An assessment of upper mantle heterogeneity based on abyssal peridotite isotopic compositions

Abstract

[1] Abyssal peridotites, the depleted solid residues of ocean ridge melting, are the most direct samples available to assess upper oceanic mantle composition. We present detailed isotope and trace element analyses of pyroxene mineral separates from Southwest Indian Ridge abyssal peridotites and pyroxenites in order to constrain the size and length scale of mantle heterogeneity. Our results demonstrate that the mantle can be highly heterogeneous to <1 km and even <0.1 m length scales. Examination of Nd isotopes in relation to modal, trace, and major element compositions indicate that the length scales and amplitudes of heterogeneities in abyssal peridotites reflect both ancient mantle heterogeneity and recent modification by melting, melt-rock reaction and melt crystallization. The isotopic and trace element compositions of pyroxenite veins in this study indicate that they are not direct remnants of recycled oceanic crust, but instead are formed by recent melt crystallization. Combined with existing data sets, the results show that the average global isotopic composition of peridotites is similar to that of mid-ocean ridge basalts, though peridotites extend to significantly more depleted 143 Nd/ 144 Nd and 87 Sr/ 86 Sr. Standard isotope evolution models of upper mantle composition do not predict the full isotopic range observed among abyssal peridotites, as they do not account adequately for the complexities of ancient and recent melting processes.

Composition of the Oceanic Crust

“…Seemingly fresh abyssal peridotites have 'depleted' Os isotopic signatures with a mean g Os of À2 and are isotopically uniform (s ¼ 2.2; n ¼ 37) compared to MORB. Since abyssal peridotites presumably represent the residues of mantle that melted to produce MORB, this difference presents something of a conundrum and contrasts with Nd isotopes, which are largely similar in MORB and abyssal peridotites (e.g., Cipriani et al, 2004; 1994), although the peridotites do extend to somewhat more depleted Nd isotopic compositions (Warren et al, 2009 , 1998). Re can also be taken up by basalts during hydrothermal alteration, which will, in turn, quickly lead to radiogenic Os isotopic compositions (Reisberg et al, 2008 Os fail to fully equilibrate with the melt.…”

Generation of permeability barriers during melt extraction at mid‐ocean ridges

“…The crystallization sequence may be perturbed by assimilation of wall rock (Figure 7d), which could increase crystallization [Kelemen and Aharonov, 1998]. Melt-rock reaction has been documented at the slow to ultraslow spreading SWIR [Seyler et al, 2007;Warren et al, 2009;Warren and Shimizu, 2010] and at ophiolites of inferred slow to ultraslow ridges [Müntener et al, 2010]. Finally, diapiric instabilities (Figure 7e) are expected to be more dominant at slower spreading rate [Lin and Phipps Morgan, 1992], and may take over as origin of focusing at ultraslow spreading [Cannat et al, 2008], although they appear less efficient than permeability barriers at focusing melt at the slow spreading Mid-Atlantic Ridge .…”

Mantle Sulfides and their Role in Re–Os and Pb Isotope Geochronology

“…In addition, compositional heterogeneities in the oceanic mantle at length scales of thousands of kilometres (e.g., Hart 1984;Meyzen et al 2007) to sub-km scales have often been reported (e.g., Shirey et al 1987;Dosso et al 1999;Meibom et al 2002;Standish et al 2002;Warren et al 2009). These observations contrast with studies of the SCLM, where Re-Os ages for the mantle often show a correspondence to the age of the overlying crust (e.g., , though examples exist of the SCLM having ages older than the overlying crust (e.g., Peslier et al 2000;and discussion above).…”

“…Blusztajn et al (2014) found considerable Pb isotope variation among BMS grains in some individual samples from the Gakkel Ridge and SWIR, including one sample with BMS grains that cover 25% of the Pb isotope range of oceanic basalts. Pyroxenes from abyssal peridotites have also been used to show considerable Nd (e.g., Cipriani et al 2004), Sr (e.g., Warren et al 2009) and Hf (e.g., Stracke et al 2011) isotope variability, but these isotopes so far have only been measured in mineral separates and not individual grains.…”