87Sr/86Sr and 143Nd/144Nd ratios constraints on the behaviour of trace elements, MORB peridotite-sea water interaction: experimental

Abstract: The geochemical changes associated with the experimental serpentinization of peridotite are due to reaction with sea water and to the growth of new mineral phases. Since the rare earth elements (REE) and Sr are primarily ensconced in clinopyroxene, it is the unreactive nature of this phase in the presence of sea water that determines the REE content, the 875r/S6Sr and ~43Nd/14aNd ratio of the experimentally produced serpentinites. Serpentinites (after lherzolite and dunite) have chondrite-normalized REE abundance patterns similar to the initial peridotite indicating that the light REE (LREE) are not selectively mobilized by peridotite-sea water interaction at 300~ However, the serpenti- nites produced as a result of harzburgite-sea water experiments show an increase in LREE content. Despite the sporadic behaviour of the LREE, the ~43Nd/~**Nd ratios in experimen- tally produced serpentinites (after lherzolite and harzburgite) are identical to primary clinopyroxene in the unaltered peridotite. In contrast, a marked change in the strontium isotopic composition of the peridotites occurs during experimental serpentinization due to the growth of hydrous and Ca-rich phases which facilitates the uptake of sea water Sr. Whereas harzburgite and dunite alter to produce serpentinites with high Sr contents and 87Sr/86Sr > 0.709, lherzolites tend to alter to serpentinites with 875r/86Sr < 0.709. This behav- iour invalidates the use of Rb-Sr data in understanding the origin of oceanic and ophiolitic peridotites, but the relative immobility of the light REE (in clinopyroxene-bearing perido- tites) and the low REE content of sea water encourages the careful use of the REE and neodymium isotopes as a petrogenetic indicator for elucidating the origin of serpentinized abyssal peridotites. Serpentinized oceanic peridotites were first reported from Atlantic ocean islands by Darwin (1845) and later studied by Tilley (1947), Hess (1954) and Melson