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

Hanyu, T Dunai, TJ Davies, GR Kaneoka, I Nohda, S Uto, K

T. Hanyu et al., Noble gas study of the Reunion hotspot: evidence for distinct less-degassed mantle sources, EARTH PLAN, 193(1-2), 2001, pp. 83-98

52

INGLESE

Article

Earth Sciences

EARTH AND PLANETARY SCIENCE LETTERS

0012-821X → ACNP

193

1-2

2001

83 - 98

ISI

0012-821X(20011130)193:1-2<83:NGSOTR>2.0.ZU;2-D

We present an extensive He. Ne and Ar isotope data set from the Reunion hot spot that demonstrates the presence of a homogeneous plume source that has unique isotopic characteristics. He-3/He-4 ratios of the two volcanoes on R eunion (Piton de la Fournaise, <0.53 Ma: Piton des Neiges, 2-0.44 Ma) are u niform 12-13.5 Ra regardless of He-4 concentration and sample age. The shie ld-building Older Series of Mauritius (5-8 Ma) has a constant He-3/He-4 rat io around 11.5 Ra. The similarity of He-3/He-4 and Sr-Nd isotope ratios amo ng them demonstrate that the volcanoes have had a common homogeneous source related to the mantle plume activity over a period of 8 Myr. Ne-20/Ne-22 a nd Ne-21/Ne-22 of these volcanoes define a linear trend on a Ne three-isoto pe diagram with a slope between the Loihi and MORB correlation lines. There is a clear correlation between Ne-20/Ne-22 and Ar-40/Ar-36. In contrast, I ntermediate (2-3 Ma) and Younger Series ( < 1 Ma) of Mauritius and Rodrigue s (1.5 Ma) have He-3/He-4 ratios similar to MORB and Sr and Nd isotope rati os closer to MORB than lavas from Reunion and Older Series of Mauritius. Th ese Intermediate and Younger Series lavas therefore record a late stage the rmal rejuvenation beneath Mauritius derived from a source that is unrelated to the mantle plume. The isotopic characteristics of the source of the Reu nion magmatism are relatively low He-3/He-4 (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 contamin ation 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 sou rce of other large plumes (Loihi and Iceland). clearly showing that the man tle contains several relatively less-degassed reservoirs and not a single p rimitive 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 experie nced 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 t he convectively stirred lower mantle. (C) 2001 Elsevier Science B.V. All ri ghts reserved.