TY - JOUR

T1 - An experimental calibration of a sulfur-in-apatite oxybarometer for mafic systems

AU - Konecke, Brian A.

AU - Fiege, Adrian

AU - Simon, Adam C.

AU - Linsler, Stefan

AU - Holtz, Francois

N1 - Funding Information: We acknowledge the experimental and analytical facilities at Leibniz University Hannover (LUH), University of Michigan (UM), American Museum of Natural History (AMNH), and GeoSoilEnviroCars (Sector 13), Advanced Photon Source (APS), Argonne National Laboratory. We thank Matt Newville and Tony Lanzarotti for assistance during XANES analyses. We acknowledge the constructive discussions with Jeremy Richards, Santiago C. Tassara, Owen Neill, Nikita La Cruz, and Tristan Childress. Finally, we thank Martin Streck, Laura Waters and an anonymous reviewer for their constructive feedback, and are grateful for the editorial handling of Ed Ripley. Use of the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. This work was funded by the NSF EAR-Grant 1524394.

PY - 2019/11/15

Y1 - 2019/11/15

N2 - The incorporation of sulfur (S) into the apatite structure and the partitioning of S between apatite and silicate melt (DS ap/m) have been proposed to vary systematically as a function of prevailing redox conditions. In this study, we experimentally equilibrated apatite with mafic silicate melt at 1000 °C, 300 MPa and a range of oxygen fugacity (fO2; log fO2 [ΔFMQ] = −1, 0, +0.3, +1.2, and +3 where FMQ is the fayalite-magnetite-quartz mineral redox buffer) to explore the partitioning behavior of S, including different oxidation states of S, between apatite and silicate melt. The data reveal that DS ap/m values increase systematically with increasing fO2, from 0.02± 0.01 at log fO2 [ΔFMQ] of −1 to 3.20 ± 0.19 at log fO2 [ΔFMQ] of +3. The bulk S content (∼0.37 and ∼0.28 wt.% S added) imparts a minor influence on DS ap/m at reducing conditions. Micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge was used to measure, in situ, the oxidation states of S in experimentally crystallized apatite. The S-XANES analyses reveal that with increasing fO2, apatite progressively incorporates S6+ ≫ S2− > S4+ > S1+. The integrated S6+/ΣS peak area ratios and centroid energies (eV) were determined for apatite crystals in apatite from experiments at all fO2 conditions. The orientation effects occurring during S-XANES analyses of apatite were considered by merging spectra from multiple grains with random crystallographic orientation. The S-XANES data reveal that S6+/ΣS peak area ratios and the centroid energies increase systematically with fO2, demonstrating that the S6+/ΣS ratio in apatite can be used as an oxybarometer. The results demonstrate that both the S6+/ΣS and CeV calibration methods are highly sensitive in the redox range of ∼FMQ to ∼FMQ + 1.2 at the conditions and compositions evaluated in this study. As a result, the S-in-apatite oxybarometer is particularly applicable to mafic systems such as mid ocean ridge basalts (MORB), relatively reduced ocean island basalts (OIB), and back-arc basin basalt (BABB). Owing to the ubiquity of apatite in magmatic and magmatic-hydrothermal systems, measuring the concentration and oxidations state(s) of S-in-apatite has the potential to serve as a powerful sulfo- and oxy-barometer for a broad range of natural systems.

AB - The incorporation of sulfur (S) into the apatite structure and the partitioning of S between apatite and silicate melt (DS ap/m) have been proposed to vary systematically as a function of prevailing redox conditions. In this study, we experimentally equilibrated apatite with mafic silicate melt at 1000 °C, 300 MPa and a range of oxygen fugacity (fO2; log fO2 [ΔFMQ] = −1, 0, +0.3, +1.2, and +3 where FMQ is the fayalite-magnetite-quartz mineral redox buffer) to explore the partitioning behavior of S, including different oxidation states of S, between apatite and silicate melt. The data reveal that DS ap/m values increase systematically with increasing fO2, from 0.02± 0.01 at log fO2 [ΔFMQ] of −1 to 3.20 ± 0.19 at log fO2 [ΔFMQ] of +3. The bulk S content (∼0.37 and ∼0.28 wt.% S added) imparts a minor influence on DS ap/m at reducing conditions. Micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge was used to measure, in situ, the oxidation states of S in experimentally crystallized apatite. The S-XANES analyses reveal that with increasing fO2, apatite progressively incorporates S6+ ≫ S2− > S4+ > S1+. The integrated S6+/ΣS peak area ratios and centroid energies (eV) were determined for apatite crystals in apatite from experiments at all fO2 conditions. The orientation effects occurring during S-XANES analyses of apatite were considered by merging spectra from multiple grains with random crystallographic orientation. The S-XANES data reveal that S6+/ΣS peak area ratios and the centroid energies increase systematically with fO2, demonstrating that the S6+/ΣS ratio in apatite can be used as an oxybarometer. The results demonstrate that both the S6+/ΣS and CeV calibration methods are highly sensitive in the redox range of ∼FMQ to ∼FMQ + 1.2 at the conditions and compositions evaluated in this study. As a result, the S-in-apatite oxybarometer is particularly applicable to mafic systems such as mid ocean ridge basalts (MORB), relatively reduced ocean island basalts (OIB), and back-arc basin basalt (BABB). Owing to the ubiquity of apatite in magmatic and magmatic-hydrothermal systems, measuring the concentration and oxidations state(s) of S-in-apatite has the potential to serve as a powerful sulfo- and oxy-barometer for a broad range of natural systems.

KW - Apatite

KW - Apatite crystallization experiments

KW - Oxybarometry

KW - Sulfur micro-XANES

KW - Sulfur oxidation states

KW - Sulfur partitioning

UR - http://www.scopus.com/inward/record.url?scp=85072564145&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2019.08.044

DO - 10.1016/j.gca.2019.08.044

M3 - Article

AN - SCOPUS:85072564145

VL - 265

SP - 242

EP - 258

JO - Geochimica et Cosmochimica Acta

JF - Geochimica et Cosmochimica Acta

SN - 0016-7037

ER -