Tungsten isotope implications for the source of ocean island basalts from the Marquesas Archipelago

Abstract

<p>The application of the short-lived radiogenic <sup>182</sup>Hf/<sup>182</sup>W-system (t<sub>1/2 </sub>= 8.9 Ma [1]) is a good approach to study early differentiation processes or potential involvement of long-term isolated and/or core-influenced mantle domains as components for ocean island basalts (OIB) [2,3].</p><p>Several examples of OIB worldwide (e.g., Hawaii, Samoa and Iceland) exhibit a negative He-W correlation [2], possibly connected to the incorporation of primordial material characterized by high <sup>3</sup>He/<sup>4</sup>He ratios and negative µ<sup>182</sup>W (<sup>182</sup>W/<sup>184</sup>W deviation of a sample from laboratory standards in parts per million). Anomalous W isotope compositions in combination with elevated <sup>3</sup>He/<sup>4</sup>He ratios have previously been connected to seismically anomalous structures in the lowermost mantle, so-called “(mega) ultra-low velocity zones” [3]. Recently, such a structure was discovered beneath the Marquesas Archipelago [4]. This volcanic island chain is located in the South Pacific, in proximity of the Marquesas Fraction Zone. Its formation process is not yet fully understood. Based on high <sup>3</sup>He/<sup>4</sup>He ratios in combination with other geochemical characteristics, such as Sr, Nd and Pb isotopes, a deep-lying mantle source has been suggested [5].</p><p>In this study, we have analysed seven samples from two islands of the Marquesas Archipelago, which exhibit <sup>3</sup>He/<sup>4</sup>He ratios up to 14.4 Ra [5]. µ<sup>182</sup>W ranges from -3.6 ±3.1 to 4.7 ±8.5. Hence, despite elevated <sup>3</sup>He/<sup>4</sup>He in some of the samples, none of them display resolved negative <sup>182</sup>W anomalies and thus, no negative He-W correlation is observed. Interpretations for the decoupling of He-W systematics in samples from the Marquesas Archipelago will be discussed.</p><p><strong> </strong></p><p><strong>References:</strong></p><p>[1] Vockenhuber et al., 2004, <em>Phys. Rev. Lett.</em></p><p>[2] Mundl et al., 2017, <em>Science</em></p><p>[3] Mundl-Petermeier et al., 2020, <em>Geochim. </em><em>Cosmochim. </em><em>Acta</em></p><p>[4] Kim et al., 2020, <em>Science</em></p><p>[5] Castillo et al., 2007, <em>Chem</em><em>.</em><em> Geol.</em></p>