Rapid Phenocryst Growth in Xenolith- Bearing Cima Basalts during Ascent: Application of Oliv-Melt Thermometry

Origin of alkali olivine basalts and hawaiites in the western Mexican arc: Evidence of rapid phenocryst growth and magma mixing during ascent along fractures

“…Another question explored in this study is the origin of an olivine population in many of the Sangangüey basalts; the olivine is too Mg-rich to be in equilibrium with the host melts, even at the liquidus (e.g., ~Fo 88-90 in samples with ~5.5 wt% MgO). This feature of the SB was first emphasized by Díaz-Bravo et al (2014) and is also seen in other intraplate hawaiites, including those from the Cima volcanic Research Paper field, California, which additionally entrained peridotite xenoliths (Brehm and Lange, 2020a). Five competing hypotheses are examined to explain the common occurrence of such a Mg-rich olivine population in intraplate lavas: (1) a surprisingly high oxidation state and thus elevated Fe 3+ /Fe T ratio in the melts; (2) the possible role of dissolved carbonate in the melt in lowering the value for the Fe 2+ -Mg exchange partition coefficient between olivine and melt (e.g., Dasgupta et al, 2007); (3) incorporation of olivine crystals from disaggregated mantle xenoliths; (4) disequilibrium due to kinetics of rapid growth; and/or (5) mixing between high-MgO and low-MgO melts.…”

“…The tectonic setting of these intraplate melts, in an extensional rift system with an abundance of extensional normal faults, may have facilitated this style of magma mixing, namely during ascent along fractures. Note that similar intraplate hawaiites (with Mg-rich olivine crystals that display rapid-growth textures and thin to absent Fe-rich rims), which additionally entrain peridotite xenoliths, are found in the Cima volcanic field (Brehm and Lange, 2020a) in the Basin and Range extensional province of the western United States. The presence of peridotite xenoliths in the Cima samples precludes storage in crustal chambers, and thus phenocryst growth and magma mixing must have occurred during ascent.…”