Onset of volatile recycling into the mantle determined by xenon anomalies

doi: 10.7185/geochemlet.1833 Noble gases serve as unique tracers of the origin and evolution of Earth’s volatile reservoirs owing to their inert nature and contribution from extinct and extant radioactivities. However, noble gases are low in abundance relative to many other elements, particularly in the Earth’s mantle. Additionally, mantle-derived samples show large post-eruptive atmospheric contamination, rendering the determination of the primary mantle composition challenging. The sources of mantle krypton and xenon remain debated due to their partially resolvable excess, if any, relative to the atmosphere. Atmospheric noble gases also appear to be recycled into the mantle via subduction, progressively overprinting the initial mantle signature. Here we develop a new protocol to accumulate non-contaminated mantle-derived xenon, in particular the low abundant 124-126-128Xe. The results show the highest excesses in 124-126-128Xe ever measured in the mantle relative to the atmosphere and point toward a chondritic origin for mantle xenon. The fissiogenic isotopes 131-132-134-136Xe allow the onset of efficient xenon recycling in the mantle to be constrained at around 3 Gyr ago, implying that volatile recycling before 3 Ga would have been negligible. Received 31 July 2018 | Accepted 24 November 2018 | Published 20 December 2018 1. Institut de Physique du Globe de Paris Sorbonne Paris Cité, UMR CNRS 7154, Université Paris Diderot. 1 Rue Jussieu, 75005, Paris, France * Corresponding author (email: peron@ipgp.fr) Letter The inert noble gases (He, Ne, Ar, Kr, Xe) serve as an invaluable tool for constraining the volatile origin and evolution of terrestrial planetary reservoirs. Among them, Xe with its nine isotopes is an ideal tracer of both volatile origin and mantle evolution because all of its isotopes represent distinct geochemical signatures (Kunz et al., 1998; Moreira et al., 1998; Caffee et al., 1999; Holland and Ballentine, 2006; Pujol et al., 2011; Mukhopadhyay, 2012; Tucker et al., 2012; Parai and Mukhopadhyay, 2015, 2018; Avice et al., 2017; Marty et al., 2017). Indeed, 124-126-128-130Xe are non-radiogenic, stable isotopes whereas 129Xe is radiogenic (decay product of the now extinct 129I radioactivity) and 131-132-134-136Xe are fissiogenic, both deriving from the fission of the now extinct 244Pu nuclide (half life 80 Myr) and the still alive 238U nuclide. Many studies have focused on the radiogenic and fissiogenic Xe compositions of the Earth’s mantle from analyses of mid-ocean ridge basalts (MORBs) (Kunz et al., 1998; Moreira et al., 1998; Parai et al., 2012; Tucker et al., 2012; Parai and Mukhopadhyay, 2015), plume-influenced back-arc basin basalts (Petö et al., 2013), oceanic island basalts (OIBs) (Poreda and Farley, 1992; Trieloff et al., 2000, 2002; Mukhopadhyay, 2012), CO2 well gases (Caffee et al., 1999; Holland and Ballentine, 2006; Holland et al., 2009) and thermal springs (Caracausi et al., 2016; Moreira et al., 2018) because these isotopes are relatively abundant. However, 124-126-128Xe isotopes are very rare and thus extremely difficult to measure. Initial studies measured the abundances of 124-126-128Xe in mantle-derived samples but proved these to be unresolvable from that of the atmosphere (Kunz et al., 1998). The only successful attempts to resolve 124-126-128Xe excess have been during the analyses of CO2 well gases and thermal springs (Caffee et al., 1999; Holland and Ballentine, 2006; Caracausi et al., 2016). However, recycling of atmospheric xenon via subduction into the convecting mantle effectively overprints and erases the primitive mantle signature, as more than 80 % of Xe in the mantle is assumed to be derived through the subduction of oceanic lithosphere (Caffee et al., 1999; Holland and Ballentine, 2006; Mukhopadhyay, 2012; Parai and Mukhopadhyay, 2015). Consequently it has long been assumed that anomalies of 124-126-128Xe were undetectable in basaltic glasses (Trieloff et al., 2000). A second issue surrounding the determination of 124-126128Xe excess is that basaltic glasses are highly susceptible to post-eruptive contamination by air (Ballentine and Barfod, 2000). Typically air can enter glass samples and be hosted within small cracks and open vesicles (Ballentine and Barfod, 2000), so that when samples are crushed, this air component mixes with mantle gases from intact bubbles, rendering determination of Xe anomalies even more difficult if not impossible. Here a new protocol is developed to accumulate air-free xenon from intact glass vesicles of MORBs and determine whether 124-126-128Xe anomalies can be detected in the convecting mantle. To this end, the gas-rich popping rock 2πD43 sample is analysed, which has been extensively studied Geochemical Perspectives Letters Letter Geochem. Persp. Let. (2018) 9, 21-25 | doi: 10.7185/geochemlet.1833 22 (Burnard et al., 1997; Kunz et al., 1998; Moreira et al., 1998). The new protocol consists of sequential crushing steps, with the neon composition of each step being used as a monitor of post-eruptive atmospheric contamination, as Ne displays distinct atmospheric and mantle compositions (20Ne/22Ne ratios of 9.8 and 12.5 respectively). If the 20Ne/22Ne ratio is higher than 11.8, then the heavy noble gases from each crush step are accumulated on activated charcoal (Fig. S-1 and Supplementary Information). The results are shown in Figure 1 and in Tables S-1 and S-2. The 129Xe/130Xe ratio of the accumulated gas is 7.41 ± 0.03 (1σ), showing limited atmospheric contamination. Indeed the upper mantle 129Xe/130Xe ratio is assumed to be 7.6 based on previous popping rock 2πD43 data (Moreira et al., 1998). The measured xenon isotopic ratios are hence corrected for this limited atmospheric contamination (17 %), assuming that the uncontaminated 129Xe/130Xe ratio is 7.6 (Table S-1) and the two data (measured and corrected) are indicated in Figure 1. The high measured 129Xe/130Xe ratio (7.41) allows checking that the new protocol is very efficient for determining the mantle xenon composition with almost no atmospheric contamination (Fig. S-2). Figure 1 Light xenon isotopic compositions for popping rock 2πD43. Measured data (blue dot) and corrected data for atmospheric contamination (orange dot) with a 129Xe/130Xe ratio of 7.6: (a) 126Xe/130Xe and (b) 128Xe/130Xe versus 124Xe/130Xe. For comparison, data of CO2 well gases from Caffee et al. (1999) (black points), Holland and Ballentine (2006) (black square) and Holland et al. (2009) (grey triangles), and thermal springs from Caracausi et al. (2016) (green square) and Moreira et al. (2018) (red square) are shown. The solid line is a fit of the data, except that of Holland and Ballentine (2006). The dotted lines indicate the 95 % confidence interval. Phase Q (Busemann et al., 2000), Solar Wind SW (Meshik et al., 2014), Xe-U and AVCC (Pepin, 2003). These new data suggest a chondritic origin (Phase Q or AVCC) for upper mantle Xe.