DI44A-02
Helium isotopes reveal a ubiquitous recycled component in the MORB mantle

Thursday, 17 December 2015: 16:15
303 (Moscone South)
Jonathan Tucker1, Charles H Langmuir2, Sujoy Mukhopadhyay3, Cedric Hamelin4 and Jocelyn Fuentes1, (1)Harvard University, Cambridge, MA, United States, (2)Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA, United States, (3)University of California Davis, Davis, CA, United States, (4)University of Bergen, Bergen, Norway
Abstract:
The region between the Kane and Hayes Fracture Zones in the northern Mid-Atlantic Ridge is an ideal laboratory to study mantle chemistry and crustal generation processes. This >1000 km length of ridge is devoid of known hotspots and major geochemical anomalies, but nonetheless exhibits systematic variations along strike in major and trace elements. To further understand the geochemistry of the depleted MORB reservoir distant from hot spots, we have carried out 57 new helium isotopic measurements along with measurements of Nd, Sr, Pb, and Hf isotopes.

The 3He/4He ratios show a regional gradient from south to north that ranges from 8.3 to 10 Ra (where 1 Ra is the atmospheric ratio). The 3He/4He ratios correlate positively with 143Nd/144Nd, such that northern lavas have the highest 143Nd/144Nd and 3He/4He. These isotope variations correspond to a broad regional geochemical gradient from more enriched compositions in the south to more depleted compositions in the north. Nd and Pb isotopes at the northern end of this region extend to compositions significantly more depleted than average MORB, raising the possibility that the high 3He/4He ratios reflect the depleted mantle end member, which may have 3He/4He higher than 10 Ra.

Other studies have also identified a depleted end member of at least ~9-10 Ra elsewhere in the Atlantic (Tucker et al., 2012), Pacific (Hamelin et al., 2011), and Indian (Graham et al. 2014) ridge systems. This suggests that globally, normal MORBs, which are often quoted as 8±1 Ra, are more radiogenic (lower 3He/4He) than the depleted end member. Because radiogenic helium comes from recycled (degassed) material, we conclude that the average MORB mantle must contain a substantial amount of the recycled material, contributing 10s of percent of the helium even in MORBs that are not particularly enriched. Furthermore, observations of helium isotopes less radiogenic than the nominal MORB range (e.g., 10-11 Ra) need not always reflect a primitive component; they may simply be due to sampling the depleted end member.

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