Using Major Element Logratios to Recognize Compositional Patterns of Basalt: Implications for Source Lithological and Compositional Heterogeneities

Understanding the source lithology of basalts can greatly impact our understanding of magmatic processes, which can help us gain insight into mantle heterogeneity and crustal material recycling. However, many uncertainties remain about how to recognize the relationship between major element patterns of basalts and source lithological and/or compositional diversities using typical petrological methods. In this study, discriminant functional analysis and multivariate regression are carried out using major element logratios for literature experimental melts of four primitive mantle-like lherzolites and six mafic lithologies (five pyroxenites and one hornblendite). A parameter called FCMS (FCMS=ln (FeO/CaO)-0.058*(ln (MgO/SiO2))(3)-0.636*(ln (MgO/SiO2))(2)-1.850*ln (MgO/SiO2)-1.170, all the major elements in weight percent) is proposed to identify source lithologies for basaltic melts. When the logratios ln(K2O/Al2O3), ln (TiO2/Na2O), and ln (Na2O/K2O) are incorporated into FCMS, the temperature and pressure effects on the compositional heterogeneity of peridotites melts can be significantly reduced; a more powerful parameter called FCKANTMS (FCKANTMS=ln (FeO/CaO)-0.08*ln(K2O/Al2O3)-0.052*ln (TiO2/Na2O)-0.036*ln (Na2O/K2O)*ln (Na2O/TiO2)-0.062*(ln (MgO/SiO2))(3)-0.641*(ln (MgO/SiO2))(2)-1.871*ln (MgO/SiO2)-1.473, all the major elements in weight percent) can be obtained. Olivine fractional crystallization and accumulation, although they can significantly change most major element contents and ratios, usually increase or decrease FCMS and FCKANTMS by only 0-0.15, which is one order of magnitude lower than their variations in the primary experimental melts of different lithologies. Importantly, approximately 80% and 50% low to moderate degree (F<60%) partial melts (n=303) of mafic lithology (with or without volatiles in source materials) can be distinguished from melts of peridotite and transitional lithologies when FCKANTMS combined with ln (CaO/TiO2) and ln (SiO2/(CaO+Na2O+TiO2)). Thus, the chemical markers could be used to constrain source lithology and origin of natural basalts. Plain Language Summary Basaltic melts can be experimentally produced from a variety of mafic and ultramafic lithologies. Understanding the source lithology of basalts can greatly impact our understanding of magmatic processes, which can help us gain insight into mantle heterogeneity and crustal material recycling. However, many uncertainties remain about how to identify the source lithology of basalts using traditional petrological methods. In this study, major element patterns of experimental basaltic melts are explored using major element logratios. We find that most peridotite melts can be distinguished from pyroxenite and hornblendite melts by the logratio-based chemical markers. Thus, the chemical markers could be used to constrain source lithology and origin of natural basalts.