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.