Decoding the interplay between magma thermodynamics and lithospheric thermal state as a possible explanation for the origin of the Daly Gap

Abstract

The mechanisms responsible for determining whether a magmatic system will generate a bimodal or monotonous intermediate volcanic suite are still widely debated in igneous petrology. Thus, the compositional characteristics of volcanic suites represent the ultimate reflection of their magmatic evolution and provide crucial insights into their remote volcanic and igneous plumbing systems. Decoding what these erupted compositions record is challenging however, and of paramount importance for volcanology, igneous petrology, and tectonic studies. This work employs an integrated petrological and thermal numerical modeling approach to identify the variables that modulate the compositional diversity registered in several volcano-tectonic settings, using two Neoproterozoic extensional rift basins in southern Brazil as case studies. Based on rhyolite-MELTS thermodynamic models of fractional crystallization, the resulting crystallization-differentiation curves of basaltic magmas indicate that the prevalence or scarcity of intermediate compositions in the volcanic record results from the thermodynamic control of differentiation patterns, whereas the numerical models also support the interplay with lithospheric heat transfer/maintenance processes. There is a well-known, yet underexplored, tendency of fractionating basaltic magmas to differentiate rapidly through intermediate compositions, possibly associated with a sudden increase in silica contents owing to Fe-Ti oxides crystallization and/or crystal productivity. This may explain the lack or scarcity of these compositions within predominantly bimodal volcanic suites, an observation known as the Daly Gap. The models presented in this contribution explore the nonlinear dependency between composition, crystallinity, and temperature (X-F-T) of differentiating basaltic magmas, which seems to be a common feature shared by alkaline, transitional, and even sub-alkaline basalts. When accounting for the considered thermic state of the medium they intrude into (ca. 300 to 800 °C) and the modeled accretion rates (ca. 0.01770 to 0.00354 m/yr) for intervals ≤ 300 ka, the models reveal that magma reservoirs undergoing crystallization-differentiation can only store intermediate compositions capable of erupting in relatively warm environments. Conversely, magma reservoirs with lower heat accumulation consist of alternating basic and silicic compositions, potentially giving rise to bimodal volcanic sequences. In these systems, compositional gaps are associated with increased crystal productivity over limited temperature intervals in the X-F-T space, coupled with relatively lower degrees of heat accumulation. Additionally, a subordinate compositional gap has been observed in the modeled silicic magmas, supporting our interpretation that increased crystal productivity results in subdued compositions along continuous liquid lines of descent. These nonlinear crystallization-fractionation patterns of basalts can also contribute to the compositional disparity between the volcanic and plutonic realms at arc settings.