Oxygen isotope trajectories of crystallizing melts: Insights from modeling and the plutonic record

Abstract

Elevated oxygen isotope values in igneous rocks are often used to fingerprint supracrustal alteration or assimilation of material that once resided near the surface of the earth. The δ 18O value of a melt, however, can also increase through closed-system fractional crystallization. In order to quantify the change in melt δ 18O due to crystallization, we develop a detailed closed-system fractional crystallization mass balance model and apply it to six experimentally-and naturally-determined liquid lines of descent (LLDs), which cover nearly complete crystallization intervals (melt fractions of 1 to <0.1). The studied LLDs vary from anhydrous tholeiitic basalts to hydrous high-K and calc-alkaline basalts and are characterized by distinct melt temperature-SiO 2 trajectories, as well as, crystallizing phase relationships.Our model results demonstrate that melt fraction-temperature-SiO 2 relationships of crystallizing melts, which are strongly a function of magmatic water content, will control the specific δ potentially by ~1.5‰, compared to hot, dry granites. Therefore, it is critical to constrain zircon-melt fractionations specific to a system of interest when using zircon δ 18O values to calculate melt δ 18 O.

Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U–Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

“…These basaltic values are shown in Fig. 8a along with a range of possible basalt oxygen isotopic compositions that could potentially be produced through fractional crystallization of a tholeiitic basalt at low pressure (after Bucholz et al 2017). It is clear that some of the calculated basalt δ 18 O values are higher than would be expected through fractional crystallization of a primitive basalt alone; therefore, they are likely recording some other processes.…”

“…The zircon δ 18 O are separated into the average value for the sample shown by filled circles and also the high values shown by open circles of the same color. The range for the δ 18 O of mantle-derived tholeiitic basalt that has undergone high degrees of fractional crystallization is shown by the gray bar (see Bucholz et al 2017). b Calculated basalt δ 18 O values for each sample are compared to the zircon εHf values.…”

Strongly Peraluminous Granites across the Archean–Proterozoic Transition

“…Magmatic d 18 O values can increase through differentiation and concomitant SiO 2 enrichment in the melt (Eiler, 2001;Bindeman et al, 2004;Bucholz et al, 2017). However, the difference in whole-rock d 18 O values is not simply a function of an increase in whole-rock SiO 2 contents.…”

The evolution of magma during continental rifting: New constraints from the isotopic and trace element signatures of silicic magmas from Ethiopian volcanoes

“…We treated the melt as a mixture of CIPW normative minerals, and calculate the temperature and melt composition-dependent mineral-melt fractionations ( at each step (i+1) and for each crystallizing mineral (n). This forward-step mass balance model (detailed in the Supplementary Information) follows equations from Bucholz et al (2017). We determine 18 Omelt at each stage of peralkaline melt genesis from primitive rift-related basalts and predict the 18 O trajectory that may plausibly represent the products of closed-system fractional crystallization.…”