4. Results
4.1 Petrography of Sulfides in Experimental Melt Inclusions
For the experiments run at ƒO2 ≤ FMQ+0.35, the quenched melt inclusions contain variable amounts of a sulfide phase, which is easily recognized in transmitted light as
Figure 3. Backscattered electron (BSE) images of experimental melt inclusions. (A) SF11- 2, homogenization experiment at 1225 °C and FMQ+0.35, (B) SF8-1, displacement method experiment cooled from 1225-1000 °C at FMQ+0.35. The vapor cavity in SF8-1 is not exposed at the sample surface. Sulfide blebs are seen as small bright spots within the inclusion glasses.
Figure 4. Optical microphotographs of experimentally homogenized and cooled melt inclusions homogenized at FMQ-1 and 1225 °C (A-E). (A) Melt inclusion with an annulus of sulfide, as well as sulfides attached to the inclusion walls. Sulfide rings or annuli are also shown in (B), and(D). Panel (E) shows an experimental inclusion containing two melt inclusions ~100 µm apart, one has no sulfides whereas the other is sulfide saturated. (C) shows a displacement method experiment that generated substantial crystal growth in the melt inclusion. (F) is a secondary electron image of the interior of a large vapor cavity in an experimental melt inclusion. There are 10-15 µm wide Fe-S-Ni blebs, as well as micrometer-sized sulfides organized in a mat on the glass wall.
Figure 5. (A) Optical microphotograph of experiment H3XC1, which was held in pure CO2 gas at 1225 °C (FMQ+4.7) for 24 hr. At the center of the image is a melt inclusion with a large vapor cavity. The surrounding olivine is pervaded by decorated dislocations (thin rust- colored lineaments). (B) Backscattered electron images reveal the growth of magnetite- pyroxene symplectites within the volume of the experimental olivine. The symplectites concentrate around and emanate from hairline cracks and traces of fractures in the olivine interior. (C) Secondary electron image of a symplectite domain in an experimentally oxidized olivine. The bright phase is magnetite, there is pyroxene in the interstices but it is almost indistinguishable from the olivine in secondary electron images. EDS spectra shows that the pyroxene has Mg:Si = 1 and for the olivine, 2. (B) Backscattered electron image of H3XR2, a reversal experiment, which was run on olivines such as that shown in (A-C) that were previously oxidized in pure CO2 and then re-homogenized at FMQ-1 for 24 hr at 1225
°C. Except for a few relict areas, the remaining symplectites dissolved back into the olivine during the reversal experiment.
opaque spherical blebs (Figure 4), and which appear as bright (high-Z) in backscattered electron (BSE) imagery (Figure 3). Within a single olivine grain, there can be melt inclusions that contain no sulfides as well as inclusions that contain sulfides (Figure 4E).
Sulfide blebs are often found to be attached to the inclusion walls (Figure 4A) or to vapor bubbles and spinels within the melt inclusions. In cooled experiments, some sulfides at the inclusion walls appeared to be embedded within the olivine that mantled the inclusion wall, indicating that they became trapped during olivine growth. In several, but not all, melt inclusions equilibrated at ƒO2 ≤ FMQ+0.35 the sulfides that were not attached to other phase surfaces and which were suspended within the melt were organized in a striking and reproducible pattern, shown in Figure 4 panels A, B, and D, in which the sulfides were arranged in an annulus or ring that overlapped with the position of the vapor bubble. The sulfide annuli were observed more frequently in the more reducing experiments and qualitatively formed a more coherent structure in experiments with lower cooling rates (in particular H11X; Saper & Stolper 2020), although the annuli were also observed in experiments that were homogenized and then quenched directly from the hotspot and thus had chemically homogeneous melt inclusions. The mechanism for forming these rings is unclear, but it is apparent that the vapor bubble exerts some control on their presence and geometry. As far as I am aware, such structures are not observed in natural melt inclusions and certainly not in the Papakōlea Beach melt inclusions that served as starting materials for these experiments. In the Papakōlea natural melt inclusions, sulfides were observed in approximately 25% of the olivine-hosted melt inclusions; there are usually fewer individual spherules in a given inclusion when compared to the experimental inclusions and they are typically larger in diameter.
The importance of the vapor bubble is further demonstrated by inspection of the interior of the vapor cavity using secondary electron (SE) imaging – in a handful of cases the vapor bubbles expanded during the experiments to a diameter that was large enough to peer into and image using the SEM, revealing phases embedded at the vapor-melt interface (Figure 4F). Energy-dispersive measurements of these phases show that they are FeS±Ni and there are at least two distinct morphologies: larger ~10 µm blebs and a mat of subequally spaced ~1 µm domains. Solid phases inside or on the walls of vapor bubbles have been observed within natural olivine-hosted melt inclusions (Kamenetsky et al. 2002, Esposito et al. 2016, Moore et al. 2018), including from Mauna Loa, Hawai‘i, as well as on vesicles in quenched submarine glasses (Mathez and Yeats 1976, Yeats and Mathez 1976, Alt et al. 1993), and so it is unclear whether these formed experimentally and/or were inherited from the natural samples. Their occurrence indicates that the fluids themselves became saturated with respect to the sulfides, and/or they formed via diffusive exchange between the silicate melt and the vapor at the interface between the two phases. It is possible that the initial presence of these sulfides on the vapor cavity wall are related to the annulus structure observed in the experimental melt inclusions, however further study is warranted to determine their precise formation mechanism.
Table 1. Experimental run conditions and Fe and S XANES and SIMS results.
