Evidence from Sardinian basalt geochemistry for recycling of plume heads into the Earth's mantle

Abstract

Up to 10 per cent of the ocean floor consists of plateaux--regions of unusually thick oceanic crust thought to be formed by the heads of mantle plumes. Given the ubiquitous presence of recycled oceanic crust in the mantle source of hotspot basalts, it follows that plateau material should also be an important mantle constituent. Here we show that the geochemistry of the Pleistocene basalts from Logudoro, Sardinia, is compatible with the remelting of ancient ocean plateau material that has been recycled into the mantle. The Sr, Nd and Hf isotope compositions of these basalts do not show the signature of pelagic sediments. The basalts' low CaO/Al2O3 and Ce/Pb ratios, their unradiogenic 206Pb and 208Pb, and their Sr, Ba, Eu and Pb excesses indicate that their mantle source contains ancient gabbros formed initially by plagioclase accumulation, typical of plateau material. Also, the high Th/U ratios of the mantle source resemble those of plume magmas. Geochemically, the Logudoro basalts resemble those from Pitcairn Island, which contain the controversial EM-1 component that has been interpreted as arising from a mantle source sprinkled with remains of pelagic sediments. We argue, instead, that the EM-1 source from these two localities is essentially free of sedimentary material, the geochemical characteristics of these lavas being better explained by the presence of recycled oceanic plateaux. The storage of plume heads in the deep mantle through time offers a convenient explanation for the persistence of chemical and mineralogical layering in the mantle.

Origin and evolution of Cenozoic magmatism of Sardinia (Italy). A combined isotopic (Sr–Nd–Pb–O–Hf–Os) and petrological view

“…Arci, Logudoro; Cioni et al, 1982;Montanini et al, 1994;Lustrino et al, 1996Lustrino et al, , 2002Lustrino et al, , 2004aLustrino et al, , 2004bLustrino et al, , 2007aGasperini et al, 2000;Fedele et al, 2007;Duncan et al, 2011;Kim et al, 2011;Figs. 1, 3).…”

“…6b,c, 11). Despite that the Sr-Nd-Pb isotopic composition of the UPV rocks has been described in detail in several papers (Montanini et al, 1994;Gasperini et al, 2000;Lustrino et al, 2000Lustrino et al, , 2002Lustrino et al, , 2004Lustrino et al, , 2007aLustrino et al, , 2007bFedele et al, 2007) a general consensus on the origin of these magmas has not yet been reached. In particular, two main models are proposed to explain the isotopic composition of these lavas: a) recycling of plume heads and b) complex styles of interaction between delaminated lower crust and ambient upper mantle.…”

“…The 143 Nd/ 144 Nd ratios show a good negative correlation with SiO 2 (R 2 = 0.80) from ~47 to ~58 wt% SiO 2 which then inverts at A C C E P T E D M A N U S C R I P T (Lustrino et al, 2000) and the field of the CiMACI rocks (Lustrino and Wilson, 2007). No new data are presented on UPV rocks because these rocks have been already fully investigated, with 23 Pb isotopic ratios being available in literature (Gasperini et al, 2000;Lustrino et al, 2000Lustrino et al, , 2002Lustrino et al, , 2007a Fig. 6f) previously defined by Gasperini et al (2000).…”

“…The recycling of plume heads in the Sardinian mantle proposed by Gasperini et al (2000) has been discussed in Lustrino et al, 2007aLustrino et al, , 2011 and therefore it will not be repeated here. We simply note that "the storage of plume heads in the deep mantle through time" as requested by Gasperini et al (2000) is unlikely from a geological point of view and hardly compatible with the composition and volume of the produced magma.…”

“…No new data are presented on UPV rocks because these rocks have been already fully investigated, with 23 Pb isotopic ratios being available in literature (Gasperini et al, 2000;Lustrino et al, 2000Lustrino et al, , 2002Lustrino et al, , 2007a Fig. 6f) previously defined by Gasperini et al (2000). 176 Hf/ 177 Hf ratios show strong positive correlation with Nd isotopes, a less distinct positive correlation with Pb isotopes and no apparent correlation with the 87 Sr/ 86 Sr isotopic system ( Fig.…”