Control of the Products of Serpentinization by the Fe2+Mg1 Exchange Potential of Olivine

It is argued that the high-Mg content (mg-number 95 3) of the serpentine minerals in serpentinized peridotite is a consequence of the environmental Fe2+Mg1 exchange potential imposed on the system by the abundance of olivine and orthopyroxene. Mass balance in the serpentinization reaction then requires the precipitation of an iron-rich mineral that in most cases is magnetite.This causes hydrogen to be evolved in an oxygen-conserved reaction.The low-variance mineral assemblage Olþ SrpþBrcþMag sets the chemical potentials of H2O, SiO2 and O2 internally at an early stage in the process, but the paragenetic assessment of serpentinites is rendered difficult by the variable and usually unknown Fe3þ content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe3+/SFe ratios404, with an average value (069) similar to that of magnetite (067). This feature may be attributed to the presence of high-Fe3+ lizardite