Near conductive cooling rates in the upper-plutonic section of crust formed at the East Pacific Rise

Abstract

A new geospeedometer, based on diffusion modeling of Mg in plagioclase, is used to determine cooling rates of the upper section of the lower oceanic crust formed at fast-spreading mid-ocean ridges. The investigated natural sample suites include gabbroic rocks formed at three different locations along the fast-spreading East Pacific Rise. These samples cover a depth interval of 0-840 m below the sheeted dike/gabbro boundary and therefore allow the variation of cooling rate as a function of depth within the upper plutonic sequence to be determined. We demonstrate that the cooling rates we obtained are robust (reproducible and consistent across different vertical sections at fast spreading ridges) and decrease significantly with increasing sample depth (covering almost 4 orders of magnitude, ranging from ∼1 • C y −1 for the shallowest samples to 0.0003 • C y −1 for the deepest samples). Both the absolute cooling rates, and the rate of decrease of cooling rate with depth, are consistent with conductive thermal models. In contrast, the absolute cooling rates determined from the deeper samples (>300 m below DGB), and the large decrease in cooling rate with depth are inconsistent with thermal models that include substantial cooling by off-axis hydrothermal circulation within the upper plutonic section of the crust.

Formation of fast-spreading lower oceanic crust as revealed by a new Mg–REE coupled geospeedometer

“…Many crustal formation models have been proposed based on petrological observations, deformation structures, and geodynamic simulations at various ocean ridge settings and ophiolites (e.g., Nicolas et al, 1988;Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Kelemen et al, 1997;Dick et al, 2008). Among these, two end-member models for fast spreading mid-ocean ridges are actively debating whether cumulates of the lower oceanic crust crystalize mainly in a shallow melt lens (e.g., Phipps Morgan and Chen, 1993;Quick and Denlinger, 1993;Coogan et al, 2007;Faak et al, 2015) or in situ solidify largely from magma sills at various depths (e.g., Nicolas et al, 1988;Kelemen et al, 1997;Lissenberg et al, 2004;Maclennan et al, 2005;VanTongeren et al, 2008VanTongeren et al, , 2015Natland and Dick, 2009). The former postulates much slower cooling in the deeper crust (i.e., near-conductive cooling), whereas the latter necessitates efficient heat removal by hydrothermal circulations throughout the entire crust.…”

“…A fixed Mg partition coefficient (K Mg = 0.01) and two plagioclase compositions (An 50 and An 80 ; An 50 denotes 50 mol% anorthite in plagioclase) were used in (a). Inset in (b) shows the silica activity model expressions of Faak et al (2015) and Carmichael (2004), in which T is temperature in K, R is the gas constant, and X denotes the oxide molar fraction. The melt in (b) used for Carmichael's model has a typical basaltic composition (i.e., 022005-1522 used in Faak et al, 2013).…”

“…Using their newly developed Mg-in-plagioclase geospeedometer, Faak et al (2015) argued that cooling rates derived from their 1-D grain-scale diffusion modeling support near-conductive cooling of the lower crust at the East Pacific Rise (EPR). However, in addition to the Dodson-type assumptions, results of Faak et al (2015) are also likely subject to uncertainties from their Mgexchange thermometer (Faak et al, 2013) and 1-D plane sheet approximation.…”

“…However, in addition to the Dodson-type assumptions, results of Faak et al (2015) are also likely subject to uncertainties from their Mgexchange thermometer (Faak et al, 2013) and 1-D plane sheet approximation. Particularly, Faak et al's thermometer has a strong dependence upon silica activities that can only be calculated using existing activity models .…”

“…Particularly, Faak et al's thermometer has a strong dependence upon silica activities that can only be calculated using existing activity models . Silica activities in a typical basaltic system buffered by olivine and orthopyroxene appear to differ by a factor of two between the models of Faak et al (2015) and Carmichael (2004) (Fig. 1b), corresponding to 100-200 • C differences in Faak et al's thermometer (cf.…”