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Abstract The SWEAP instrument suite on Parker Solar Probe (PSP) has detected numerous proton beams associated with coherent, circularly polarized, ion-scale waves observed by PSP’s FIELDS instrument suite. Measurements during PSP Encounters 4−8 revealed pronounced complex shapes in the proton velocity distribution functions (VDFs), in which the tip of the beam undergoes strong perpendicular diffusion, resulting in VDF level contours that resemble a “hammerhead.” We refer to these proton beams, with their attendant “hammerhead” features, as the ion strahl. We present an example of these observations occurring simultaneously with a 7 hr ion-scale wave storm and show results from a preliminary attempt at quantifying the occurrence of ion-strahl broadening through three-component ion VDF fitting. We also provide a possible explanation of the ion perpendicular scattering based on quasilinear theory and the resonant scattering of beam ions by parallel-propagating, right circularly polarized, fast magnetosonic/whistler waves. 

Here we present the first thermodynamic models that predict the full range of possible S-liberating reactions during the subduction of mafic oceanic crust. Models for MORB and AOC were created in Perple_X utilizing the combined thermodynamic databases of [1] and [2]. Transitions from pyrrhotite to pyrite and pyrite to anhydrite are observed with increasing P-T. The pressure of the pyrite-anhydrite transition depends on initial Fe3+/6Fe: 3.2 GPa for MORB (Fe3+/6Fe = 0.16) and 2.3 GPa for AOC (Fe3+/6Fe = 0.28) at 650 °C. Reactions were monitored along slab-top geotherms for Honshu and Cascadia (D80, [3]). Above the pyrite-anhydrite transition HSO4-, SO42-, and HSO3- are the dominant fluid species (<0.9 mol/kg), whereas HS- is dominant in the sulfide fields (<0.1 mol/kg). Along the Honshu path, oxidized Sspecies increase from 0.05 to 0.4 mol/kg over 550 to 625 °C (82-84 km depth), concurrent with an increase of 70 % in the total fluid volume due to reactions such as lawsonite-out. Sulfur oxidation is balanced by the reduction of Fe3+, with a 48 % decrease in Fe3+/6Fe. In contrast, oxidized S-species increase from 0.0 to 0.4 mol/kg over 600 to 675 °C (68-77 km depth) along the Cascadia path, concurrent with a 15 % increase in total fluid volume. Nearly 85 % of the total fluid released along the Cascadia path occurs where HS- dominates and S concentrations in fluid are low. Our data suggest that slab-derived S-bearing fluids are a viable mechanism for oxidation of arc magmas. Coeval sulfur and water loss along cold P-T paths are expected to result in high fluxes of oxidized sulfur to volcanic arcs, whereas significant dehydration prior to sulfur oxidation will result in low sulfur fluxes along hot P-T paths. This discrepancy is expected to be accentuated by the less oxidized nature of younger oceanic crust at hot subduction zones. [1] Holland & Powell (2011) JMG [2] Servjensky et al. (2014) GCA [3] Syracuse et al. (2010) PEPI 

Titanite and apatite can incorporate significant amounts of common Pb (204Pb) into their mineral structures, which leads to uncertainty when applying the U-Pb decay series for geochronology. The isobaric interference between 204Pb and 204Hg creates an additional complexity when calculating common lead corrections. Here we investigate the removal of 204Hg interferences during titanite U-Pb dating using reaction cell gas chemistry via triple quadrupole mass spectrometry compared with traditional methods that calculate U-Pb ages using a common lead correction. U-Pb dates for titanite natural reference materials MKED-1 and BLR-1 were determined using an ESI NWR193UC excimer laser coupled with an Agilent 8900 ‘triple quadrupole’ mass spectrometer. The 8900 is equipped with an octopole collision/reaction cell, which enables online interference removal. In order to compare traditional methods for U-Pb dating with interference removal methods, two experiments were run, one in which data was collected in NoGas mode, and one in which the 8900 was run in MS/MS mode, in order to assess the feasibility of determining U/Pb ratios with mass shifted isotopes. In MS/MS mode, NH3 was flowed through the reaction cell in order to enable a charge transfer reaction between NH3 and Hg+, effectively neutralizing Hg. During spot analyses in NoGas mode, masses 202Hg, 204Hg, 204Pb, 206Pb, 207Pb, 208Pb, 232Th, 235U, and 238U were monitored. For spot analyses in MS/MS mode, Th and U isotopes were measured on-mass at 232Th, 235U, 238U and mass-shifted to 247Th, 250U, and 253U. Pb isotopes were measured on-mass since Pb does not react with NH3. Ratios for 207Pb/235U, 206Pb/238U, and 207Pb/206Pb were calculated in Iolite (v.3.7.1) using the Geochron4 DRS using MKED-1 as the primary reference material and BLR-1 as a secondary reference material. Dates were calculated using IsoplotR. Weighted mean ages for titanite BLR-1 in MS/MS mode are 1043.8 ± 10.5 Ma (2σ, MSWD=1.08) for U isotopes measured on mass, and 1039.7 ± 8.3 Ma (2σ, MSWD=1.08) for mass-shifted U isotopes. These dates are both in agreement with the TIMS 206Pb/238U date for the BLR-1 titanite of 1047.1 ± 0.4 Ma. The use of NH3 for reaction cell chemistry has the potential to enable measurement of 204Pb without needing to correct for Hg interferences.