Isotope Geochemistry of Leg 115 Basalts and Inferences on the History of the Réunion Mantle Plume

Abstract

Strontium, neodymium, and lead isotope ratios are reported for 13 Leg 115 basalts as well as 3 basalts from Texaco drill hole SM-1 on the Mascarene Plateau. Pb ratios, typical of Indian Ocean mid-ocean ridge (MORB) and oceanic-island basalts (OIB). Isotopic compositions of Leg 115 basalts generally fall between fields for MORB and Reunion Island basalts, consistent with the conclusion drawn from geochronological studies that Deccan flood basalt volcanism, the Chagos-Laccadive Ridge, and the Mascarene Plateau are all products of the Reunion mantle plume. Isotopic compositions of magmas produced by this plume have varied systematically with time in the direction of less "depleted," less MORB-like isotopic signatures. This compositional change has been accompanied by a decrease in eruption rate. We interpret Deccan volcanism as the voluminous beginning of the plume. Reduced entrainment of asthenosphere following melting of the plume head resulted in less MORB-like isotope ratios in magmas and a decrease in eruptive activity with time.

Geochemistry of Lavas from Mauritius, Indian Ocean: Mantle Sources and Petrogenesis

“…Values for the youngest drill site, Site 706 (33 Ma), are closest to those of Mauritius and Réunion. White et al (1990) argued that, as a group, the ε Nd values of the Leg 115 basalts and Réunion showed an overall decrease, and 87 Sr/ 86 Sr a rough increase, with decreasing age. Within the framework of the mantle plume model for ocean island volcanism (e.g., Richards et al, 1989), they interpreted this trend as reflecting progressively decreasing entrainment of asthenospheric (MORB-source) mantle by the upwelling plume.…”

“…Data sources for Mauritius are this study, Mahoney et al (1989), and Peng and Mahoney (1995). Fields for CIR and Carlsberg Ridge MORB (Ito et al, 1987;Mahoney et al, 1989) and Réunion (Fisk et al, 1988) are shown for comparison, as are data for ODP Leg 115 and Texaco drill site SM-1 (White et al, 1990). Ages for the Leg 115 and SM-1 basalts are from Duncan and Hargraves (1990). post-shield Intermediate and Younger Series lavas of Mauritius are more MORB-like in Nd-Sr isotopic characteristics than the shield-stage Older Series, it is possible, perhaps likely, that the drill-site lavas represent post-shield volcanism and that shieldstage lavas were not penetrated during drilling.…”

“…Morgan (1981) Mahoney et al (2002) and . ODP Leg 115 sites (706,107,713,715) and Texaco drill site SM-1 are indicated by open circles; the ages of the basalts are from McDougall and Chamalaun (1969) and Duncan and Hargraves (1990). have been studied extensively (e.g., McDougall, 1971;Fisk et al, 1988;Albarède and Tamagnan, 1988;Gillot and Nativel, 1989;Staudacher et al, 1990;White et al, 1990;Albarède et al, 1997;Hanyu et al, 2001). In comparison, few studies of Rodrigues Island lavas exist (e.g., McDougall et al, 1965;Baxter et al, 1985).…”

A boundary exchange influence on deglacial neodymium isotope records from the deep western Indian Ocean

“…Cretaceous basalts also occur along the southeast Madagascan coast, with instead more radiogenic isotopic compositions of ε Nd = +4.6 ± 3.0 (2σ, n = 3) (Mahoney et al, 1991). The geology of the Mascarene Plateau is varied and Nd isotope measurements are available from southern Mascarene Plateau volcanics with ε Nd = +6.3 ± 1.6 (2σ, n = 8) (White et al, 1990) and Reunion Island volcanics with ε Nd = +3.1-4.5 (Bosch et al, 2008). In the global compilation of Jeandel et al (2007), the margin of the western Indian Ocean in this region is assigned a value of ε Nd = -14, but this clearly represents an average of old continental crustal sources and younger volcanics rather than a uniform source of this composition.…”

Tracing the Indian Ocean Mantle Domain Through Time: Isotopic Results from Old West Indian, East Tethyan, and South Pacific Seafloor

“…Lawver & Gahagan, 1993) (slightly before the 150 Ma Masiran origin of the 120 Ma Masiran rocks remains problematic, because although the isotopic signature of the main rocks were formed), and northward flow of Indian-Oceantype mantle may have begun earlier there. If so, to reach component in the Réunion plume appears to have changed little in the last 66 my (White et al, 1990; Peng the paleolatitude of the Samail crust by 95 Ma, such asthenospheric flow must have occurred at rates of~70-& , the Réunion hotspot is believed by most workers to have appeared only shortly before 66 100 mm/yr; in comparison, rates of 25-40 mm/yr are indicated for westward flow of Pacific-type asthenosphere Ma, the Deccan Traps event being interpreted as the hotspot's initial, plume-head phase [e.g. Basu et al (1993) into the southeastern Indian Ocean between Antarctica and Australia since 43 Ma (Pyle et al, 1992(Pyle et al, , 1995.…”