Intellectual Merit. A fundamental problem in the origin of silicic magmas in oceanic environments (both oceanic arc and mid-ocean ridge) concerns the relative roles of fractional crystallization of basaltic magmas vs. partial melting of hornblende bearing lower crustal grabbro or gabbroic cumulate rocks. This issue bears on basic interpretations of the inner workings of oceanic magmatic systems and, ultimately, their impact on continental growth. The PI has proposed recently a geochemical "test" (based on the co-variation between SiO2 and the REE) that might permit discrimination of these two processes in oceanic environments (Brophy, 2008, 2009). In short, fractional crystallization of parental basalt oceanic island arc (OIA) or mid-ocean ridge (MOR) settings should lead to a positive SiO2-REE correlation while dehydration melting of amphibolite (OIA environment) or hydration-induced melting of gabbro (MOR environment) should lead to a negative SiO2-REE correlation. If correct, these predicted co-variations represent a straightforward geochemical test for the genesis of silicic magmatism in oceanic environments. This proposal aims to evaluate the robustness of this model via both experimental studies and petrologic studies of natural rock suites. The experimental work comprises two parts: (1) dehydration melting of oceanic arc crustal amphibolite at 7.5 Kb (~base of crust); and (2) hydration-induced melting of mid-ocean ridge crustal gabbro at 2.0 Kb (~shallow crust). These experiments will be designed to insure global equilibrium and development of sufficiently large melt pools to permit in situ major element (EMP) and REE (and other trace element) (SIMS) analysis. Experiments will be conducted at three different locations including The Institute for Study of the Earth's Interior, Okayama University, Japan, Hannover University in Hannover, Germany, and the University of Oregon. An added benefit will be the determination of partition coefficients for the REE and other trace elements between high-SiO2 liquid (> 65% SiO2) and the minerals hornblende, plagioclase, augite, and Fe-Ti oxides. Such data are sorely lacking and will fill an important void in our overall knowledge of solid-liquid partition coefficients. The field based work comprises studies of natural examples of both dehydration melting of lower OIA crustal amphibolite (the Asago Body of the Yakuno Ophiolite, Honshu Island, Japan) and hydration melting of MOR gabbro (the Fournier Oceanic Crust, New Brunswick, Canada). Through the direct analysis of major element and trace element abundances in both unmelted host rock and crystallized silicic melt (i.e. leucosomes) it is anticipated that the proposed REE-SiO2 systematics can be directly evaluated in the natural setting.
Broader Impacts. The proposed study has the potential to verify and, therefore, provide a straightforward means of determining a crustal melting versus fractionation origin for silicic magmas in oceanic volcanic systems. At the more practical level, the proposed study will initiate (and hopefully perpetuate) collaborative research with several international scientists including Dr. Eizom Nakamura (Director, Institute for Study of the Earth's Interior), Dr. Akira Ishiwatari (Tohoku University), Dr. Juergen Koepke (University of Hannover) and Dr. John Spray (University of New Brunswick). The work will support the training of one graduate student in geology. The Department of Geological Sciences at Indiana University strongly encourages the participation of undergraduate students in faculty research. The PI currently has two undergraduate students working in his lab and expects to "carve out" additional individual projects suitable for undergraduate level research.
A long standing controversy in the study of intra-oceanic silicic magmatism is whether or not such magmas are generated by fractional crystallization or crustal melting. Brophy (2008, 2009) proposed that the Rare Earth Element (REE) – SiO2 systematics of natural felsic lavas could be used to assess a fractional crystallization versus crustal melting. Specifically, it was proposed that, for felsic lavas with greater than 65% SiO2, fractional crystallization should yield a positive REE-SiO2 correlation while crustal melting should yield a negative correlation. This study was designed to verify these predictions through a combined field and experimental study. Field study #1, located in the Fournier Oceanic fragment, New Brunswick, Canada, evaluated the REE-SiO2 systematics of natural melts generated by the hydration melting of mid-ocean ridge crust. The results fully confirm the predicted negative REE-SiO2 correlation. Field study #2, located in the Asago body of the Yakuno Ophiolite, Honshu Island, Japan, evaluated the REE-SiO2 systematics of natural melts generated by the dehydration melting of lower island arc crustal amphibolite. The results fully confirm the predicted negative REE-SiO2 correlation. A high pressure-temperature experimental study of amphibolite dehydration melting, conducted in a piston-cylinder apparatus at 0.4 GPA pressure over a temperature range of 795 to 970 C has yielded partial melts that range in composition from 66 to 78 wt. % SiO2. Laser-ablation ICPMS analysis of several of the experimental glasses display a negative correlation between SiO2 and REE abundances thus further confirming the predicted negative REE-SiO2 correlation. In summary, the combined field and experimental study has confirmed the theoretical predictions of Brophy (2008, 2009). REE-SiO2 systematics can now be used with confidence as a new geochemical tool for discerning a fractional crystallization versus crustal melting origin for natural intra-oceanic silicic rocks. Brophy JG (2008) Contributions to Mineralogy and Petrology, 156: 337-357 Brophy JG (2009) Contributions to Mineralogy and Petrology, 158: 99-111
