Control of the Products of Serpentinization by the Fe2+Mg−1 Exchange Potential of Olivine and Orthopyroxene

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⁺Mg−1 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 Ol + Srp + Brc + Mag 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 Fe³⁺ content of the serpentine minerals, particularly lizardite. Whole-rock analyses of highly to completely serpentinized peridotites reveal Fe³⁺/ΣFe ratios > 0·4, with an average value (0·69) similar to that of magnetite (0·67). This feature may be attributed to the presence of high-Fe³⁺ lizardite, as has been found in Mössbauer spectroscopy studies. Electron microprobe and scanning electron microcope analyses in the literature exhibit element trends (e.g. decreasing Si vs ΣFe a.p.f.u.) for olivine-pseudomorph lizardite and, with some exceptions, for bastite lizardite, that show a substitution of the cronstedtite component (Fe³⁺ charge-balanced on T and M sites). Cronstedtite substitution will be favoured at low temperature and/or low hydrogen fugacity, and in these circumstances less magnetite will be evolved during serpentinization, in some cases none at all. Some bastite lizardites from sea-floor settings show evidence of M-site vacancy substitution of Fe³⁺ for Fe²⁺. In the course of progressive serpentinization, micrometer to millimeter-scale variations in SiO2 potential may well be present, but their influence on Fe in lizardite seems to be limited to a few cases of lizardite associated with orthopyroxene. Chrysotile is on average more Mg-rich and less variable in Fe/Mg ratio than lizardite, facts that may be attributed to the greater Fe³⁺ content of lizardite. Chrysotile veins provide the best record available to us of the environmental Fe²⁺Mg−1 exchange potential in the pore fluid attending serpentinization. This potential serves as a robust control on serpentine and brucite compositions, although it may fail after olivine and orthopyroxene have been armoured or eliminated, and in more open-system environments (high water/rock ratio) such as on the sea floor or at serpentinite host-rock contacts. The default assumption in microprobe analyses that measured iron is all Fe²⁺ can lead to inappropriate petrological conclusions in the case of serpentinites.