Insights into mantle composition and mantle melting beneath mid‐ocean ridges from postspreading volcanism on the fossil Galapagos Rise

Abstract

[1] New major and trace element and Sr, Nd, and Pb isotope data, together with 39 Ar-40 Ar ages for lavas from the extinct Galapagos Rise spreading center in the eastern Pacific reveal the evolution in magma compositions erupted during slowdown and after the end of active spreading at a mid-ocean ridge. Lavas erupted at 9.2 Ma, immediately prior to the end of spreading are incompatible element depleted mid-ocean ridge tholeiitic basalts, whereas progressively younger (7.5 to 5.7 Ma) postspreading lavas are increasingly alkalic, have higher concentrations of incompatible elements, higher La/Yb, K/Ti, 87 Sr/ 86 Sr, and lower 143 Nd/ 144 Nd ratios and were produced by smaller degrees of mantle melting. The large, correlated variations in trace element and isotope compositions can only be explained by melting of heterogenous mantle, in which incompatible trace element enriched lithologies preferentially contribute to smaller degree mantle melts. The effects of variable degrees of melting of heterogeneous mantle on lava compositions must be taken into account when using mid-ocean ridge basalt (MORB) to infer the conditions of melting beneath active spreading ridges. For example, the stronger "garnet signature" inferred from Sm/Nd and 143 Nd/ 144 Nd ratios for postspreading lavas from the Galapagos Rise results from a larger contribution from enriched lithologies with high La/Yb and Sm/Yb, rather than from a greater proportion of melting in the stability field of garnet peridotite. Correlations between ridge depth and Sm/Yb and fractionation-corrected Na concentrations in MORB worldwide could result from variations in mantle fertility and/or variations in Copyright 2011 by the American Geophysical Union 1 of 21 the average degree of melting, rather than from large variations in mantle temperature. If more fertile mantle lithologies are preferentially melted beneath active spreading ridges, then the upper mantle may be significantly more "depleted" than is generally inferred from the compositions of MORB.

Progressive Changes in Magma Transport at the Active Serreta Ridge, Azores

“…Lower La/Yb ratios additionally indicate that lavas from unit 2 may have originated from a higher degree of partial melting than unit 1 lavas (Bourdon et al, ). We note, however, that the degree of partial melting and the mantle source composition may be linked, that is, differences in trace element ratios such as La/Yb may result from both, different degrees of enrichment in the mantle source and changes in degree of partial melting (Haase et al, ).…”

The size of plume heterogeneities constrained by Marquesas isotopic stripes

“…Authors such as Saal et al [1998, 2005], Jackson and Hart [2006], Paul et al [2011] or Sobolev et al [2011] demonstrated that the isotopic variability in melt inclusions from oceanic island magmas was much larger than in the lavas themselves and they attributed the isotopic diversity to small scale sampling of the source material. In contrast, at the scale sampled by mid‐ocean ridge basalts or ocean island basalts, most authors suggest that the observed isotopic diversity results from melting of a heterogeneous source in which low degree melts preferentially sample the enriched material, while higher melting degrees dilute this enriched material with larger proportions of more “normal” mantle [ Niu et al , 2002; Donnelly et al , 2004; Castillo et al , 2010; Haase et al , 2011]. Using petrological arguments instead of isotopic constraints, Sobolev et al [2005, 2007] also suggested that low‐degree melts sampled fertile pyroxenite while the signature of the peridotitic source swamped high‐degree melts.…”

Selenium and tellurium systematics in MORBs from the southern Mid-Atlantic Ridge (47–50°S)

“…The presence of various amounts of metasomatic/recycled components within the SMAR mantle sources can be traced back to the influence of the nearby Discovery plume (e.g., Douglass et al, 1999;le Roux et al, 2002b,c). Pyroxenite layers/lenses in a "marble-cake" like mantle have been suggested to be the enriched component in the SMAR mantle (le Roux et al, 2002c) as in other heterogeneous mantle sources of global oceanic basalts (Stracke, 2012;Stracke et al, 2003;Haase et al, 2011;Lambart et al, 2013;Gurenko et al, 2013;and references therein). Le Roux et al (2002c) estimated a maximum of 20-40% contribution from pyroxe- Fig.…”