Noble gas study of the Reunion hotspot: evidence for distinct less-degassed mantle sources

摘要

We present an extensive He, Ne and Ar isotope data set from the Reunion hotspot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. ~3He/~4He ratios of the two volcanoes on Reunion (Piton de la Fournaise, < 0.53 Ma; Piton des Neiges, 2-0.44 Ma) are uniform 12-13.5 Ra regardless of ~4He concentration and sample age. The shield-building Older Series of Mauritius (5-8 Ma) has a constant ~3He/~4He ratio around 11.5 Ra. The similarity of ~3He/~4He and Sr-Nd isotope ratios among them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. ~(20)Ne/~(22)Ne and ~(21)Ne/~(22)Ne of these volcanoes define a linear trend on a Ne three-isotope diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between ~(20)Ne/~(22)Ne and ~(40)Ar/~(36)Ar. In contrast, Intermediate (2-3 Ma) and Younger Series (<1 Ma) of Mauritius and Rodrigues (1.5 Ma) have ~3He/~4He ratios similar to MORB and Sr and Nd isotope ratios closer to MORB than lavas from Reunion and Older Series of Mauritius. These Intermediate and Younger Series lavas therefore record a late stage thermal rejuvenation beneath mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reunion magmatism are relatively low ~3He/~4He (13 Ra), an intermediate slope in a Ne three-isotope diagram and relatively radiogenic Sr isotope ratios. These source characteristics cannot be explained by either crustal contamination or MORB source mixing with Loihi-type primitive mantle. Thus He-Ne-Ar-Sr-Nd isotopes demonstrate that this plume source is distinct from the source of other large plumes (Loihi and Iceland), clearly showing that the mantle contains several relatively less-degassed reservoirs and not a single primitive source. Two possible models can account for the different isotopic signature of Reunion and Hawaii hotspots; (1) the Reunion source contains more recycled material than Loihi source and (2) the Reunion source experienced stronger degassing/differentiation than the Loihi source in the early stage of mantle evolution. In both cases a convection mode in the mantle is required that isolates and preserves several less-degassed reservoirs in the convectively stirred lower mantle.