Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earth's mantle conditions Journal Article uri icon

DCO ID 11121/4807-6729-9698-4816-CC

is Contribution to the DCO

  • YES

year of publication

  • 2015

abstract

  • Halogenshave, because of theirvolatile behavior and incompatibility, thepotential to act as key tracers of volatiletransportprocesses within theEarth's mantle. A better understanding of halogen behavior duringpartial meltingprocesses will improve our understanding of volatile input mechanisms into the Earth's mantle and give insightinto its evolution over the Earth's history. This study introduces time of flight secondary ion mass spectrometry(TOF-SIMS) as an analytical method for the determination of halogen partition coefficients and significantly ex-tends the available datasetfor fluorine and chlorine partitioning between mantle minerals and silicate melts toconditions of partial melting processes in Ocean Island Basalt (OIB) source regions.Halogen partitioning between olivine, orthopyroxene and silicate melt has been determined in experiments at1.0–2.3 GPa and 1350–1600 °C. Combining our data with results of recent studies (O'Leary et al., 2010; Beyeret al., 2012;Dalou et al., 2012, 2014; Rosenthal et al., 2015)shows that fluorine and chlorine partitioning betweenolivine andmelt increases by about 1.5–2orders of magnitude between 1350 °Cand 1600 °C (fluorine: 0.005(3)–0.31(16); chlorine: 0.005(45)–0.17(9)) and does not show any pressuredependence between 1.0 and 2.3 GPa.Chlorine partitioning between orthopyroxene and melt increases by about 1 order of magnitude between1450 °C and 1600 °C (0.015(8)–0.16(9)) at a constant pressure of 2.3 GPa. Fluorine partitioning betweenorthopyroxene and melt increases by 1.5 orders of magnitude between 1250 °C and 1600 °C (0.029(6)–0.20(14)) and does not show any pressure dependence.Transmission electron microscopy measurements show that halogens are not incorporated in the form ofhumite-type defects in olivine. The most reasonable incorporation mechanism for halogens is via point defectsin the olivine and orthopyroxene lattice, where they are inferred to be charge-balanced via oxygen defects.By combining our partitioncoefficients withnatural halogenconcentrations inoceanic basalts, weare able to giveestimatesfor fluorine and chlorine abundances in Mid OceanRidge Basalt (MORB) (F = 3–14; Cl = 0.6–14 ppm)and OIB (F = 34–76; Cl = 21–71 ppm) mantle source regions. Comparing these with estimates of bulk silicateEarth (BSE) concentrations (F = 18 ± 8 ppm, Lyubetskaya and Korenaga, 2007; F = 25 ± 10 ppm, Palme andO'Neill, 2003;Cl=30±12ppmPalme and O'Neill, 2003) indicates that the upper mantle is degassed by 22–88% in fluorine and22–99% in chlorine relative to the primitive mantle. The OIB source mantleregion has a chlo-rine concentration that is similar to primitive mantle estimates, but is enriched in fluorine by a factor of 1.4–4.2relative to the primitive mantle. An explanation for the relative fluorine enrichment in the OIB source region isthat compared to chlorine, fluorine may be incorporated to a greater extent into the crystal structure of mineralsthat are stable at high P–T conditions and may thus be recycled more efficiently into the deeper mantle throughsubduction of oceanic crust.

volume

  • 416