A thoughtful review by Class (2008) and a new article by Albarede (2008) provide a new context for re-examining potential relationships between excess temperatures (Tex) and 3He/4He (Putirka, 2008). The issue is to explain why high 3He/4He mantle (HHM) is tapped by ocean island basalts (OIB), but not by the depleted mantle (DM) that feeds mid-oceanic ridge basalts (MORB). Two possibilities (not mutually exclusive) are HHM is physically segregated from DM and tapped only by OIB-specific dynamics (e.g., plumes), or HHM is intermixed within a DM matrix but partially melts at a lower or higher temperature.
A new merged data set (Putirka, 2008, Figure 3; Abedini et al., 2006; Jackson et al., 2007) yields nine new 3He/4He maxima compared to my 2008 data (Putirka, 2008): Azores, 11.3; Cape Verde, 15.7; Galapagos, 27.4; Hawaii, 35; Iceland, 37.7; Kerguelen, 18.3; Reunion, 14.9; Samoa, 33.8; and Tahiti, 17. Also added are data from the Cook-Austral chain: 3He/4Hemax = 7 (Mukhopadhyay, 2007), Tp = 1535 °C, Tex = 139 °C, F = 8.03%, and H2O = 0.93%. The correlation coefficient (R) between Tex and 3He/4He is now +0.67 (Fig. 1A). Galapagos is excluded in this new data set, as in Putirka (2008), because its Tex and parental melt fraction (F) are not well known. We also now compare 3He/4He to F, a proxy for Tex calculated independent of T (Putirka, 2008), and positively correlate F with 3He/4He (Fig. 1B), allowing a quantitative illustration of certain premises in Putirka (2008).
Class noted that “because enriched components contribute radiogenic 4He” they “allow for any combination of Tex and 3He/4Hemax.” However, if enriched components are fertile, only partial melts generated at low T and low F can have high 3He/4He (upper dashed curve, with negative slope; Fig. 1), even while some low-F melts may have low 3He/4He (lowermost dashed curve). High-F melts should also merge toward DM (3He/4He = 8) or some DM-HHM mixture, depending on whether HHM (the fertile source) is exhausted. But OIB with high F and Tex are observed to have high 3He/4He.
What if HHM is the most refractory mantle component (Albarede, 2008)? Could selective fusion of a refractory component explain Tex-3He/4He-F relationships? Partial melts should then follow the solid curves, with positive slopes. The HHM-as-refractory model appears to explain the OIB pattern (Fig. 1), but can only match the data if HHM has 3He/4He = 120 (Jackson et al., 2008), and is >400 times less fertile than DM (FDM/FHHM = 425) (Fig. 1). Given that HHM and DM have comparable 87Sr/86Sr and 143Nd/144Nd, a stark difference in fertility seems unlikely.
In contrast, global Tex-3He/4He-F relation-ships are consistent with a model whereby HHM is transported into the melting region by a thermally activated process, e.g., a thermal plume. Because thermal plumes are expected to cool in transit (Albers and Christensen, 1996), plumes with low Tex and high 3He/4He can be explained. While there is no necessary assumption that HHM is homogeneous (e.g., Jackson et al., 2008), if no plumes with Tex>200 °C and low 3He/4He are discovered, HHM might not only be deep, but resistant to mixing. The rest of the mantle also need not be singular in its 3He/4He: some low Tex OIB plausibly derive from a fertile source with low to moderate 3He/4He (Fig. 1A). Of course, the expected relationships between 3He/4He-F for a non-layered mantle (Fig. 1B) are invalid if plumes do not partially melt ambient mantle, or if ambient mantle is not DM, options which are perhaps worth considering. In the meantime, global Tex-3He/4He-F relationships may be telling us something about the petrologic character of isotopic mantle components. To the extent that they do, models that exclude compositional layering appear less probable, at least at this stage of the investigation.
Thanks to M. Jackson and S. Hurwitz for help in understanding 3He/4He-related issues and to C. Herzberg for a helpful review.