Heterogeneous upper mantle Ne, Ar and Xe isotopic compositions and a possible Dupal noble gas signature recorded in basalts from the Southwest Indian Ridge

Variations in heavy noble gas (Ne, Ar, Xe) isotopic compositions provide unique insights into the nature of heterogeneities in the mantle. However, few precise constraints on mantle source heavy noble gas isotopic compositions are available due to ubiquitous shallow-level atmospheric contamination. As a result, the extent of heterogeneity in mid-ocean ridge basalt (MORB) mantle source Ne, Ar and Xe isotopic compositions is unknown. Basalts from the ultra-slow spreading Southwest Indian Ridge (SWIR) between 7°E and 25°E exhibit remarkable variability in He isotopic composition: SWIR ⁴He/³He spans half the total range observed in all mantle-derived basalts. Therefore, basalts from the SWIR provide a unique window into upper mantle heterogeneity and present an ideal opportunity to characterize variations in upper mantle heavy noble gas isotopic composition. Here we present new high-precision Ne, Ar and Xe isotopic compositions as well as He, CO₂, Ne, Ar and Xe abundances measured in basalt glasses from the SWIR. After correcting the measured values for shallow-level atmospheric contamination, significant and systematic variations in mantle source Ne, Ar and Xe compositions are observed. We note that large variations in source ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³⁰Xe are observed in basalts removed from the influence of known hotspots, indicating a heterogeneous mid-ocean ridge basalt source. Thus, SWIR heavy noble gas data reveal a greater degree of source heterogeneity than is evident in the ⁴He/³He systematics alone. The observed heavy noble gas isotopic heterogeneities imply that the average MORB source ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³⁰Xe ratios are not yet well-determined.Variation in MORB source ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³⁰Xe at a given ⁴He/³He and ²¹Ne/²²Ne may reflect heterogeneous recycling of atmospheric Ar and Xe. In particular, we find low mantle source ⁴⁰Ar/³⁶Ar and ¹²⁹Xe/¹³⁰Xe ratios in the eastern region of the study area, which may reflect the noble gas signature of the Dupal mantle domain. Our observations require that the sampled mantle domain either is very ancient (>4.45Ga) or has been metasomatized by subduction zone fluids carrying recycled atmospheric Ar and Xe. However, our Xe isotopic measurements indicate that differences between MORB and ocean island basalt (OIB) source noble gas compositions cannot be explained by recycling of atmospheric noble gases alone. Instead, a relatively undegassed mantle reservoir is required to account for OIB noble gases. The SWIR data demonstrate that the reservoir supplying primordial noble gases to mantle plumes differentiated from the MORB source early in Earth history, and the two reservoirs have not been homogenized over 4.45Ga of mantle convection.