Helium isotopes in ferromanganese crusts from the central Pacific Ocean S. Basu a, * , F.M. Stuart a , V. Klemm b , G. Korschinek c , K. Knie c , J.R. Hein d a Isotope Geosciences Unit, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK b Institute for Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH-Zurich, ETH Zentrum, Sonneggstrasse 5, CH+8092 Zurich, Switzerland c Technische Universita ¨t Mu ¨ nchen, Fakulta ¨t fu ¨ r Physik, James-Franck-Straße 1, D-85747 Garching, Germany d U.S. Geological Survey, 345 Middlefield Road, MS-999, Menlo Park, CA 94025, USA Received 14 December 2005; accepted in revised form 25 May 2006 Abstract Helium isotopes have been measured in samples of two ferromanganese crusts (VA13/2 and CD29-2) from the central Pacific Ocean. With the exception of the deepest part of crust CD29-2 the data can be explained by a mixture of implanted solar- and galactic cosmic ray-produced (GCR) He, in extraterrestrial grains, and radiogenic He in wind-borne continental dust grains. 4 He concentrations are invariant and require retention of less than 12% of the in situ He produced since crust formation. Loss has occurred by recoil and dif- fusion. High 4 He in CD29-2 samples older than 42 Ma are correlated with phosphatization and can be explained by retention of up to 12% of the in situ-produced 4 He. 3 He/ 4 He of VA13/2 samples varies from 18.5 to 1852 R a due almost entirely to variation in the extra- terrestrial He contribution. The highest 3 He/ 4 He is comparable to the highest values measured in interplanetary dust particles (IDPs) and micrometeorites (MMs). Helium concentrations are orders of magnitude lower than in oceanic sediments reflecting the low trapping effi- ciency for in-falling terrestrial and extraterrestrial grains of Fe-Mn crusts. The extraterrestrial 3 He concentration of the crusts rules out whole, undegassed 4–40 lm diameter IDPs as the host. Instead it requires that the extraterrestrial He inventory is carried by numerous particles with significantly lower He concentrations, and occasional high concentration GCR-He-bearing particles. Ó 2006 Elsevier Inc. All rights reserved. 1. Introduction Ferromanganese (Fe-Mn) crusts form on ocean-floor rock substrates as hydrogenous precipitates and preserve long-term records of the chemical composition of seawater. In particular, the isotopic compositions of Pb, Nd, Hf, and Os in Fe-Mn crusts record changes in the delivery of terres- trial material to the oceans that allow constraints to be placed on ocean circulation and climate over the last few tens of million years (e.g. Ling et al., 1997; Burton et al., 1999; Frank et al., 1999; Frank, 2002; Van de Flierdt et al., 2003). The reconstruction of past changes in seawater composition is predicated on accurate chronologies for the growth of the crusts that are provided by Co concentra- tions (e.g. Halbach et al., 1983), 10 Be (e.g. Segl et al., 1984; Ling et al., 1997), and Os isotopes (Klemm et al., 2005). Helium isotopes in oceanic sediments provide useful information on the flux of terrestrial and extraterrestrial debris to the sea floor over geological time. 4 He is sup- plied by detrital minerals and is a proxy for the delivery of continental material to the deep oceans in the form of wind-blown debris (e.g. Patterson et al., 1999). In con- trast, 3 He provides a record of the accumulation of in- falling extraterrestrial dust-sized particles that can be used to identify major cometary shower and asteroidal break-up events, associated with increasing dustiness in the inner solar system (e.g. Farley, 1995). 3 He can also be used to determine sedimentation rates beyond the range possible using conventional radiometric techniques (e.g. Marcantonio et al., 1996). The potential to use He isotopes in Fe-Mn crusts to trace long-term fluctuations in the delivery of extraterrestrial and 0016-7037/$ - see front matter Ó 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.05.015 * Corresponding author. Fax: +44 0 1355 229898. E-mail address: s.basu@suerc.gla.ac.uk (S. Basu). www.elsevier.com/locate/gca Geochimica et Cosmochimica Acta 70 (2006) 3996–4006