Identification, classification, and interpretation of boninites from Anthropocene to Eoarchean using Si-Mg-Ti systematics

Abstract

Boninites are rare, high-Si, high-Mg, low-Ti lavas that have considerable tectonic significance, especially for recognizing and interpreting episodes of subduction initiation in the geologic record. Formal identification and classification of boninites may be carried out using MgO-SiO2 and MgO-TiO2 diagrams to find compositions that satisfy modified International Union of Geological Sciences (IUGS) criteria of Si8 > 52 and Ti8 < 0.5, where Si8 and Ti8 refer to concentrations of the oxides at 8 wt% MgO. However, screening of highly metasomatized rocks and accurate classification require precautions, including normalization to a 100% volatile-free basis. The MgO-SiO2 diagram can also be used for subdivision into low-Si boninites (Si8 < 57) and high-Si boninites (Si8 > 57). Satisfying one but not both of the boninite criteria are rocks with Si8 > 52 but Ti8 ≥ 0.5 (siliceous high-magnesium basalts) and rocks with Si8 ≤ 52 but Ti8 < 0.5 (low-Ti basalts). We tested the classification methodologies using ∼100 low-Ti lava suites dating from the present-day back to the Eoarchean. We conclude that, of those classifying as “boninite series,” Izu-Bonin-Mariana arc–type subduction initiation terranes provide the dominant setting only back as far as ca. 2 Ga, which marks the maximum age of extensive clinopyroxene-undersaturated melting and eruption of high-Si boninites. From 2 to 3 Ga, most boninites formed in intraplate settings by melting of refertilized, depleted cratonic roots. Prior to 3 Ga, hot, depleted mantle plumes provided the main boninite sources. Nonetheless, arc-basin boninites, though rare, do extend back to 3.8 Ga, and, together with the inherited subduction component in intracratonic boninites, they provide evidence for some form of subduction during the Archean.

The inception of plate tectonics: a record of failure

“…Moreover, Turner et al [33] showed that, not only do the Nuvvuagittuq mafic rocks overlie modern arc rocks on a plot of Th/Yb versus Nb/Yb, but they also record the same geochemical stratigraphy as the lowermost sequences in modern subduction zones. The recent analysis of Pearce & Reagan [93] further reinforces the subduction-like nature of the mafic Nuvvuagittuq mafic rock sequences ( figure 7), and so there is an increasingly strong case that these reflect some form of subduction at, or prior to, 4.1 Ga. Before 4.1 Ga, we unfortunately have a dearth of rocks and rock relationships to study and so the main record of crust from these times is contained in detrital zircons. Belousova et al [100] have argued that the excursion to low ε Hf isotope values in greater than 3 Ga detrital zircons requires that some silicic crust had developed much earlier in order to be recycled.…”

“…As such, these observations are most consistent with an association of these particular rocks with mature subduction that has evolved past the initiation stage. [93]). Panels (a)-(e) demonstrate the importance of chemically screening the altered rocks using the methodology detailed in Pearce & Reagan [93].…”

An andesitic source for Jack Hills zircon supports onset of plate tectonics in the Hadean

“…Although some remain sceptical, plots like the Th/Yb versus Nb/Yb diagram shown in Fig. 2a have been repeatedly appraised to help distinguish the tectonic setting in which ancient rocks formed 25,26 . As can be seen the protoliths of the Jack Hills zircons clearly plot well above the fields for oceanic rocks and in or above the field for modern arc rocks 3 .…”

Reply to referee Y. Kusano's comment

“…The definition of boninite-series lavas has been incrementally expanded to include fractionation series which pass through the IUGS fields, extending from low-Si compositions to high-magnesium andesites (HMA) at < 8 wt % MgO (Fig. 6; Pearce and Reagan, 2019;Pearce and Robinson, 2010;Reagan et al, 2017). Boninites can be further divided into low-Si boninite (LSB) and high-Si boninite (HSB) series, with a dividing composition along an olivine fractionation line which passes through SiO 2 = 57 wt % at MgO = 8 wt % .…”