TEMPORAL EVOLUTION OF THE KERGUELEN PLUME - GEOCHEMICAL EVIDENCE FROMAPPROXIMATE-TO-38 TO 82 MA LAVAS FORMING THE NINETYEAST RIDGE - DISCUSSION

In ''Temporal evolution of the Kerguelen plume: geochemical evidence f rom similar to 38 to 82 Ma lavas forming the Ninetyeast Ridge'' by F. Frey and D. Weis (1995), the literature geochemical data on the Ninety east Ridge are summarized, new trace element and isotopic data are pre sented, and isotope data on Ninetyeast Ridge lavas are compared with t hose of the Kerguelen Archipelago. Frey and Weis (1995) assume the com position of the Kerguelen plume source is nearly homogeneous and is th e same as the youngest lavas of Kerguelen, in agreement with conclusio ns of some previous studies (Storey el al, 1988; Gautier et al. 1990; Weis et al. 1993). Class et al, (1993) concluded that the Ninetyeast R idge-Kerguelen plume source has a unique composition based on evaluati on of the isotopic evolution of plume-related lavas through time, but identified an entirely different composition for it (to be consistent with Frey and Weis, 1995, we call it Kerguelen plume instead of Ninety east Ridge plume). In this comment we review the two contrasting appro aches to distinguishing the composition of the Kerguelen plume source. To simplify matters, we term the Kerguelen plume composition used by Frey and Weis (1995) and the others cited above as the ''static'' plum e composition, and the one of Class et al. (1993) as the ''evolving'' plume composition. We discuss some of our concerns about the foundatio ns for the ''static'' composition, considering in addition to Kerguele n data the constraints afforded by lavas from nearby Heard Island. Fre y and Weis (1995) take strong issue with the interpretation of Class e t al. (1993) and contend that the Kerguelen plume does not represent a n ''evolving'' or ''aging'' plume source. We find their arguments to b e unconvincing, calling for clarification on our part.