TRACE-ELEMENT BEHAVIOR IN THE ALKALI BASALT-COMENDITIC TRACHYTE SERIES FROM MURUROA-ATOLL, FRENCH-POLYNESIA

Numerous drill holes have penetrated close to 1000 m of the volcanic pile of Mururoa atoll. The rocks are a typical example of mildly alkaline intraplate basaltic volcanics ranging from Mg-rich compositions to comenditic trachytes. Evolved basalts and hawaiites are the dominant rock types. Benmoreites and trachytes are relatively uncommon. The available K-Ar ages indicate a long period of activity between 11.8 and 10.7 Ma. Nevertheless, all the volcanic rocks studied are cogenetic in a broad sense (identical Nd-143/Nd-144 and initial Sr-87/Sr-86 ratios, constancy of ratios of highly incompatible elements). No indications of the occurrence of assimilation or magma mixing have been found. The Mururoa series is thus suitable for the study of fractionation-related processes. Three crystallization/fractionation models have been tested using trace elements, the mineral proportions taken from mass-balance calculations on major elements and individual distribution coefficients determined from trace element data on phenocrysts/host rock pairs. Closed-system fractional crystallization (CSF) is consistent with all of the trace element data. It satisfactorily reproduces the trends observed for compatible and incompatible elements, including the complex behavior of Y and rare-earth elements which are fractionated by kaersutite and apatite in mugearitic and benmoreitic magmas. In situ crystallization and equilibrium crystallization produce trends very close to Rayleigh's law model for most incompatible elements. In contrast, in situ and equilibrium crystallization models produce significant underdepletions (compared to CSF) in compatible elements (Sc, Cr, Co, Ni). The corresponding patterns are not consistent with our analytical data in intermediate and evolved rocks. Cooling calculations indicate, however, that the length of volcanic activity in Mururoa (greater-than-or-equal-to 1 Ma) is hardly compatible with closed-system fractional crystallization of a single magma batch. Various models of open-system fractionation have thus been tested, but generally they do not fit the observed trace element patterns. However, one peculiar case of open-system fractionation in a periodically replenished magma chamber is consistent with the data. In this model, batches of mantle-derived parent liquids filling up the chamber at the beginning of each cycle evolve by fractional crystallization. All the corresponding residual liquids either crystallize or erupt at the end of each cycle, before the next replenishment. Thus, the apparently cogenetic Mururoa series is likely to result from fractional crystallization occurring under similar conditions through time, either in several magma chambers or in a single periodically refilled reservoir.