Origin of negative Cerium anomalies in subduction-related volcanic samples: constraints from Ce and Nd isotopes Nina Bellot, Maud Boyet, Régis Doucelance, Pierre Bonnand, Ivan Savov, Terry Plank, Tim Elliott To cite this version: Nina Bellot, Maud Boyet, Régis Doucelance, Pierre Bonnand, Ivan Savov, et al.. Origin of nega- tive Cerium anomalies in subduction-related volcanic samples: constraints from Ce and Nd isotopes. Chemical Geology, Elsevier, 2018, 500, pp.46-63. 10.1016/j.chemgeo.2018.09.006. hal-02110867 HAL Id: hal-02110867 https://hal.uca.fr/hal-02110867 Submitted on 25 Apr 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. *Revised manuscript with no changes marked Click here to view linked References 1 Origin of negative Cerium anomalies in subduction-related 1 2 3 2 volcanic samples: constraints from Ce and Nd isotopes 4 5 6 3 7 8 4 9 10 a a a a 11 5 Nina Bellot , Maud Boyet , Régis Doucelance , Pierre Bonnand , 12 13 b c d 6 14 Ivan P. Savov , Terry Plank , Tim Elliott 15 16 7 17 18 8 a Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F- 19 20 9 63000 Clermont-Ferrand, France. 21 22 10 b School of Earth and Environment, Institute of Geophysics and Tectonics, University of 23 24 11 Leeds, Leeds LS2 9JT, UK. 25 26 12 c Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, New 27 28 13 York 10964, USA. 29 30 14 d Bristol Isotope Group, School of Earth Sciences, University of Bristol, Wills Memorial 31 32 15 Building, Queen’s Road, Bristol BS8 1RJ, UK. 33 34 16 35 36 17 Corresponding author: [email protected] 37 38 18 nina.bellot.gmail.com / [email protected] / [email protected]/ 39 40 19 [email protected] / [email protected] / [email protected] 41 42 20 43 44 21 Main text: 8952 words (without abstract, references, table and figure captions) 45 46 22 Figures: 11 47 48 23 Tables: 3 49 50 24 Supplementary files: 7 51 52 25 53 54 55 26 ABSTRACT 56 57 27 Negative Cerium (Ce) anomalies are observed in chondrite-normalized rare earth element 58 59 28 patterns from various volcanic arc suites. These anomalies are well defined in volcanic rocks 60 61 62 63 1 64 65 29 from the Mariana arc and have been interpreted as the result of addition of subducted 1 143 144 138 142 2 30 sediments to the arc magma sources. This study combines Nd/ Nd and Ce/ Ce 3 4 31 isotope measurements in Mariana volcanic rocks that have Ce anomalies ranging from 0.97 5 6 32 to 0.90. The dataset includes sediments sampled immediately before subduction at the 7 8 33 Mariana Trench (Sites 801 and 802 of ODP Leg 129) and primitive basalts from the Southern 9 10 34 Mariana Trough (back-arc basin). Binary mixing models between the local depleted mantle 11 12 35 and an enriched end-member using both types of sediment (biosiliceous and volcaniclastic) 13 14 36 found in the sedimentary column in front of the arc are calculated. Marianas arc lavas have 15 16 37 Ce and Nd isotopic compositions that require less than 2.5% of a sediment component 17 18 38 derived from the volcaniclastics. With this proportion of sediment, most of the Ce/Ce* range 19 20 39 measured in lavas is reproduced. Thus, this study confirms that the origin of the Ce 21 22 40 anomalies in the Mariana arc magmas can be principally attributed to recycling of trench 23 24 41 sediments through active subduction. The participation of a component derived from 25 26 42 biosiliceous sediments does not explain the Ce-Nd isotope composition of the lavas because 27 28 43 the involved proportion is too high (up to 8%) in comparison to results obtained from other 29 30 44 geochemical proxys. Using this end-member, the modeled Ce anomalies are also too high 31 32 45 (0.91-0.84) in comparison to those measured in lavas. Various processes and conditions are 33 34 46 able to generate Ce anomalies: oxygen fugacity, residual mineral phases, partial melting, 35 36 47 fractional crystallization and tropical weathering. Their influence in the case of Mariana 37 38 48 volcanic arc magmas seems to be very limited but partial melting effect may explain the 39 40 49 lowest measured Ce/Ce* values. Magmatic processes cannot be definitely ruled out in 41 42 50 producing Ce anomalies in other arc system environments. Additional experimental data, 43 44 51 however, are needed for a better understanding of the behavior of cerium relative to its 45 46 52 neighboring elements. Also, this study highlights the importance of using local depleted 47 48 53 mantle and sediments to model the isotopic compositions of arc lavas. 