RE-EVALUATING THE UTILITY OF SPINEL-GROUP MINERAL CHEMISTRY AS A GEOTECTONIC INDICATOR

Ultramafic rocks form in a diverse suite of petrogenetic environments, including: as part of oceanic lithosphere (ophiolites), lavas derived from high degrees of partial melting (komatiites), and cumulate products (e.g., layered intrusions). Determining the petrologic and tectonic processes responsible for their formation and preservation in the rock record can provide critical insight into lithospheric evolution. However, ultramafic rocks are susceptible to chemical alteration that often obscures their primary origin.

Spinel-group minerals exhibit a broad range of solid solution and are relatively resistant to alteration compared to other high temperature minerals. Consequently, they have been used to distinguish between geotectonic environments for over 50 years (Irvine 1965). In a comprehensive compilation of chemical analyses from almost 1000 studies, Barnes and Roeder (2001) developed broad geotectonic fields in bivariate and ternary space, using Fe²⁺# (Fe²⁺/[Fe²⁺+Mg]), Fe³⁺# (Fe³⁺/[Fe³⁺+Al+Cr]), Cr# (Cr/[Cr+Al]) and TiO₂ as proxies.

Here, we present an internally consistent major element dataset collected by electron microprobe. The dataset has been filtered (based on textural and chemical constraints; see Guice et al. 2022) to isolate primary chemical compositions, with the aim of re-assessing the utility of spinel-group mineral chemistry for distinguishing between geotectonic environments. Our dataset of over 2300 analyses includes samples from classic komatiites (Komati, South Africa; Munro, Canada; Gorgona, Colombia) and ophiolites (Semail, Oman), and altered komatiites (Morro do Onça, Brazil) and ophiolites (Baltimore Mafic Complex; Stateline Complex, USA).

Results and associated statistical analysis demonstrate that TiO₂ is the only proxy capable of distinguishing ophiolites (< 0.16 wt. %) from komatiites (0.22–0.74 wt. %). All other chemical proxies show significant overlap between ophiolite (e.g., Cr# = 19–85) and komatiite (Cr# = 39–86) samples. We also show that the range of altered compositions is significantly greater than previously characterized, and demonstrate that rigorous petrographic assessment is vital to avoid incorrectly classifying altered komatiites as ophiolites or vice versa.