Timescales of Magmatic Differentiation
Crystal-liquid separation is one of the fundamental processes that drives changes in magma composition. On a large-scale this process contributes to the differentiation of continental crust, while on the scale of an individual intrusion it can lead to a greater potential to dike or erupt. However, the timescales over which this process occurs over a range of crustal depths and magma SiO2 content remain largely unconstrained. Recently, I've become interested in better constraining these timescales. This research has involved work on crystal-liquid separation in large, upper-crustal, silicic magma reservoirs as well as quantifying the rates in layered mafic intrusions. I'm currently exploring techniques and potential localities to expand this research to the middle and lower crust.
Coupled geochronologic and geochemical constraints on the timescale of magmatic differentiation from alkaline basalt to quart monzonite in the Dariv Complex, Mongolia from Bucholz et al. (2017).
Dariv Complex, Mongolia
The Cambrian Dariv Igneous Complex in western Mongolia preserves both cumulates and residual melts related to differentiation of a parental high-K arc basalt. High Zr concentrations in the parental magma resulted in early zircon saturation, and in conjunction with Claire Bucholz (Cal Tech) and Oli Jagoutz (MIT), I've dated zircons from rocks ranging in composition from ultramafic cumulates (biotite wherlite) to silicic granitoids (quartz monzonite and late stage felsic dikes). The zircon dates suggest that differentiation occurred in ≤ 590 ± 350 kyr, consistent with other estimates for the timescales of differentiation (Bucholz et al., 2017). This dataset represents the first time that zircon has been recognized as an early crystallizing phase in a layered mafic intrusion and consequently provides the first direct age constraints on the duration of magmatic differentiation. We are currently looking for younger alkaline intrusive complexes that can provide higher-resolution age constraints.
Back scattered electron (BSE) image of zircon included in clinopyroxene in a monzogabbro within the Dariv igneous complex. Textural constraints on the position of zircon within this suite of rocks allowed us to assess whether it represented an early or late crystallizing phase.
We concluded that early zircon saturation in the Dariv complex is related to elevated Zr concentrations in the parental basalt. In this graph reproduced from Bucholz et al. (2016), we show that high Zr content may be common in primitive alkaline melts.