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

Abstract

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/SiO 2 )) 3 − 0.636 * (ln (MgO/SiO 2 )) 2 − 1.850 * ln (MgO/SiO 2 ) − 1.170, all the major elements in weight percent) is proposed to identify source lithologies for basaltic melts. When the logratios ln(K 2 O/Al 2 O 3 ), ln (TiO 2 /Na 2 O), and ln (Na 2 O/K 2 O) 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(K 2 O/Al 2 O 3 ) − 0.052 * ln (TiO 2 /Na 2 O) − 0.036 * ln (Na 2 O/K 2 O) * ln (Na 2 O/TiO 2 ) − 0.062 * (ln (MgO/SiO 2 )) 3 − 0.641 * (ln (MgO/ SiO 2 )) 2 − 1.871 * ln (MgO/SiO 2 ) − 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/TiO 2 ) and ln (SiO 2 / (CaO + Na 2 O + TiO 2 )). 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.

Melt Inclusions in Plagioclase Macrocrysts at Mount Jourdanne, Southwest Indian Ridge (~64° E): Implications for an Enriched Mantle Source and Shallow Magmatic Processes

“…Thus, caution should be taken in identifying the source lithology of basalts with low FC3MS values as well. The uncertainties in CaO-MgO and FC3MS, however, can be resolved by the newly proposed FCKANTMS parameter, especially when combined with ln (SiO 2 /(CaO + Na 2 O + TiO 2 ) and ln (CaO/TiO 2 ) [61]. In general, the 61-67 • E lavas (including the Mount Jourdanne basalts) show a wide range of FCKANTMS (0-0.6) (Figure 7c-e).…”

“…For this reason, many chemical markers of pyroxenite (e.g., MgO-CaO trend, Fe/Mn ratio, and FC3MS parameter) have been proposed based on major element compositions of basaltic lavas [60]. Recently, a more powerful parameter called FCKANTMS was obtained to constrain the mantle source lithology [61]. On the other hand, ratios of moderately incompatible trace elements and major elements, such as Zn/Fe and Zn/Mn [62,63], can be good indicators of the lithology and residual mineral assemblages of the magma sources.…”

“…FC3MS = FeO/CaO − 3 × MgO/SiO2, all in wt %[65]. 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 in wt %[61]. The solid line in (a) is an experimental divide between peridotite and pyroxenite partial melts[64].…”

“…The red dashed line in (b) is upper limit for peridotite melts[65]. The dashed lines in (c)-(e) are boundaries for melts derived from different lithologies[61]. The dashed line in (d) marked with Mg# = 0.7 indicates the upper boundary of normal peridotite and harzburgite melts when their Mg# = 0.7.…”

Olivine in picrites from continental flood basalt provinces classified using machine learning

“…However, we need to emphasize that the link between an olivine population and a source depends on the choice of markers of source lithology, and many markers have their own limitations (e.g. Yang et al, 2019Yang et al, , 2016. Thus, we also applied the last chemical marker suggested by Yang et al (2019).…”