Abstract
Microtextural and chemical evidence from gabbros indicates that melts may react with the crystal framework as they migrate through crystal mushes beneath mid-ocean ridges; however, the importance of this process for the compositional evolution of minerals and melts remains a matter of debate. Here we provide new insights into the extent by which melt-rock reaction process can occur in oceanic gabbros by conducting a detailed study of cryptic reactive melt migration as preserved in an apparently unremarkable, homogeneous olivine gabbro from deep within a section of the plutonic footwall of the Atlantis Bank core complex on the Southwest Indian Ridge (International ocean discovery program Hole U1473A). High-resolution chemical maps reveal that mineral zoning increases toward and becomes extreme within a cm-wide band that is characterized by elevated incompatible trace element concentrations and generates extreme more/less incompatible element ratios. We demonstrate that neither crystallization of trapped melt nor diffusion can account for these observations. Instead, taking the novel approach of correcting mineral-melt partition coefficients for both temperature and composition, we show that these chemical variations can be generated by intergranular reactive porous flow of a melt as it migrated through the mush framework, and whose composition evolved by melt-rock reaction as it progressively localized into a cm-scale reactive channel. We propose that the case reported here may represent, in microcosm, a preserved snapshot of a generic mechanism by which melt can percolate through primitive mafic (olivine gabbro) crystal mushes, and be modified toward more evolved compositions via near-pervasive reactive transport.
Highlights
Seismic investigations of mid-ocean ridges have shown that the large, km-scale melt-filled magma chambers originally inherent in ophiolite-derived models do not exist: at even the most magmatically robust fast-spreading ridges most of the lower ocean crust beneath the ridge axis transmits shear waves and is largely solid, albeit normally with a tiny melt-rich lens or sill ≤10 s of meters thick present at the very top of the lower crust; instead, it is generally believed that the broader lower crustal region must be comprised of a “crystal mush”, apparently consisting of no more than a few percent interstitial melt overall (e.g. Detrick et al, 1987; Sinton and Detrick, 1992)
We focus on an apparently mundane, texturally homogeneous olivine-gabbro from the plutonic lower crustal section of the Atlantis Bank oceanic core complex Southwest Indian Ridge; International Ocean Discovery Program (IODP) Hole U1473A; (MacLeod et al, 2017)
We show that the reaction band most likely represents a high local melt flux channel, and demonstrate that the mineral chemical modifications throughout the sample are explained by dissolution-reprecipitation melt-rock reactions that occurred under decreasing temperature conditions as melt migrated through the host crystal framework
Summary
Seismic investigations of mid-ocean ridges have shown that the large, km-scale melt-filled magma chambers originally inherent in ophiolite-derived models do not exist: at even the most magmatically robust fast-spreading ridges most of the lower ocean crust beneath the ridge axis transmits shear waves and is largely solid, albeit normally with a tiny melt-rich lens or sill ≤10 s of meters thick present at the very top of the lower crust; instead, it is generally believed that the broader lower crustal region must be comprised of a “crystal mush”, apparently consisting of no more than a few percent interstitial melt overall (e.g. Detrick et al, 1987; Sinton and Detrick, 1992). In this study we hereinafter focus on an apparently unremarkable “typical” coarse-grained olivine gabbro, in which the potentially complicating effects of deformation are absent and which lacks any visible textural evidence of melt-rock interaction This sample (360-U1473A-84R-4, 118–122 cm; 746 m below seafloor; Figure 3) has some of the most primitive mineral compositions encountered at Hole U1473A and contains an extremely low proportion of accessory phases, so much so that it was initially selected for study as a typical representative of the predominant olivine gabbro facies and pre-supposed to be the least modified representative of the original cumulate crystal mush. Precision and accuracy of the REE concentration values were assessed through repeated analysis of the BCR2-g standard to be better than ±7% and ±10%, respectively, at the ppm concentration level
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