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| Titel |
Bromine partitioning between olivine and melt at OIB source conditions: Indication for volatile recycling |
| VerfasserIn |
Bastian Joachim, Lorraine Ruzié, Ray Burgess, Alison Pawley, Patricia L. Clay, Christopher J. Ballentine |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250127089
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| Publikation (Nr.) |
 EGU/EGU2016-6913.pdf |
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| Zusammenfassung |
| Halogens play a key role in our understanding of volatile transport processes in
the Earth‘s mantle. Their moderate (fluorine) to highly (iodine) incompatible and
volatile behavior implies that their distribution is influenced by partial melting,
fractionation and degassing processes as well as fluid mobilities. The heavy halogens,
particularly bromine and iodine, are far more depleted in the Earth’s mantle than
expected from their condensation temperature (Palme and O’Neill 2014), so that their
very low abundances in basalts and peridotites (ppb-range) make it analytically
challenging to investigate their concentrations in Earth’s mantle reservoirs and
their behavior during transport processes (Pyle and Mather, 2009). We used a new
experimental technique, which combines the irradiation technique (Johnson et al.
2000), laser ablation and conventional mass spectrometry. This enables us to present
the first experimentally derived bromine partition coefficient between olivine and
melt.
Partitioning experiments were performed at 1500˚ C and 2.3 GPa, a P-T condition
that is representative for partial melting processes in the OIB source region (Davis
et al. 2011). The bromine partition coefficient between olivine and silicate melt
at this condition has been determined to DBrol∕melt = 4.37•10−4± 1.96•10−4.
Results show that bromine is significantly more incompatible than chlorine (∼1.5
orders of magnitude) and fluorine (∼2 orders of magnitude) due to its larger ionic
radius.
We have used our bromine partitioning data to estimate minimum bromine abundances in
EM1 and EM2 source regions. We used minimum bromine bulk rock concentrations
determined in an EM1 (Pitcairn: 1066 ppb) and EM2 (Society: 2063 ppb) basalt (Kendrick
et al. 2012), together with an estimated minimum melt fraction of 0.01 in OIB
source regions (Dasgupta et al. 2007). The almost perfect bromine incompatibility
results in minimum bromine abundances in EM1 and EM2 OIB source regions
of 11 ppb and 20 ppb, respectively. The effect on the partitioning behaviour of
other minerals such as pyroxene, mantle inhomogeneity, incongruent melting, a
potential effect of iron, temperature, pressure or the presence of fluids, would be to
shift the estimated bromine mantle source concentration to higher but not to lower
values. Comparing our minimum bromine OIB source region estimate with the
estimated primitive mantle bromine abundance (3.6 ppb; Lyubetskaya and Korenaga,
2007) implies that the OIB source mantle is enriched in bromine relative to the
primitive mantle by at least a factor of 3 in EM1 source regions and a factor of
5.5 in EM2 source regions. One explanation is that bromine may be efficiently
recycled into the OIB source mantle region through recycling of subducted oceanic
crust.
Dasgupta R, Hirschmann MM, Humayun, ND (2007) J. Petrol. 48, pp. 2093-2124.
Davis FA, Hirschmann MM, Humayun M (2011) Earth Planet. Sci. Lett. 308, pp.
380-390.
Johnson L, Burgess R, Turner G, Milledge JH, Harris JW (2000) Geochim. Cosmochim.
Acta 64, pp. 717-732.
Kendrick MA, Woodhead JD, Kamenetsky VS (2012) Geol. 32, pp. 441-444.
Lyubetskaya T, Korenaga J (2007) J. Geophys. Res.-Sol. Earth 112, B03211.
Palme H, O’Neill HStC (2014). Cosmochemical Estimates of Mantle Composition. Treat.
Geochem. 2nd edition, 3, pp. 1-39.
Pyle DM, Mather TA (2009) Chem. Geol. 263, pp. 110-121. |
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