THE OXIDATION STATE OF SULFUR IN APATITE AS A FUNCTION OF THE REDOX CONDITIONS BRIAN A. KONECKE1, ADRIAN FIEGE1,2, ADAM C. SIMON1 1Department
of Earth and Environmental Sciences, University of Michigan, Ann Arbor, USA 2Department of Earth and Planetary Sciences American Museum of Natural History, New York, New York 10024-5192, USA. Contact E-Mail: [
[email protected]]
The oxygen fugacity (ƒO2) of magmatic systems is a fundamental variable that influences and is influenced by crystallization and degassing processes, as well as ore metal ratios in porphyry ore deposits [1-3]. Apatite—commonly Ca5(PO4)3(F, Cl, OH)—is a resistant, ubiquitous mineral in magmatic systems and can contain up to several thousand µg/g of S [4]. In this study, the hypothesis whether apatite can be used as a redox tracer is investigated, with one outcome being the ability to constrain the variability of predegassed fO2 in the source magmas of magmatic-hydrothermal ore fluids. Ultimately, this new tool will allow us to test the hypothesis that variation in S-speciation and fO2 are a master control on ore metal variability.
(4) Peak Area Fitting & Integration
A)
B)
C)
D)
(6) Assessment of Beam Damage
• The open source curve and peak fitting software Fityk [7] were used for the peak area integration of corrected and merged S-XANES spectra (Figure 2 A-D).
• The integrated peak area ratios of the S6+, S4+ and S2- peaks were used in order to evaluate relative
• Apatite crystallization experiments (Table 1) were conducted in rapid quench internally heated pressure vessels (IHPV) at Leibniz University Hannover (LUH), Germany. • Gold capsules were loaded with 40 mg of mafic starting material (Table 2) and 7-8 wt.% H2O. Table 1: Experimental conditions Run name
Temperature (oC)
Pressure (MPa)
fO2 (FMQ)
Duration (days)
S added (wt. %)
LA45-IH1 LA45-IH7 LA45-IH13
1,000 1,000 1,000
300 300 300
0 1.2 3
3 5 5
1.0 (po.) 1.0 (po.) 1.0 (*po.)
changes in S-oxidation state in the sample.
Figure 4: Sulfur XANES spectrum of hydrous glass demonstrating beam damage, which is characterized by the systematic reduction of the S6+ peak and development of a S4+ peak with increasing analytical time (e.g., from scan 1 to scan 3).
Figure 2 A-D: Illustration of the Fityk peak fitting procedure for selected S XANES spectra collected on apatites. [A] Durango apatite; [B] LA45-IH1 (FMQ); [C] LA45-IH7 (FMQ+1.2); [D] LA45-IH13 (FMQ+3). The lower panels of A-D show the residual of the fitting after subtracting from the raw spectrum as a function of energy
Figure 5: One-hour S XANES time-series on Durango apatite to test for possible irradiation damage of apatite during analysis by evaluating the integrated S6+/STotal peak area ratios for significant deviation (e.g., within ±1σ standard deviation; gray box). The average (dashed line) = 0.956 ±0.002 (2σ).
(7) Orientation Effects
*0.35 wt.% S (elemental) + 0.92 wt.% Fe2O3, where the Fe/S ratio corresponds to pyrrhotite (po.) Table 2: Starting mafic composition
P2O5
H2O
F
Total
4.37 8.57 8.75 0.20 9.21 16.24 0.50 5.29 3.81 Lamproite 40.13 Analyzed by XRF; n.d.: not determined *Lamproite: Vestfjella, Dronning Maud Land, Antarctica; fused at 1,200oC; FMQ+1.5
n.d.
n.d.
97.07
wt%
SiO2
TiO2
Al2O3
FeO MnO MgO CaO Na2O K2O
• Sulfur X-ray absorption nearedge structures (S XANES; [5]) spectroscopy. • Sulfur XANES is an in situ, qualitative method used to determine the formal charge (i.e., oxidation state) of elements in minerals and glasses [6]. • Electron probe micro-analysis (EPMA) for major and trace elements was performed using a Cameca SX-100. • The crystallographic orientations of apatites were determined by electron backscatter diffraction (EBSD) using a Zeiss EVO 60 Variable Pressure scanning electron microscope (SEM).
• Crystallographic orientation effects have been observed previously in other minerals [8].
(5) S XANES Results A)
B)
glass
C)
glass
glass
• Presence and extinction of S6+, S4+ and S2peaks a function of the perceived orientation of the crystal (Figure 6). • Several apatites grains were analyzed and merged to achieve a representative distribution within each sample. • Crystallographic orientation effects were not observed in the apatites crystallized from intermediate (FMQ+1.2) and oxidized melts (FMQ+3).
apatite
apatite
apatite
Figure 6 A-D: (A) is S-XANES spectra for apatite (LA45-IH1) partially parallel c-axis; (B) partially parallel to c-axis; (C) perpendicular to c-axis; and (D) merged spectra of all apatites measured in the sample. Crystal geometries were perceived optically during XANES analysis and subsequently correlated using EBSD.
(8) Conclusions 1. To our knowledge, this observation makes apatite the first mineral to incorporate reduced (S2-), intermediate (S4+), and oxidized (S6+) species of S in variable proportions as a function of the prevailing fO2 of the system. 2. Orientation effects may provide information pertaining to the local bonding environment of S6+, S4+, and S2- in the apatite structure (e.g., the existence and/or extinction of S-species in the XANES spectra). Figure 1: S-XANES spectra of different sulfur compounds. Peak positions for S6+ (2482 eV), S4+ (2478 eV) and S2(2470 eV) are consistent with known standards.
Figure 3 A-C: S XANES analysis of quenched glass (gl; top) and apatite (ap; bottom) from experiments performed at different fO2 conditions: (A) FMQ, (B) FMQ+1.2 and (C) FMQ+3.
3. Upon calibration over a range of geologically relevant T-P-X-fO2-fS2, S-in-apatite can serve as a powerful oxybarometer used to quantify the fO2 and redox evolution of magmatic systems.
References: [1] G. Faure, Principles of Isotope Geology. John Wiley and Sons, New York (1986). [2] P. Candela, P. Piccoli, Magmatic Processes in the Development of Porphyry-Type Ore Systems. Econ. Geol., 25–37 (2005). [3] A. C. Simon, E. M. Ripley, The Role of Magmatic Sulfur in the Formation of Ore Deposits. Rev. Mineral. Geochemistry. 73, 513–578 (2011). [4] J. D. Webster, P. M. Piccoli, Magmatic Apatite: A Powerful, Yet Deceptive, Mineral. Elements. 11, 177–182 (2015). [5] This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357. [6] M. E. Fleet, XANES SPECTROSCOPY OF SULFUR IN EARTH MATERIALS. Can. Mineral. 43, 1811-1838 (2005). [7] M. Wojdyr, Fityk: A general-purpose peak fitting program. J. Appl. Crystallogr. 43, 1126–1128 (2010). [8] K. A. Evans et al., Variation in XANES in biotite as a function of orientation, crystal composition, and metamorphic history. Am. Mineral. 99, 443–457 (2014).
