Rocks mineralogy controls the metasomatic activities: An indication from water/rock ratio modeling

The final ratio equation of an isotopic element ina rock, derived from water/rock formula of McCulloch etal. Earth Planet Sci Lett 46:201-211, 1980, McCulloch etal. J Geophys Res 86:B4 2721-2735, 1981 is used to assessthe behavior of diverse suites of rocks towards thealteration effect, and what implications can give abouthydrothermal alteration in terms of isotopic compositions.Due to their higher Sr and lower Nd initial ratios thanseawater, rocks of metamorphic and sedimentary signaturessuch as carbonates and Precambrian basement rocks showsimilar but inverse mixing curves compared with igneousrocks. Sr composition of rocks immediately alters byseawater, while Nd composition keeps unchanged untillarge volumes of water are added. Although, this can beattributed to the very low Nd concentration in seawater, itindicates that Nd-exchange may only take place underseawater, possibly hydrothermally by circulated seawater,and Nd-concentration of less altered crustal rocks areapparently primary. The isotopic composition and rockmineralogy seem to be the main factors controlling thevolume of water required to cause isotopic alteration in rocks.Crustal rocks require higher water volumes due to theirrelatively low temperature minerals, whereas, mantle peridotites mainly consist of residual olivine minerals that are highlysusceptible to alteration and lack of Sr and Nd compositions,and so need less amount of water for metasomatism. Thisproperty reduces the limited penetration effect as the maficaffinity increases at depth in the oceanic crust, and enablesmodified (probably acidified) circulated fluids to maintain ionexchanging and leaching throughout their passageway.