The Halogen Composition of the Proto-Iceland Plume Source Mantle

• Main Author: O'FARRELL, D?NAL JAMES
• Format: Thesis
• Language: English
• Published: Trinity College Dublin. School of Natural Sciences. Discipline of Geology 2018
• Subjects: Baffin Island; Greenland; Vaigat; Kendrick; Baffin; Iceland
• Online Access: http://hdl.handle.net/2262/82826; http://people.tcd.ie/dofarre

APPROVED The study of halogens in mantle-derived melts is allowing for new and unique insights into mantle geodynamics (e.g. Kendrick et al., 2017). The current dataset of mantle halogens record the compositions of both the depleted MORB mantle and the major re-enriched OIB deep mantle reservoirs (EMI, EMII and HIMU). However our understanding of the halogen geochemical cycle suffers from a major limitation; the lack of a direct and measurable constraint on the Bulk Silicate Earth halogen composition of the Earth. Without this constraint in place, it is impossible to assess properly, the overall mass balance of halogens between the major geochemical reservoirs. This study has aimed to address this issue by characterising the halogen composition of the primordial ?undegassed? mantle reservoir. This ?undegassed? high 3He/4He mantle is sampled in the Vaigat formation; the oldest rocks of the 62-58Ma Baffin Island-West Greenland lavas; products of the ancestral Iceland plume (Stuart et al., 2003; Starkey et al., 2009). Based on tungsten isotopic measurements in lavas from the same locality; the formation of this reservoir is time constrained to the first fifty million years of solar system history (Rizo et al., 2016). It records primordial compositions with respect to Pb and Nd (Jackson et al., 2010); and has previously elucidated the primordial water content of the Earth (Hallis et al., 2015). As such characterising the halogen composition of this reservoir represents a unique opportunity to assess the halogen inventory of the earliest Earth. Modelling for crustal contamination revealed that none of the inclusions studied display any evidence of having been obviously crustally contaminated. The most primitive inclusions, yielded near-chondritic trace element compositions, while more enriched inclusions, record comparative light-ion lithophile element enrichment consistent with their respective whole rock?s more radiogenic Sr-Nd composition. Cl/K ratios suggest that only one of the inclusions studied records potential seawater contamination. This enriched inclusion also records very incompatible element (Nb, Th, U Ba) enrichment suggestive of derivation from a source with a more enriched (EMI or EMII) mantle-like derivation; present within younger Icelandic plume lavas (Stracke et al., 2003; Thirlwall et al., 2004; Halldorsson et al., 2016). This primary enriched melt then interacted with a deep contaminant such as altered oceanic crust or iv seprentinized mantle endemic to the asthenospheric mantle beneath Baffin-Island and West Greenland. Elemental (Cl,Br) halogen compositions were calculated for the primitive mantle using experimentally derived partition coefficients (Joachim et al., 2015; Joachim et al., 2016).The primitive mantle Cl composition is suggested to lie within the range of 8.38-20.84 ppm; generally lower than previous estimates for primitive mantle with the upper range within range of previous estimates (Sun and McDonough, 1995, Palme and O?Neill, 2013, Kendrick et al., 2017). As such these results must prefer lower estimates for the Cl composition of the depleted MORB mantle in agreement with recent estimates (eg. Urann et al., 2017). Br values range from 1.15 to 21.84 ppb agreeing well with the values of O?Neill and Palme (2003). Remarkably, δ37Cl compositions of the most primitive inclusions yield values averaging - 0.21?0.4?, which agrees closely with the average value (-0.2?), previously inferred for the Bulk Silicate Earth (Sharp et al., 2007) and that of type 3 chondrites at -0.3?0.5? (Sharp et al., 2013). Considering the Baffin Island melt inclusions record primitive trace element and 3He/4He characteristics in combination with other primordially determinant geochemical proxies; the values presented here may represent the combined Br, Cl and δ37Cl compositions of the Bulk Silicate Earth.