49 50 54 51 52 55 53 54 55 56 Highlights: 56 57 57 Origin of negative Ce anomalies in Mariana arc magmas. 58 59 60 61 62 63 64 2 65 58 Coupled 138Ce/142Ce and 143Nd/144Nd isotope measurements in Mariana arc magmas, 1 2 59 sediments and back-arc basalts. 3 4 60 Ce-Nd isotopic binary mixing models coupled with Ce/Ce* prove that volcaniclastic 5 6 61 sediments control the Ce/Ce* in Mariana lavas. 7 8 62 9 10 63 Keyword: Ce anomalies, 138Ce/142Ce, sediment recycling, Mariana volcanic arc, Rare Earth 11 12 64 Elements. 13 14 65 15 16 66 17 18 67 1. Introduction 19 20 21 68 22 23 69 Arc magmas record chemical signatures associated with subduction processes. The 24 25 70 subducted slab inventory is made up of variously altered and different in age subducted 26 27 71 oceanic crust and its sedimentary cover. The sediments are diverse in type and origin 28 29 72 depending on the age of the subducting plate, proximity to a continent, and the presence or 30 31 73 absence of an accretionary prism (Plank and Langmuir, 1998a; von Huene and Scholl, 1991). 32 33 74 Cerium anomalies have been measured in arc rocks from different localities, particularly the 34 35 75 New Britain, Mariana, Tonga, Central America and the Lesser Antilles (Carr et al., 1990; 36 37 76 Dixon and Batiza, 1979; Ewart et al., 1973; Jakes and Gill, 1970; White and Patchett, 1984). 38 39 77 A negative Ce anomaly means that the Ce concentration normalized to the chondritic value is 40 41 78 lower than the value interpolated from the two neighboring Rare Earth Elements (REE) 42 43 79 Lanthanum (La) and Praseodymium (Pr). The origin of negative Ce anomalies in arc settings 44 45 80 has been attributed to the addition of sedimentary component to the arc magma source 46 47 81 (Dixon and Batiza, 1979; Elliott et al., 1997; Hole et al., 1984; Woodhead, 1989). Cerium is 48 49 82 the only REE that exists in either 3+ or 4+ oxidation states and in nature Ce fractionations are 50 51 83 related to the changes of the redox conditions. The short residence time of Ce4+ in seawater 52 53 84 relative to the trivalent Ce3+ ions explains the large negative Ce anomaly in the seawater REE 54 55 85 pattern (Elderfield and Greaves, 1982). Fe-Mn crusts and MnO clays preferentially scavenge 56 57 86 Ce4+ relative to other REE3+ and thus have positive Ce anomalies (Amakawa, 1991; Bau et 58 59 87 al., 2014). Conversely, negative Ce anomalies are generally identified in authigenic clays, 60 61 62 63 3 64 65 88 hydrothermal sediments, nannofossil ooze, or fish debris (Moiroud et al., 2015; Picard et al., 1 2 89 2002; Plank and Langmuir, 1998). The geochemical composition of subducting sediments is 3 4 90 now fairly well known globally (Plank, 2013). However, the average “global subducting 5 6 91 sediment” reservoir (GLOSS, see Plank and Langmuir (1998) and Plank (2013)) does not 7 8 92 show significant Ce anomaly with Ce/Ce* values of 0.97 and 1.02 for GLOSS I and GLOSS II, 9 10 93 respectively. The majority of the mean trench sedimentary piles in individual subduction 11 12 94 zones used for the GLOSS reservoir calculation have negative Ce/Ce* values (57% of them 13 14 95 based on the weighted mean composition of each pile). The lack of anomaly in GLOSS 15 16 96 reflects the dominance of volcanic ashes and turbidites in some trench sediments that 17 18 97 represent large masses in the total budget, which are deposited too quickly to fractionate Ce 19 20 98 from other REE. 21 22 99 23 138 138 24 100 The La- Ce isotope systematics (T1/2 = 292.5 Ga; Tanimizu, 2000) is an interesting 25 26 101 tool to trace the recycling of sediments in subduction zones since material characterized by 27 28 102 Ce anomalies have fractionated La/Ce ratios and then will develop by radiogenic ingrowth 29 30 103 different 138Ce/142Ce ratios. Calculations show that significant deviations of the 138Ce/142Ce 31 32 104 ratio from the chondritic reference can be generated in less than 100 Ma in sediments 33 34 105 characterized by highly fractionated La/Ce ratios (3-8) as those measured in seawater (see 35 36 106 Figure 1 in Bellot et al., 2015).