Rare Glass in Deep Mantle Hot-Spots on the
South Atlantic Ridge

Philippe Sarda Laboratoire de Géochimie, Institute de Physique du Globe, 4 place Jussieu,

75252 Paris Cedex 05, France


Manuel Moreira Laboratoire de Géochimie, Institute de Physique du Globe, 4 place Jussieu,

75252 Paris Cedex 05, France

Thomas Staudacher Laboratoire de Géochimie, Inst. de Physique du Globe, 4 place Jussieu,

75252 Paris Cedex 05, France

Jean-Guy Schilling School of Oceanography, University of Rhode Island, Nagaransett, RI, USA

Claude J. Allègre Laboratoire de Géochimie, Institute de Physique du Globe, 4 place Jussieu,

75252 Paris Cedex 05, France

In a recent paper, we presented rare gas data demonstrating that the southern end of the South Atlcantic Ridge is affected by interaction of a deep mantle plume with the ridge, in the so-called Shona ridge anomaly area, at 51-52°S (Moreira et al., 1995). Helium and neon isotopically typical of a poorly degassed reservoir were found in the Shona samples, with 4He/3He ratios down to 55,000, high 20Ne/22Ne ratios of up to 12.6 and intermediate 21Ne/22Ne ratios of 0.03 -0.05. Here, we present a complete noble gas study of a nearby zone of the South Atlantic Ridge, namely the Discovery zone at 48-52°S, where one expects to see the influence of the plume which generated the Discovery line of seamounts located between the ridge and the South African continent. Our measurements show that the Discovery hot spot is the brother of the Shona one, with again high amounts of 3He relative to 4He. The neon also shows the same kind of systematics than at Shona, with a sharply defined primitive component of virtually solar isotopic composition, and some kind of mixing trend between the plume and the local MORB component. Argon is compromised by some contamination process. Xenon has some isotopic anomalies in the zones where the plume influence is small suggesting that the isotopic composition of plume xenon has only weak anomalies if any.

The samples were collected by dredging during a cruise of the R/V Maurice Ewing in November-December 1993. Helium from the Discovery zone exhibits a large and well defined isotopic anomaly centered on the ridge segment comprised between the Falkland-Aghulas fracture zone to the North and the small offset to the South. Close to the nothern fracture zone, helium is close to normal MORB with a 4He/3He of 86,000. Next to the south, four dredges show much smaller, relatively constant 4He/3He ratios of 45,000 - 50,000. South of the ridge offset, the 4He/3He ratio becomes surprisingly radiogenic, with a maximum value of about 125,000. Theses samples are supposed to be DUPAL like by Douglass et al. (1995) based on the Pb-Sr isotopic ratios. Further South, the 4He/3He decreases back to more normal MORB values, then decreases again toward plume-like values at the Shona zone.

Neon also exhibit large plume-type isotopic anomalies, and for the very same samples which define the low 4He/3He isotopic anomaly. The maximum 21Ne/22Ne values obtained from stepwise degassing display a pattern very similar to the helium one versus latitude. The maximum 20Ne/22Ne ratio measured is 13.1±0.2 (21Ne/22Ne=0.042), very close to the solar value in the three neon isotope diagram. In the same diagram, the Discovery data define a bridge between the solar-like plume component and the normal MORB alignment, where the maximum values are relatively low for both 20Ne/22Ne and 21Ne/22Ne. This is analoguous to what was observed at Shona, and is important because this area of the ridge is the only one known at present where the neon data do not define a linear trend passing through the point for atmospheric neon. One possible interpretation is that we actually observe mixing between an upper mantle degassed component typical of MORB and the plume component, with only minor perturbation by atmospheric contamination. If this is true, the local MORB component must be characterised by relatively low 20Ne/22Ne and 21Ne/22Ne. Samples with high 4He/3He are below the MORB line (Sarda et al., 1988) in the neon diagram and thus it reflects the neon isotopic signature of the Dupal Anomaly which is more nucleogenic than the North Atlantic.