Domains of depleted mantle: New evidence from hafnium and neodymium isotopes

Abstract

[1] Isotope systematics of basalts provide information on the distribution of mantle components and the length scale of mantle heterogeneity. To obtain this information, high data and sampling density are crucial. We present hafnium and neodymium isotope data on more than 400 oceanic volcanics. Over length scales of several hundred to over one thousand kilometers hafnium and neodymium isotopes of mid-ocean ridge basalts are correlated and form an array of parallel trends on a global scale. On a larger scale these domains differ in the amount of highly depleted mantle material with radiogenic hafnium and neodymium isotope ratios. Compared to the Atlantic and Indian Ocean basins the asthenosphere of the Pacific basin seems to have a more uniform and a less radiogenic Hf isotopic composition for a given Nd isotopic composition. The parallel arrays of mid-ocean ridge basalts provide strong constraints on the makeup of the MORB mantle and are evidence for the presence of a highly depleted and highly radiogenic neodymium and hafnium component. This component, because of its highly depleted character, is unrecognized in the strontium-neodymium-lead isotope systems alone. Alternatively, the parallel arrays can have an ancient origin of systematic variations in the degree of depletion. Each array then represents the variations in this fossil melting regime. Individual ocean island basalt suites display different slopes in hafnium-neodymium isotope space, which are also best explained by varying amounts of highly residual mantle rather than isotopic differences in enriched mantle components as previously invoked. The ocean island basalt arrays diverge at

Magma dynamics of ancient Mt. Etna inferred from clinopyroxene isotopic and trace element systematics

“…Etna through time. Lassiter et al, 2003;Stracke et al, 2003;Gaffney et al, 2004;Huang et al, 2005;Xu et al, 2007;Sims et al, 2008;Blichert-Toft and Albarède, 2009;Yamasaki et al, 2009;Garcia et al, 2010;Peate et al, 2010;Chekol et al, 2011;Salters et al, 2011;Viccaro et al, 2011); mantle components from Zindler and Hart, 1986;Salters and White, 1998;Workman et al, 2004;Stracke et al, 2005;Workman and Hart, 2005. Hafnium isotopic values for Italian sediments (Conticelli et al, 2002;Brems et al, 2013) are calculated from Nd isotopic data and both cases following the seawater array (SA) and the terrestrial array (TA) of Vervoort et al (2011) Trua et al (1998).…”

Atypically depleted upper mantle component revealed by Hf isotopes at Lucky Strike segment

“…Anomalously high values of 177 Hf/ 176 Hf for a given 143 Nd/ their mantle source across the garnet-spinel transition beneath the mid-oceanic ridge (Blichert-Toft et al, 2005;Debaille et al, 2006). More recently, a systematic study of Hf-Nd isotopes in oceanic basalts has shown an array of parallel trends on a global scale (Salters et al, 2011). These authors attributed these sub-parallel arrays to the involvement of varying amounts of highly residual peridotites within the upper mantle.…”

Siderophile Elements in Tracing Planetary Formation and Evolution

“…Further, some refractory portions of the DMM may not be accessed at all in the production of MORB. Evidence for considerable heterogeneity in the DMM, beyond the range sampled by MORB, includes the fact that some peridotites that directly sample the DMM have much more radiogenic 143 Nd/ 144 Nd and 176 Hf/ 177 Hf than any known MORB (e.g., Sharma et al, 1995;Warren et al, 2009a;Salters et al, 2011;Stracke et al, 2011). Thus, the nature and scale of chemical heterogeneity in the modern DMM may be utilised to reveal important secrets of the mantle.…”

“…Such research has been fruitful because of the extent to which MORBs have been sampled throughout all of the ocean basins (e.g., Klein and Langmuir, 1987;Workman and Hart, 2005;Salters et al, 2011). Thus, MORB research has elucidated global variations in the chemical and isotopic nature of the DMM.…”

“…So, even with this living, breathing, nonsedentary portion of the mantle that we can touch on occasion, it is not so easy to arrive at complete characterisation of its siderophile element budget. The heterogeneity is most amply evidenced by the diverse lithophile element compositions (chemical and isotopic) of derivative melts, such as those sampled by MORB (e.g., Langmuir and Hanson, 1980;Cohen and O'Nions, 1982;Dupré and Allègre, 1983;Shirey et al, 1987;Salters et al, 2011), as well as by data generated through the direct analysis of mantle rocks accessible in the form of abyssal (oceanic) peridotites (Snow et al, 1994;Warren et al, 2009a;Stracke et al, 2011), and the mantle sections of ophiolites (Sharma et al, 1995;Büchl et al, 2002;Dilek and Furnes, 2011). The processes that resulted in the creation of heterogeneities in the oceanic mantle most likely were dominated by multiple stages of partial melting and metasomatism, as well as lithospheric recycling, yet the relative impact of each of these processes, the timing of the processes, and the length scales and longevity of the resulting heterogeneities remain topics of considerable debate.…”