Water contents of pyroxenes in intraplate lithospheric mantle

Abstract

Water contents of clinopyroxene and orthopyroxene in mantle peridotites from various xenolith occurrences in intraplate settings (both oceanic and continental) were determined by Fourier Transform Infrared Spectroscopy (FTIR). The localities are as follow: Sal Island (Cape Verde Archipelago); Baker Rocks and Greene Point (Northern Victoria Land, Antarctica); Panshishan and Lianshan (Subei Basin, Eastern China). They represent well-known localities where detailed petrographical and geochemical studies have already been carried out or areas which are currently under investigation. The water incorporated in these pyroxenes is low (cpx, 37-399ppm; opx: 9-166ppm)(or very low as in Greene Point, Antarctica; cpx, 5-16ppm; opx: 9-16ppm) and, among each population, no clear correlation with melting parameters (MgO contents) in single mineral is evident. Results are compared with the available literature data on water contents in mantle pyroxene which includes peridotites from on-craton (hosted by kimberlitic-type magmas) and off-craton (hosted by alkaline basic magmas), as well as subarc mantle settings. The "relatively dry" (cpx: 140-528 ppm; opx: 38-280 ppm) sub-arc mantle xenoliths (Peslier et al., 2002) are shown to be wetter than the intraplate (off-craton) xenoliths. Cratonic mantle pyroxenes are only represented by a few determinations on garnet peridotites and eclogite from Kaapvaal and Colorado Plateau. They record the highest water contents (cpx: 342-1012 ppm; opx: 180-491 ppm) so far measured in mantle pyroxenes from various tectonic settings. Despite the limited data set, the indication that the cratonic mantle is strongly hydrated is compelling. Rehydration for the Colorado Plateau craton may be due to the Farallon plate subduction (Li et al., 2008), while for Kaapvaal Craton it might be related to young (<100Ma) metasomatic enrichments (Griffin et al., 2003a; Kobussen et al., 2008). If this is the case then the Archean mantle water content needs to be determined; this may be solved by analysing highly depleted unmetasomatized lithologies. However, assuming that the water content was initially very low, it is hard to believe that metasomatic events, similar to those observed in the intraplate settings studied in this work, would be able to produce a significant water content. According to literature and our own data it appears that water rehydration may substantially occur at convergent margins.

Water contents of the Cenozoic lithospheric mantle beneath the western part of the North China Craton: Peridotite xenolith constraints

“…Rare opx grains have a band at 3100 cm − 1 . The positions of the absorption bands are similar to those reported in previous studies on both natural and experimental minerals and interpreted as the vibration of structural OH (Skogby and Rossman, 1989;Skogby et al, 1990;Bell and Rossman, 1992;Ingrin and Skogby, 2000;Peslier et al, 2002;Grant et al, 2007;Li et al, 2008;Yang et al, 2008;Bonadiman et al, 2009;Gose et al, 2009;Green et al, 2010;Xia et al, 2010;Hao et al, 2011;Sundvall and Stalder, 2011). The relative absorbance of these bands varies among grains in a given sample due to variable orientation of grains with respect to the IR beam direction.…”

“…The water content of ol from spinel peridotite xenoliths hosted by alkaline basalts is typically lower than 10 ppm, possibly due to low initial water contents and/or diffusive H-loss during xenolith ascent to the surface (e.g., Demouchy et al, 2006;Peslier and Luhr, 2006). In contrast, pyroxenes coexisting with olivine appear to retain their initial H contents to a much larger extent (Peslier et al, 2002;Bell et al, 2004;Grant et al, 2007;Yang et al, 2008;Bonadiman et al, 2009;Gose et al, 2009;Xia et al, 2010;Sundvall and Stalder, 2011). In our study, the OH contents measured in ol (~0 ppm) are unlikely to represent their source value; the latter can be recalculated for ol by H 2 O partition between pyroxene and olivine.…”

“…H 2 O contents of the Cenozoic lithospheric mantle beneath ENCC revealed by mantle xenoliths hosted by alkali basalts have been well studied, including the localities of Penglai, Qixia, Changle, Hebi, Nushan, Panshishan, Lianshan, Fangshan and Beiyan (Yang et al, 2008;Bonadiman et al, 2009;Xia et al, 2010;Hao et al, 2011). Still, the H 2 O content data of the Cenozoic WNCC lithospheric mantle are only reported for peridotites from Hannuoba (Aubaud et al, 2007;Yang et al, 2008) and Yangyuan .…”

On water in nominally anhydrous minerals from mantle peridotites and magmatic rocks

“…Laboratory experiments demonstrate that the difference of hydrogen diffusivities between olivine and pyroxene (Ingrin Histograms of water contents in NAMs from craton, off-craton, oceanic lithosphere, mantle wedge and lava. References: craton (Skogby et al, 1990;Rossman, 1992a, 1992b;Kurosawa et al, 1997;Matsyuk et al, 1998;Bell and Ihinger, 2000;Bell et al, 2004b;Matsyuk and Langer, 2004;Mosenfelder et al, 2006a;Grant et al, 2007;Matveev and Stachel, 2007;Li et al, 2008;Peslier et al, 2008Peslier et al, , 2010Peslier et al, , 2012Yang et al, 2008;Bonadiman et al, 2009;Xia et al, 2010Xia et al, , 2013aSundvall and Stalder, 2011;Baptiste et al, 2012;Hao et al, 2012;Doucet et al, 2014;Li et al, 2015;Pintér et al, 2015); off-craton Rossman, 1992a, 1992b;Yu et al, 2011;Hao et al, 2014); oceanic lithosphere (Bonadiman et al, 2009;Bizimis and Peslier, 2015;Demouchy et al, 2015); mantle wedge (Kurosawa et al, 1997;Peslier et al, 2002;Peslier and Luhr, 2006); phenocrysts from lava (Skogby et al, 1990;Bell and Rossman, 1992b;Jamtveit et al, 2001;Matsyuk and Langer, 2004;Matveev et al, 2005;Koch-Müller et al, 2006;…”

“…There are a considerable number of studies on the water content in NAMs of mantle xenoliths from different cratons, including Kaapvaal craton (e.g., Peslier et al, 2010Peslier et al, , 2012Baptiste et al, 2012), Siberian craton (e.g., Matsyuk and Langer, 2004;Doucet et al, 2014), Tanzanian craton (e.g., Baptiste et al, 2012;Hui et al, 2015), Colorado plateau (e.g., Li et al, 2008), and NCC (e.g., Yang et al, 2008;Bonadiman et al, 2009;Xia et al, 2010Xia et al, , 2013a. Previous studies have shown that NCC has lost its ancient lithospheric mantle root (e.g., Fan et al, 2000).…”