Nuclear microprobe analysis of serpentine from the Mid-Atlantic Ridge

At mid-ocean ridges, ultramafic rocks are serpentinized by interaction with seawater-derived fluids. Elements, dissolved in large quantities in seawat er, e.g., Na, K, Cl, Br, Ca and Sr, can be, in small amounts, incorporated as traces into the crystal structure of the various serpentine minerals (Mg 3Si2O5(OH)(4)) These trace elements can be used to track the composition of the reacting fluids and to constrain physico-chemical conditions. This pap er represents the first application of particle-induced X- and gamma-ray em ission (PIXE/PIGE) analysis to serpentine using the nuclear microprobe at t he Laboratoire Pierre Sue (CEA-CNRS). Three types of serpentine, belonging to two different serpentinization generations, have been analysed in sample s collected from the Mid-Atlantic Ridge (14 degrees 45'N/45 degrees W) that exposes serpentinized peridotites on which the Logachev black smoker is pl aced. The trace elements Cl, F, S, Cu, Zn, Ca, K, Ni, Cr and Mn were detect ed from several tens to several thousands of ppm. Bromine, As and Sr are cl ose to the detection limit of about 5 ppm. The trace element concentrations and interelement relationships in serpentines vary (a) with the serpentine type and (b) with the geographic location to the black smoker. Chlorine an d in part S originated from seawater, whereas Cu, Zn, Ca, K, Ni, Cr and Fe and the major amount of S were mobilized from the unaltered host rock and p artitioned between the serpentine and the aqueous solution. (C) 1999 Elsevi er Science B.V. All rights reserved.