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.