Experimental petrology constraints on the recycling of mafic cumulate: a focus on Cr-spinel from the Rum Eastern Layered Intrusion, Scotland

Abstract

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Partial Melting of Lower Oceanic Crust Gabbro: Constraints From Poikilitic Clinopyroxene Primocrysts

“…A selection of major elements is presented in Figure 8. The MELTS algorithm gives reasonable estimates of the melt, olivine and clinopyroxene Mg# (Leuthold et al, 2015), and the lower oceanic crust primitive gabbro bulk rock Mg# composition (Gillis et al, 2014a) overlaps with our gabbro cumulate MELTS calculations (Figure 8). The MELTS calculations and geochemical results appear identical at 150 MPa, at least until saturation of Fe-Ti oxides (ca.…”

“…Thus, they represent ideal elements to calculate clinopyroxene parental magma chemistry, using partition coefficients. Based on high MgO-basalt and basalt equilibrium and fractional crystallization experiments at one atmosphere and 200, 700, and 800 MPa (Grove and Bryan, 1983;Grove et al, 1992;Villiger et al, 2007;Bédard, 2014;Leuthold et al, 2015), the clinopyroxenemelt partition coefficient for Cr 2 O 3 decreases upon cooling from ca. 20 [at oxygen fugacity <NNO, typical for MOR lavas (Righter et al, 2006) Ghiorso et al (2002); based on KLB-1 spinel lherzolite of Hirose and Kushiro (1993), see composition in Table A.3] [note: partial melting of olivine-poor lherzolite produces a melt with lower Mg#, as seen in experiments by Hirose and Kushiro (1993)].…”

“…However, higher degrees of peridotite partial melting, either resulting from higher liquid fraction (F) and/or from partial melting of residual depleted lherzolite/harzburgite, produces a melt with higher Cr 2 O 3 (Hirose and Kushiro, 1993) and lower Sr content, which thus fails to explain the core1 to core2 evolution. Melt compatible Cr 2 O 3 , Al 2 O 3 , Mg#, Sr/Sr * regularly decrease during fractionation of a gabbroic mineral assemblage, while incompatible TiO 2 , Zr, Y and REE increase (e.g., Grove et al, 1992;Leuthold et al, 2015). Simple equilibrium or fractional crystallization thus fails to explain the geochemical zoning observed, which is characterized by decreases in Cr, Al, Zr, Y, REE and increases in Mg# and Sr/Sr * (Figures 3, 4), thus pointing to incoherent behavior of compatible and incompatible elements.…”

“…Upon hybridization of anatectic gabbro melt with MOR primitive magma, the hybrid melt Sr/Sr * increases. The plagioclase stability field also expands (Leuthold et al, 2015) and results in an initial rapid Sr depletion along the hybrid melt liquid line of descent. We can speculate on the Sr/Sr * peak observed at the Hess Deep core1 HD -core2 HD transition.…”

“…Assimilation of crustal partial melt in deep crustal hot zones (i.e., lower crust mafic sill complexes) can play a major role in magma differentiation (e.g., De Paolo, 1981;Hildreth and Moorbath, 1988;Solano et al, 2012). As a result, liquid and host rock/crystal mush chemistry and textures are modified, and new and secondary mineral phases crystallize from the hybrid melt (Eason and Sinton, 2009;Leuthold et al, 2014aLeuthold et al, , 2015. Melting and dissolution reactions are efficient and rapid (Donaldson, 1985;Tsuchiyama, 1986;Kvassnes and Grove, 2008), and have started to be recognized as an important MORB crustal differentiation process through the oceanic crustal filter (Bédard et al, 2000), based on studies of oceanic glasses (O'Neill and Jenner, 2012), plutonic rocks (e.g., Coogan et al, 2000;Gao et al, 2007;Lissenberg and Dick, 2008;Lissenberg et al, 2013;Lissenberg and MacLeod, 2016), phenocryst melt inclusions (Laubier et al, 2012;Coumans et al, 2016), and plutonic xenoliths in MORB (Ridley et al, 2006).…”