Freund S, Beier C, Krumm S, Haase K (2013)
Publication Status: Published
Publication Type: Journal article, Original article
Publication year: 2013
Book Volume: 347
Pages Range: 271-283
Andesitic to dacitic lavas occur along a 300km long portion of the Pacific-Antarctic Rise close to the intersection of the spreading axis with the Foundation seamount chain. The fresh silicic glasses have low δO isotope values between 5.6‰ and 5.1‰ whereas basaltic glasses from the same ridge section have normal MORB δO values. Additionally, two FeTi basaltic and all silicic glasses have high Cl (up to 1.1wt.%) and KO (up to 1.6wt.%) contents, indicating assimilation of hydrothermally altered material. Modelling suggests that the fractionating magma assimilated up to 30% of hydrothermally altered material after 57% fractional crystallisation of the basaltic magma in a melt lens at less than 2km depth. In contrast, the basaltic glasses show little assimilation and clinopyroxene-melt barometry indicates crystal fractionation in deeper melt sills. Relatively low HO/Ce and Li/Ce ratios as well as the low δO values and high Cl and K concentrations in the silicic glasses suggest assimilation of altered crustal rock rather than a brine. While some of the variability in highly incompatible element ratios is best explained by crystal fractionation processes of FeTi-oxides (e.g., decreasing Nb/U) others require a reaction of the melt with residual amphibole and clinopyroxene (Cl/K, Tb/Yb, Hf/Sm, Ce/Yb). The initial onset of FeTi oxide crystallisation is associated with a reduced oxygen fugacity causing sulphide saturation and significant loss of S, Cu, and Co from the evolved melts. This change to more reducing melts as a result of oxide crystallisation is supported by the strong development of a negative Eu anomaly in the andesites and dacites indicating stronger partitioning of Eu into plagioclase. Reverse mineral zoning in the evolved lavas also indicates that replenishment by mafic melts occurs in the shallow melt lens. © 2013 Elsevier B.V.
Freund, S., Beier, C., Krumm, S., & Haase, K. (2013). Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation. Chemical Geology, 347, 271-283. https://dx.doi.org/10.1016/j.chemgeo.2013.04.013
Freund, Sarah, et al. "Oxygen isotope evidence for the formation of andesitic-dacitic magmas from the fast-spreading Pacific-Antarctic Rise by assimilation-fractional crystallisation." Chemical Geology 347 (2013): 271-283.