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Helium-Osmium Systematics

Anders Meibom

Laboratoire d'Etude de la Matiere Extraterrestre, USM 205 (LEME), Case Postale 52, Museum National d'Histoire Naturelle, 61 rue Buffon, 75005 Paris

An increasingly large body of isotopic and trace-element analyses of mid-ocean ridge basalts (MORBs) demonstrates that the upper mantle is not homogenous but contains several distinct geochemical domains on a variety of length scales [Graham et al., 2001; Hofmann, 1997; Meibom et al., 2002; Zindler and Hart, 1986]. However, the physical properties of these domains, including their exact location, size, temperature and dynamics, remain largely unconstrained.

He and Os isotopic distributions of unfiltered, global spreading ridge data

Perhaps the most important example of an inferred genetic relationship between a geochemical signature and a specific geophysical phenomenon is provided by He isotopes and conjectured deep-rooted mantle plumes. Preponderance of opinion among geochemists is that the distinct He isotopic signatures (i.e. high 3He/4He ratios) associated with certain locations, such as Hawaii, Yellowstone and Iceland, indicate a link, via such mantle plumes, to a primordial and undegassed reservoir isolated in the lower mantle. In this interpretation, the He isotopic signatures of “hotspot” basalts provide geochemical evidence for large scale stratification of the mantle and the presence of an “undegassed, primordial lower mantle reservoir” [Farley and Neroda, 1998; Kellogg and Wasserburg, 1990; Porcelli and Wasserburg, 1995]. Recently, however, an alternative to this interpretation has been explored [Meibom et al., 2003].

The unfiltered He isotope data set for MORB displays a wide, nearly Gaussian, distribution [Anderson, 2000], qualitatively similar to the Os isotopic (187Os/188Os) distribution of mantle-derived Os-rich alloys [Meibom et al., 2002] (see figure). Both distributions could result from shallow mantle processes involving the mixing of different proportions of recycled, radiogenic and unradiogenic domains. These could be generated as follows.

In the case of the (U+Th)-He isotope system, the capture of He-rich bubbles by growing olivine phenocrysts in crystallizing magma chambers [Natland, 2003] effectively separates He gas from U+Th. This prevents additional 4He, produced by the radiogenic decay of U+Th, from adding to the He gas in the bubbles and reducing its 3He/4He. In this way, old, unradiogenic (high) 3He/4He is “frozen in” and preserved. The higher-than-chondritic (U+Th)/He elemental ratio in the MORB melt, however, results in the relatively rapid growth of 4He, and this, coupled with the partially degassed nature of the MORB melt, causes 3He/4He to reduce rapidly, and thereby provide the radiogenic (i.e. low 3He/4He) endmember.
During partial melting, Re is mildly incompatible, whereas Os is strongly compatible. This results in high Re/Os ratios in basalts, and correspondingly low Re/Os in the refractory, depleted solid residue left behind in the mantle [Shirey and Walker, 1998]. Thus, 187Os/188Os in basalts and the residue rapidly diverges after melting and separation. Radiogenic (MORB-rich) and unradiogenic (depleted mantle residue) 187Os/188Os endmembers are constantly produced by partial melting events.

If this model is correct, the assumption that high 3He/4He is diagnostic of a plume component in oceanic basalts is not justified.



last updated 30th March, 2005