Significance of Lower Mantle Entrainment or Plume-Lithosphere Interaction in Oceanic Island Basalts: Helium Isotope Vvidence from Grande Comore

Cornelia Class Lamont-Doherty Earth Observatory of Columbia University, P.O. Box 1000, Palisades, NY 10964, USA

Steven L. Goldstein Max-Planck Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

Mark D. Kurz Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Many oceanic islands display large isotopic variations, showing that erupted lavas contain contributions from different sources. There is still no consensus whether such variations reflect intrinsic heterogeneity of the deep mantle plume source or contamination of plume material during its rise to the surface of the Earth. Two recent suggestions have focused on the importance of entrainment of surrounding mantle material in a rising plume (Hart et al., 1992; Hauri et al., 1994) and interaction between the plume and the intruded oceanic lithospheric mantle (Class and Goldstein, 1994, 1995). Grande Comore, comprised of La Grille and Karthala volcanoes, provides a special opportunity to assess the relative importance of mantle entrainment versus plume-lithosphere interactions. Class and Goldstein (1994, 1995) have argued, using a trace element approach, that the mantle source of lavas from La Grille volcano on Grande Comore contains amphibole, which is stable in the lithospheric mantle but not in a rising thermal plume from the deep mantle. Class and Goldstein interpreted the isotopic variations of Grande Comore lavas as reflecting mixing between components from the deep plume and the oceanic lithosphere. The Sr-Nd-Pb isotopic composition of their lithospheric end-member coincides with a mantle component suggested to be lower mantle entrained by rising plumes (Hart et al., 1992; Hauri et al., 1994). Thus, both models have strong but conflicting implications concerning locations and compositions of mantle reservoirs.

Helium in Grande Comore volcanics

The interpretations of plume-lithosphere interaction and lower mantle entrainment both conclude that Karthala lavas have the largest component from the deep mantle plume source in Grande Comore. It has been previously suggested that the Comoro plume is a 'low 3He' hotspot, based on helium isotopes of a sample from Karthala (Kaneoka et al., 1986). Combining our preliminary helium data with the published sample shows a 3He/4He range for Karthala of R/Ra = 5.2-6.7, thus extending from approximately MORB-like (R/Ra = 8 ± 1) to lower values. Such low values might indicate presence of recycled crustal material in the deep mantle source of the Comoro plume, or contamination of plume-derived melts by the intruded oceanic crust. Samples with Pb-Sr-Nd isotope ratios closer to the proposed plume-endmember still have to be measured, as it will require more effort to separate olivines from these samples than for the ones used to obtain the preliminary data. As a result, it is not yet clear whether the low 3He/4He ratios reflect the isotopic composition of the deep Comoro plume or indicate shallow-level contamination of the helium.

The models of plume-lithosphere interaction versus mantle entrainment by the rising plume can be tested using helium isotopes on La Grille lavas, which have Sr-Nd-Pb ratios of the "lithosphere or lower mantle" endmember. The lower mantle entrainment model assumes that primordial helium is stored in the lower mantle. If 3He/4He ratios of the "lithosphere or lower mantle" endmember are higher than typical MORB upper mantle (R/Ra ~8), this would support the lower mantle entrainment model. If they are around the range of MORB, it would support the plume-lithosphere interaction model, as this range characterizes lithospheric mantle samples worldwide (Porcelli et al., 1986; Vance et al., 1989). The sample thus far analyzed from La Grille has R/Ra = 6.83. To the extent that this sample is representative of the La Grille, it indicates that this endmember of the Grande Comore isotope array does not represent a 'high 3He/4He' lower mantle component, but rather supports the suggestion of Class and Goldstein (1994, 1995) of a lithospheric origin for these lavas.


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