Variations in the H₂O Content and H₂O/Ce Ratio of Mantle Pyroxenites: Implications for Enriched Components in the Mantle

Abstract

Mantle pyroxenite xenoliths were collected from Hannuoba, East China, and investigated for their H2O content and trace element contents, with particular regard to the H2O/Ce ratio. The pyroxenites were divided into three groups: Cr‐pyroxenites, Al‐pyroxenites, and garnet pyroxenites. The H2O contents and H2O/Ce ratios of the pyroxenites in this study showed much wider ranges than those previously reported for the whole rock pyroxenites. The Cr‐pyroxenites had an H2O content ranging from 23 to 339 ppm and an H2O/Ce ratio between 32 and 212; the Al‐pyroxenites had an H2O content ranging from 42 to 106 ppm and an H2O/Ce ratio between 1 and 22, and the garnet pyroxenites had an H2O content ranging from 37 to 69 ppm and an H2O/Ce ratio between 5 and 54. These systematic variations in the H2O contents and H2O/Ce ratios for the Hannuoba pyroxenites are associated with their origins. Both the H2O content and H2O/Ce ratio of the Al‐pyroxenites were low compared to those of Hawaiian pyroxenite, enriched mantle, and depleted MORB mantle sources. The high H2O content and H2O/Ce ratio of the melt equilibrated with the Cr‐pyroxenites indicate that a water‐rich component was involved in its formation. Their large variations in the H2O contents and H2O/Ce ratios may indicate that pyroxenites are essential enriched components in the sources of continental basalts.

Mantle metasomatic influence on water contents in continental lithosphere: New constraints from garnet pyroxenite xenoliths (France & Cameroon volcanic provinces)

“…Lastly, clinopyroxenes in FMC and AVP pyroxenites contain 1.5-3.5 times more water than orthopyroxenes from the same sample (Fig. 9), consistent with the range of water J o u r n a l P r e -p r o o f partition coefficients between clinopyroxene and orthopyroxene (𝐷 H 2 O Cpx;Opx ) determined in natural samples: 1.5-3.5 (Warren and Hauri, 2014;Bizimis and Peslier, 2015;Demouchy and Bolfan-Casanova, 2016;Demouchy et al, 2017;Peslier et al, 2017;Xia et al, 2017;Hao et al, 2018;Li et al, 2018). Hence, clinopyroxene and orthopyroxene water contents are considered to be in equilibrium for all samples studied herein.…”

“…However, contrary to the observations of Patkó et al (2019), there is no correlation of type-2 spectra (and related low water content) with anomalously high hydroxyl partition coefficients between clinopyroxene and orthopyroxene: the ratio of clinopyroxene to orthopyroxene water contents in LN78 and LP27 are within 1.5-3.5, matching literature data (Fig. 9; Peslier et al, 2002Peslier et al, , 2012Peslier, 2010;Xia et al, 2010Xia et al, , 2013Yu et al, 2011;Hao et al, 2012Hao et al, , 2014Hao et al, , 2016aHao et al, , 2016bDoucet et al, 2014;Warren and Hauri, 2014;Bizimis and Peslier, 2015;Demouchy et al, 2015;Hui et al, 2015;Li et al, 2015;Peslier and Bizimis, 2015;Denis et al, 2018;Hao et al, 2018).…”

“…It follows that, if pyroxenites display different H 2 O/Ce ratios than peridotites, the H 2 O/Ce ratio can be used as a tracer of pyroxenites in the mantle source, and thus to attempt to quantify the involvement of pyroxenites during magmas genesis. However, both the AVP and FMC pyroxenites have H 2 O/Ce ratios spanning a wider range of values (from 30 ;5 :8 to 2,388 ;512 :945 ) than in previous studies (Bizimis and Peslier, 2015;Hao et al, 2018;Li et al, 2018), and H 2 O contents varying from 58 ;9 :15 to 480 ;108 :64 ppm (Fig. 13).…”

“…This was confirmed by Hauri et al (2006) > 350 ppm) in mantle melting could account for the low H 2 O/Ce ratios of some fertile magmas (Hawaii, Samoa, Society). Relying on those important results and based on the observation that pyroxenites from other settings are also characterized by low H 2 O/Ce ratios (e.g., Hao et al, 2018;Li et al, 2018;Fig. 13), it seems plausible that the H 2 O/Ce ratio of basalts might track the involvement of pyroxenite in their genesis.…”

“…where garnet pyroxenites are stable (deeper than 45 km), those lithologies are essential to our understanding of magma genesis. Nevertheless, few data are available on the water contents of garnet pyroxenites (Bizimis and Peslier, 2015;Hao et al, 2018;Li et al, 2018).…”