Abstract
A new geospeedometer is developed based on the differential closures of Mg and rare earth element (REE) bulk-diffusion between coexisting plagioclase and clinopyroxene. By coupling the two elements with distinct bulk closure temperatures, this speedometer can numerically solve the initial temperatures and cooling rates for individual rock samples. As the existing Mg-exchange thermometer was calibrated for a narrow temperature range and strongly relies on model-dependent silica activities, a new thermometer is developed using literature experimental data. When the bulk closure temperatures of Mg and REE are determined, respectively, using this new Mg-exchange thermometer and the existing REE-exchange thermometer, this speedometer can be implemented for a wide range of compositions, mineral modes, and grain sizes.Applications of this new geospeedometer to oceanic gabbros from the fast-spreading East Pacific Rise at Hess Deep reveal that the lower oceanic crust crystallized at temperatures of 998–1353 °C with cooling rates of 0.003–10.2 °C/yr. Stratigraphic variations of the cooling rates and crystallization temperatures support deep hydrothermal circulations and in situ solidification of various replenished magma bodies. Together with existing petrological, geochemical and geophysical evidence, results from this new speedometry suggest that the lower crust formation at fast-spreading mid-ocean ridges involves emplacement of primary mantle melts in the deep section of the crystal mush zone coupled with efficient heat removal by crustal-scale hydrothermal circulations. The replenished melts become chemically and thermally evolved, accumulate as small magma bodies at various depths, feed the shallow axial magma chamber, and may also escape from the mush zone to generate off-axial magma lenses.
Highlights
Earth’s lower oceanic crust is mainly composed of mafic cumulate rocks, solidifying from mantle-derived melts beneath the spreading mid-ocean ridges
When considered together with geochemical and geophysical observations, results of the Mg–rare earth element (REE) speedometry on oceanic cumulates from the lower crust of East Pacific Rise (EPR) at Hess Deep reveal that crustal formation at fast-spreading ocean ridges proceeds with hydrothermal circulations across the entire crust, replenishment of mantle-derived melts, and in situ solidification of many small magma bodies at various depths in a crustal-scale mush zone beneath the ridge axis
As cooling rates and initial temperatures derived from this speedometer are mean quantities based on bulk diffusion of Mg and REE between coexisting plagioclase and clinopyroxene, average mineral compositions are required for better representing the bulk closure concentrations
Summary
Earth’s lower oceanic crust is mainly composed of mafic cumulate rocks (e.g., gabbros and gabbronorites), solidifying from mantle-derived melts beneath the spreading mid-ocean ridges. An analogue of fast-spreading mid-ocean ridges, but obtained highly controversial results: one showed slower cooling rates consistent with near-conductive cooling (e.g., Coogan et al, 2007), whereas the other found higher cooling rates supporting deep hydrothermal cooling (VanTongeren et al, 2008) The reasons for their discrepancies remain unclear, but their approaches may involve uncertainties arising from the Dodson-type assumptions: (1) negligible influences of initial temperatures and (2) surrounding clinopyroxene acting as an infinite reservoir. The bulk closure temperatures of REE can be calculated using the REE-exchange thermometer of Sun and Liang (2017), while those of Mg can be determined using a new Mg-exchange thermometer calibrated at 800–1430 ◦C Applying this speedometer to oceanic gabbros from the Hess Deep rift valley (Lissenberg et al, 2013), we show that the lower crust of EPR at Hess Deep formed by in situ solidification of axial and off-axial magma intrusions at various depths with efficient heat removal through crustal-scale hydrothermal circulations
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