Highly heterogeneous depleted mantle recorded in the lower oceanic crust

Abstract

The Earth's mantle is heterogeneous as a result of early planetary differentiation and subsequent crustal recycling during plate tectonics. Radiogenic isotope signatures of midocean ridge basalts have been used for decades to map mantle composition, defining the depleted mantle end member. These lavas, however, homogenize via magma mixing and may not capture the full chemical variability of their mantle source. Here we show that the depleted mantle is significantly more heterogeneous than previously inferred from the compositions of lavas at the surface, extending to highly enriched compositions. We perform high spatial resolution isotopic analyses on clinopyroxene and plagioclase from lower crustal gabbros drilled on a depleted ridge segment of the northern Mid-Atlantic Ridge. These primitive cumulate minerals record nearly the full heterogeneity observed along the northern Mid-Atlantic Ridge, including hotspots. Our results demonstrate that substantial mantle heterogeneity is concealed in the lower oceanic crust and that melts derived from distinct mantle components can be delivered to the lower crust on a centimetre scale. These findings provide a starting point for re-evaluation of models of plate recycling, mantle convection, and melt transport in the mantle and the crust. The mantle, Earth's largest geochemical reservoir and main source of volcanism, records the timeintegrated history of recycling of oceanic lithosphere and its overlying sediments during the plate tectonic cycle 1,2. The compositional heterogeneity thus generated within the mantle reflects this

Trace Element and Isotopic Characteristics of Plagioclase Megacrysts in Plagioclase Ultraphyric Basalts (PUB)

“…Given that plagioclase is able to record the effects of crystallization from distinct melts present at small scales, the observed range of trace element and isotopic heterogeneity in megacrysts is arguably a more realistic indication of the heterogeneity of mantle melts as one may sample them in the upper mantle compared to the local diversity of erupted lavas, both because of mixing during transport. This is consistent with the observed patterns seen in lower crustal gabbros (Coogan, 2014;Lambart et al, 2019) and olivine-hosted inclusions (e.g., Jones et al, 2019).…”

“…Unlike phyric MORBs containing plagioclase with complex major element zonation patterns reported at some locations (Kuo & Kirkpatrick, 1982;Lambart et al, 2019;Meyer & Shibata, 1990), the phenocrysts from the PUB lavas from this study have homogeneous cores with occasional thin lower anorthite rims (Figure 3). Plagioclase megacrysts with unzoned cores have been reported at several other locations (Sasha Seamount in Lamont Seamount chain; Allan et al, 1989;Siqueiros Transform, Batiza et al, 1977;ODP Hole 648B, Serocki volcano, Mid-Atlantic Ridge, Costa et al, 2010: Gorda Ridge, Nielsen et al, 1995.…”

“…A largely untapped source of information on the diversity of MORB magmas is the trace element and isotopic composition of plagioclase (e.g., Lambart et al, 2019;Lange et al, 2013;Lissenberg et al, 2019;Valer et al, 2017;Winpenny & Maclennan, 2014). However, to use phenocryst compositions to evaluate the original characteristics of the array of melts produced in the mantle requires that we understand the partitioning behavior of the elements of interest and that we know how the phenocrysts compositions might have changed due to interaction with the magmas in which they were transported, which are often not the magmas from which they crystallized .…”

Novel insights from Fe-isotopes into the lithological heterogeneity of Ocean Island Basalts and plume-influenced MORBs

“…This diagram also shows a key feature of our model, in that the sub-ridge mantle contains streaks and/or blebs of pyroxenitic material set in a peridotite matrix. Melt mixing and homogenisation is believed to occur in sub-ridge magma storage regions as recent studies have shown that the isotopic heterogeneity of melt entering the crust beneath oceanic spreading centres is very large (Lambart et al, 2019 Hart, 2005), as their estimate for the trace-element composition of the average DMM was not able to reproduce the highly depleted nature of D-MORBs from the GSC.…”

The Neodymium Stable Isotope Composition of the Oceanic Crust: Reconciling the Mismatch Between Erupted Mid-Ocean Ridge Basalts and Lower Crustal Gabbros

“…The clinopyroxene and whole rock samples analyzed herein are generally indistinguishable from the Hole 735B average, except sample MS20-10 (ε 143 Nd = +6.8) which is the least radiogenic so far measured but remains well within the range of heterogeneity observed along the SWIR (Figure 7). The preservation of extreme isotopic heterogeneity in oceanic mantle between constituent phases and across small length is increasingly being recognized as a common phenomenon (Burton et al, 2012;Lambart et al, 2019). This is not an unexpected results given the depleted mantle represents a mixture of materials at different stages in their evolution, and is therefore unlikely to be a isotopically homogeneous (e.g., Salters and Stracke, 2004).…”