Oxygen Isotope Trajectories of Crystallizing Arc Magmas

Wednesday, 17 December 2014: 11:20 AM
Claire E Bucholz1, Oliver E Jagoutz1, Jill A VanTongeren2 and Zhengrong Wang3, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)Rutgers University New Brunswick, New Brunswick, NJ, United States, (3)Yale University, Geology and Geophysics, New Haven, CT, United States
Oxygen isotopes are essential to quantify mantle-derived versus ‘recycled’ crustal contributions to arc magmas. High δ18O values in igneous rocks (i.e., δ18OSMOW > ~5.7) are generally used to identify supra-crustal inputs, but a melt can also become enriched in 18O due to magmatic differentiation [1,2]. To assess magmatic δ18O values of plutonic rocks, δ18Ozircon values, which are resilient to secondary alteration, are often used. Thus, to disentangle the effects of assimilation versus fractionation, both the absolute increase in melt δ18O due to differentiation and ∆18O(WR-zircon) must be determined. However, existing constraints on the effect of magmatic fractionation on melt δ18O are model-based [2] and calculated relationships between WR SiO2, δ18Ozircon, and δ18Omelt do not incorporate complex melt SiO2, H2O, and temperature (T) relationships [3]. To build upon these initial constraints, we combine the first high-precision δ18O data set on natural samples documenting changes in δ18O melt values with increasing extent of differentiation and modeling which incorporates experimentally constrained melt SiO2, H2O, and T relationships. We analyzed 55 mineral separates with infrared laser-fluorination [4] across large fractionation intervals of two well-studied cumulate sequences: (I) a relatively dry (~1 wt.% H2O initial) tholeiitic sequence (analyzed minerals include plag, opx, cpx, & Fe-rich ol) from the Bushveld Complex and (II) a hydrous high-K sequence (analyzed minerals include ol, cpx, bt, fsp, & qtz) from the Dariv paleoarc in Mongolia. Our results indicate that multiple per mil increases in melt δ18O can occur during magmatic fractionation that in detail depend strongly on melt composition and T. Calculated relationships between WR SiO2 and δ18Ozircon for experimental melt compositions show that wet, ‘cool’ and dry, ‘hot’ melts are characterized by larger and smaller ∆18O (melt-zircon) fractionations, respectively. Applying our results to the O isotope record of the Sierra Nevadas [3], we show that some trends in δ18Ozircon from individual intrusive suites can be explained by fractional crystallization alone and do not require temporal variations in crustal contamination.

[1] Eiler 2001 [2] Bindeman et al. 2004 [3] Lackey et al. 2008 [4] Wang et al. 2011