Plume‐cratonic lithosphere interaction recorded by water and other trace elements in peridotite xenoliths from the Labait volcano, Tanzania

Abstract

Water and other trace element concentrations in olivine (1-39 ppm H 2 O), orthopyroxene (10-150 ppm H 2 O), and clinopyroxene (16-340 ppm H 2 O) of mantle xenoliths from the Labait volcano, located on the edge of the Tanzanian craton along the eastern branch of the East African Rift, record melting and subsequent refertilization by plume magmas in a stratified lithosphere. These water contents are at the lower end of the range observed in other cratonic mantle lithospheres. Despite correlations between water content and indices of melting in orthopyroxene from the shallow peridotites, and in both olivine and orthopyroxene from the deep peridotites, water concentrations are too high for the peridotites to be simple residues. Instead, the Labait water contents are best explained as reflecting interaction between residual peridotite with a melt having relatively low water content (<1 wt.% H 2 O). Plume-derived melts are the likely source of water and other trace element enrichments in the Labait peridotites. Only garnet may have undergone addition of water from the host magma as evidenced by water content increasing toward the kelyphite rim in one otherwise homogeneous garnet. Based on modeling of the diffusion profile, magma ascent occurred at 4-28 m/s. In summary, plume-craton interaction appears to result in only moderate water enrichment of the lithosphere.

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

“…Olivine water contents in mantle xenoliths measured so far (Figure 1) are always much lower than its water storage capacity (e.g., Smyth et al, 2006). Almost all of them are below 400 ppmw (e.g., Beran and Libowitzky, 2006;Hui et al, 2015), and most below 200 ppmw (e.g., Matsyuk and Langer, 2004;Peslier, 2010;Hui et al, 2015). The low water content of mantle-derived olivine implies that water is not saturated in mantle olivine, or the olivine has lost its water due to degassing.…”

“…Olivine is the most important mineral in the upper mantle, and therefore several different laboratories have calibrated its molar absorption coefficient, which varies significantly Koch-Müller et al, 2006;Aubaud et al, 2009;Thomas et al, 2009;Withers et al, 2012). The difference between the earliest (also the most used in literature; and the most recent (Withers et al, 2012) calibrations could result in as much as twice the difference in olivine water content (Doucet et al, 2014;Hui et al, 2015). The difference of calibrations could either come from the systematic errors between different experimental methods, or the compositional difference between the standards.…”

“…In addition, water in NAMs from most mantle xenoliths could be analyzed using FTIR. Therefore, a great number of mantle peridotite xenoliths have been systematically studied for water in NAMs (e.g., Li et al, 2008;Yang et al, 2008;Peslier et al, 2010Peslier et al, , 2012Xia et al, 2010Xia et al, , 2013aDoucet et al, 2014;Warren and Hauri, 2014;Hui et al, 2015;.…”

“…A recent study on mantle peridotites from the Labait volcano in Tanzania, however, has shown that water is heterogeneously distributed in some garnets with enrichments towards the rim, as water diffusion profiles (Hui et al, 2015). The garnet is enclosed by the water-rich kelyphite, a product of decompression breakdown of garnet during magma ascent Koornneef et al, 2009).…”

Nephelinite lavas at early stage of rift initiation (Hanang volcano, North Tanzanian Divergence)

“…phlogopite, amphibole) and primary calcite related to the presence of complex mantle metasomatism events (Dawson and Smith 1988;Koornneef et al 2009;Lee et al 2000;Rudnick et al 1993). The refractory mantle beneath the East African Craton records deep fluidrock interaction at >90 km with a large amount of Ca-carbonatitic fluid (2 % of carbonatitic fluid, Jones et al 1983;Rudnick et al 1993) and minor H 2 O-rich fluid (<50 ppm wt H 2 O whole-rock peridotite; Baptiste et al 2015;Hui et al 2015) and Fe-rich, Ca-rich and Si-rich metasomatic events leading to a heterogeneous lithospheric mantle (Dawson 2012;Gibson et al 2013). The silicic nephelinite from Hanang corroborates the presence of CO 2 -rich H 2 O-poor melilititic liquid in the southern part of the NTD and melt percolations after deep partial melting of CO 2 -rich oxidized mantle source.…”