[SIT26-06] Tungsten isotopic compositions of Ethiopian and Aden Bay basalts and the Samoan ocean island: implications for core-mantle interaction in their source
Keywords:core-mantle interaction, 182W/184W isotope, Ethiopian basalt, Samoan basalt, Hf-W decay system, He isotope
Tungsten has five stable isotopes, which have masses of 180, 182, 183, 184, and 186. Tungsten-182 is produced by β-decay of the extinct nuclide 182Hf, which has a relatively short half-life of 8.9 million years (Vockenhuber et al. 2004). Because the abundance of 182Hf is low, the expected variation of 182W in natural samples is extremely small. The 182W/184W isotope ratio is, therefore, commonly shown as m182W in parts per million (ppm) relative to the value for a standard: μ182W (ppm) = (182W/184Wsample/182W/184Wstandard − 1) × 106.
Hafnium is a lithophile and W is a siderophile; therefore, during segregation of the Earth’s core, Hf remained in the silicate phase and W preferentially partitioned to the metal phase. It is expected that such Hf-W fractionation occurred prior to the extinction of 182Hf on the early Earth and in lunar and meteorite bodies. Various investigators have reported positive and negative anomalies in μ182W values relative to the present-day mantle value (μ182W = 0) in terrestrial rocks. Most ancient rocks older than 2.5 Ga generally show relatively uniform μ182W values of +10 to +15 (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). Some komatiites such as Schapenburg and Komati have negative μ182W value and μ182W that is unresolved from modern value, respectively (Touboul et al. 2012, Puchtel et al. 2018). On the other hand, certain ocean island basalts have negative μ182W values (−25 to 0; e.g., Mundl et al. 2017, Mundl-Petermeier et al. 2019, Rizo et al. 2019, Mundl-Petermeier et al. 2020).
In this study, we obtained the highly precise W isotope data for the Ethiopian and Samoan basalts to discuss core-mantle interaction beneath these locations. To identify the small variations in the μ182W values of natural samples, an extremely precise method is required. Recent improvements in our techniques for analysis of W isotopes, including both the chemical procedures used for sample treatment and the mass spectrometric methods (Takamasa et al., under review), may facilitate the detection of small variations in W isotope ratios. We obtained accurate μ182W for a basalt reference material (JB-2). Analysis of the Loihi basalt, which has a high 3He/4He ratio (35Ra), yielded a μ182W value lower than the present-day mantle value, which is consistent with the results of previous study and demonstrate the reliability of the method we developed.
We applied our analytical technique for Samoan basalts. Though Mundl et al. (2017) mentioned that the W and He isotope data are also located on the line produced by the Hawaiian basalts, our data possess a different correlation curve from the Hawaiian one. It indicates that the isotopic signatures of the source of the Samoan in the deep mantle are different from those of the Hawaiian one or that the He and W fractionation occurs in the source regions, probably the core-mantle boundary. Tanaka (2002) reported the existence of Ultra Low Velocity Zone (ULVZ) at the lowermost mantle beneath Samoa and unordinary scatter in the lower mantle beneath the Samoan islands, which implies the ascent of lowermost mantle to the surface.
We also analyzed the W isotope of the Ethiopian basalts and the Aden Bay mid-ocean ridge basalts and found the small negative μ182W for them. Similar to the Hawaiian and Samoan basalts, Afar plume, the source of the Ethiopian basalts, which also affects the Aden Bay basalts are likely to be derived from the core-mantle boundary and may contain the core material with the negative μ182W.
Hafnium is a lithophile and W is a siderophile; therefore, during segregation of the Earth’s core, Hf remained in the silicate phase and W preferentially partitioned to the metal phase. It is expected that such Hf-W fractionation occurred prior to the extinction of 182Hf on the early Earth and in lunar and meteorite bodies. Various investigators have reported positive and negative anomalies in μ182W values relative to the present-day mantle value (μ182W = 0) in terrestrial rocks. Most ancient rocks older than 2.5 Ga generally show relatively uniform μ182W values of +10 to +15 (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). Some komatiites such as Schapenburg and Komati have negative μ182W value and μ182W that is unresolved from modern value, respectively (Touboul et al. 2012, Puchtel et al. 2018). On the other hand, certain ocean island basalts have negative μ182W values (−25 to 0; e.g., Mundl et al. 2017, Mundl-Petermeier et al. 2019, Rizo et al. 2019, Mundl-Petermeier et al. 2020).
In this study, we obtained the highly precise W isotope data for the Ethiopian and Samoan basalts to discuss core-mantle interaction beneath these locations. To identify the small variations in the μ182W values of natural samples, an extremely precise method is required. Recent improvements in our techniques for analysis of W isotopes, including both the chemical procedures used for sample treatment and the mass spectrometric methods (Takamasa et al., under review), may facilitate the detection of small variations in W isotope ratios. We obtained accurate μ182W for a basalt reference material (JB-2). Analysis of the Loihi basalt, which has a high 3He/4He ratio (35Ra), yielded a μ182W value lower than the present-day mantle value, which is consistent with the results of previous study and demonstrate the reliability of the method we developed.
We applied our analytical technique for Samoan basalts. Though Mundl et al. (2017) mentioned that the W and He isotope data are also located on the line produced by the Hawaiian basalts, our data possess a different correlation curve from the Hawaiian one. It indicates that the isotopic signatures of the source of the Samoan in the deep mantle are different from those of the Hawaiian one or that the He and W fractionation occurs in the source regions, probably the core-mantle boundary. Tanaka (2002) reported the existence of Ultra Low Velocity Zone (ULVZ) at the lowermost mantle beneath Samoa and unordinary scatter in the lower mantle beneath the Samoan islands, which implies the ascent of lowermost mantle to the surface.
We also analyzed the W isotope of the Ethiopian basalts and the Aden Bay mid-ocean ridge basalts and found the small negative μ182W for them. Similar to the Hawaiian and Samoan basalts, Afar plume, the source of the Ethiopian basalts, which also affects the Aden Bay basalts are likely to be derived from the core-mantle boundary and may contain the core material with the negative μ182W.