flood basalts, as well as samples from the 3.3 Ga Singhbhum and Dharwar komatiites in India, in order to further constrain the processes involved in the generation of μ182W anomalies in the Earth’s mantle.
The Singhbhum and Dharwar individual samples yield no μ182W anomalies ranging from -0.5 to +5.6 (n = 3) and from -1.4 to +5.0 (n = 4), respectively. These values are much less than the range of the uniform μ182W values of rocks older than 2.5 Ga. The results imply that deep mantle already had a low μ182W value during 3.5 to 3.0 Ga, probably because of the efficient mixing of early differentiated mantle and late accreted material. Additionally, our results for the Singgalo-type OJP basalt yield the μ182W value indistinguishable from the present-day mantle, which is similar to the ones of Kroenke-type OJP basalts with the different isotopic signature from the Singgalo-type. Also, the Emeishan flood basalts possess the no μ182W anomalies relative to the value of the present-day mantle. Previously, Rizo et al., (2019) mentioned that the lavas from the Canary Islands are the only OIB to display resolvable 182W excesses, but the basaltic rock seems to originate at shallower depths (King and Ritsema, 2000; Courtillot et al., 2003) and various geochemical tracers suggest these magmas were contaminated by the ancient continental root (e.g., Thirlwall et al., 1997). Except for the Canary Islands, no resolvable 182W excess have been found in the mantle since 250 Ma.
To identify the small variations in the μ182W values of natural samples, an extremely precise method is required. More recently, our analytical improvements in multi-collector inductively coupled plasma mass spectrometry have enable the resolution of 182W/184W measurements on the order of 6~9 ppm (2SD) with small amount of sample (~0.4 g) (Takamasa et al., under review). By using the newly developed analytical technique, we determined the W isotopic compositions from plume-related volcanic rocks from the Ontong Java Plateau (OJP), 250 Ma Emeishan flood basalts, as well as samples from the 3.3 Ga Singhbhum and Dharwar komatiites in India, in order to further constrain the processes involved in the generation of μ182W anomalies in the Earth’s mantle.
The Singhbhum and Dharwar individual samples yield no μ182W anomalies ranging from -0.5 to +5.6 (n = 3) and from -1.4 to +5.0 (n = 4), respectively. These values are much less than the range of the uniform μ182W values of rocks older than 2.5 Ga. The results imply that deep mantle already had a low μ182W value during 3.5 to 3.0 Ga, probably because of the efficient mixing of early differentiated mantle and late accreted material. Additionally, our results for the Singgalo-type OJP basalt yield the μ182W value indistinguishable from the present-day mantle, which is similar to the ones of Kroenke-type OJP basalts with the different isotopic signature from the Singgalo-type. Also, the Emeishan flood basalts possess the no μ182W anomalies relative to the value of the present-day mantle. Previously, Rizo et al., (2019) mentioned that the lavas from the Canary Islands are the only OIB to display resolvable 182W excesses, but the basaltic rock seems to originate at shallower depths (King and Ritsema, 2000; Courtillot et al., 2003) and various geochemical tracers suggest these magmas were contaminated by the ancient continental root (e.g., Thirlwall et al., 1997). Except for the Canary Islands, no resolvable 182W excess have been found in the mantle since 250 Ma.