The Halogen Composition of the Proto-Iceland Plume Source Mantle
Citation:
O'FARRELL, D?NAL JAMES, The Halogen Composition of the Proto-Iceland Plume Source Mantle, Trinity College Dublin.School of Natural Sciences.GEOLOGY, 2018Download Item:
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Abstract:
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.
Sponsor
Grant Number
Science Foundation Ireland (SFI)
Author's Homepage:
http://people.tcd.ie/dofarreDescription:
APPROVED
Author: O'FARRELL, DÓNAL JAMES
Advisor:
Tomlinson, EmmaPublisher:
Trinity College Dublin. School of Natural Sciences. Discipline of GeologyType of material:
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