Magnesium isotope compositions of Mauna Kea lavas: Crystal fractionation effect

Recently, sub-per mil of δ^26/24Mg variations have been found in oceanic and continental basalts, and they are attributed to mantle source heterogeneity. For example, Teng et al. (2016) attributed the high δ^26/24Mg, 0.15 higher than typical mantle value, in some Lesser Antilles arc lavas to contribution of fluid released from the altered subducting slab. Low δ^26/24Mg have been reported in continental basalts from eastern China (Li et al., 2017b) and oceanic basalts from Pitcairn (Wang et al., 2018), which have been attributed to recycled carbonates in their mantle sources. In all the studies, it is assumed that basaltic magma evolution does not fractionate Mg isotopes, a conclusion reached by Teng et al. (2007; 2010) based on the measurements of 14 lavas from Kilauea Iki lava lake at Hawaii. However, our Mg isotope study on Mauna Kea lavas challenges this assumption. The studied Mauna Kea lavas include both shield and postshield stage lavas, with MgO ranging from 30% to 2%, recording the whole crystallization history of basaltic magma. ⁸⁷Sr/⁸⁶Sr decreases and La/Yb increases during the shield to postshield transition, because the volcano sampled more of a depleted source component, and its degree of partial melting decreased during this transition. Mauna Kea shield stage lavas have mantle-like δ^26/24Mg, but postshield stage lavas have both higher and lower δ^26/24Mg, ranging from -0.1 to -0.4. Within Mauna Kea lavas, δ^26/24Mg is not correlated with either ⁸⁷Sr/⁸⁶Sr or La/Yb, implying that δ^26/24Mg is not controlled by either source heterogeneity or degree of partial melting. In contrast, δ^26/24Mg is controlled by crystal fractionation. At MgO > 8% when olivine was the only crystal phase, there is no measurable δ^26/24Mg variation in Mauna Kea lavas. In contrast, at MgO < 8%, magma compositions are controlled by multiple phase crystallization, including pyroxenes, magnetite, and ilmenite, and their δ^26/24Mg range from higher to lower than typical mantle values. Compared to melt, magnetite has a higher δ^26/24Mg and ilmenite has a lower δ^26/24Mg. Consequently, the large δ^26/24Mg variation in low MgO lavas may reflect crystal fractionation effects of magnetite and ilmenite.