Experimental olivine-hosted melt inclusions.
Name ΔFMQa Homogenization T °C (24 hr) Quench
T °C Cooling typeb Fe3+/FeT c S6+/ST d S
(ppm) H2O
(ppm) CO2
(ppm)
H3XC1 +4.7 1225 1225 Homogenization 0.688 1025
HTOX2 +2.8 1225 1225 Homogenization 0.299 851
SF11-5 +0.35 1225 1225 Homogenization 0.103 0.03-0.06 800 526 48
SF1-2 +0.35 1225 1225 Homogenization 0.117 823
SF13-1 +0.35 1225 900 Displacement 0.118 769 53
SF8-3 +0.35 1225 1000 Displacement 0.108 0
SF3-2 +0.35 1225 1150 T.C. Controlled 0.137 0.04-0.10
SF4-3 +0.35 1225 1000 T.C. Controlled 0.129 0 1041
H11X1B -1.0 1225 1225 Homogenization 0.076 0 842 526 48
H7XC1 -1.0 1225 1225 Homogenization 0.065 788 188 15
H3XR2e and -1 +4.7 1225 1225 Homogenization 783 376 54
Natural unheated olivine-hosted melt inclusions from Papakōlea, Mauna Loa, Hawai’i (starting materials for experiments)
Name Calculated ΔFMQf
Reconstructed T °C prior to
eruptiong
T °C (MgO in liquid at inclusion
wall)g
Cooling Rateh Fe3+/FeT c S6+/ST d S
(ppm) H2O
(ppm) CO2
(ppm)
HIGS11 +0.10 to +1.22 1176 1115 8117±800°C/hr 0.184 0.45-0.66 1900i
HIGS14 +0.10 to +1.23 1154 881 55±5°C/hr 0.199 1.0 154.0 (20001874 i) 19
HIGS20 +0.10 to +1.33 (1175) 0.185 1035 2700i
HIGS21 -0.02 to
+1.12 (1175) 974 0.165 129 1600i
Caption to Table 1.
a - Nominal ƒO2 of the experiments, based on H2-CO2 gas mix in the furnace atmosphere. ΔFMQ is the difference in log units between the experimental ƒO2 and the FMQ data of O'Neill (1987).
b - Method by which the experiments were cooled. See section 3.1 in text for details.
c - Based on the average of 2-3 XANES measurements of each glass. Repeat of the same glass differed by 0.01-0.02 and the estimated 1σ uncertainty is 0.015 (Brounce et al. 2017).
d - The lower value refers to using the unscaled integrated peak area ratios, whereas the higher value used the generic scaling factors of Nash et al. 2019.
e - H3XR2 was a reversal experiment, first run for 24 hr at 1225°C and FMQ+4.7, quenched, and then re-run for 24 hr at 1225 °C and FMQ-1.
f - The range of ƒO2 - expressed relative to FMQ (O'Neill 1987) - calculated based on the Fe3+/FeT measured by XANES, the reconstructed melt T, and the inclusion compositions, using the six Fe oxybarometers described in the text.
g - See Saper & Stolper 2020 for details. Reconstructed T based on integrating the zoned melt inclusion compositions and adding equilibrium olivine back to the melt until it was in equilibrium with olivine adjacent to the melt inclusion walls. The inclusions HIGS20 and HIGS21 were not completely exposed at their centers, and so this calculation was not performed and instead a T = 1175°C was assumed, based on the T implied from MgO in the central plateaus of other melt inclusions from the same locality.
h - Cooling rate determined by MgO diffusion speedometry in the melt inclusions (Saper & Stolper 2020).
i – H2O determined by FTIR (Saper & Stolper 2020), otherwise it was determined by SIMS.
Table 2. Major and minor element compositions of the centers of experimental and natural olivine-hosted melt inclusions.
Name ΔFMQa Visible
Sulfides SiO2
(wt%) TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Totals S
(ppm) XFo Olivineb H3XC1 +4.7 N 52.66 1.58 10.76 13.97 0.15 10.06 8.58 1.80 0.24 0.15 99.93 1025 89.2 HTOX2 +2.8 N 52.36 1.65 12.86 10.92 0.16 9.64 10.10 1.90 0.26 0.38 100.23 851 87.1 SF11-5 +0.35 Y 52.56 2.01 14.17 7.78 0.13 8.93 10.90 2.24 0.33 0.25 99.30 800 88.6 SF1-2 +0.35 Y 51.71 1.94 13.66 8.36 0.13 9.10 10.81 2.16 0.30 0.16 98.35 823 88.4 SF13-1 +0.35 Y 51.55 1.88 13.29 9.33 0.15 8.94 10.48 2.09 0.31 0.27 98.27 86.5 SF8-3 +0.35 Y 50.71 2.11 14.15 7.87 0.13 8.81 11.51 2.02 0.33 0.28 97.91 87.9
SF3-2 +0.35 N 51.57 2.06 13.25 9.73 8.72 10.83 2.11 0.29 0.21 98.94 88.5
SF4-3 +0.35 Y 52.31 2.12 13.41 0.10 9.25 0.13 7.62 11.70 2.09 0.32 0.19 99.23 1041 88.4 H11X1B -1 Y 52.72 2.08 13.26 9.21 0.15 9.32 10.26 2.24 0.43 0.22 99.89 842 87.4
H7XC1 -1 Y 53.23 2.15 14.05 7.66 0.13 9.01 10.53 2.42 0.33 0.28 99.79 788
H3XR2e +4.7 & -1 Y 783
H11X1B_2 -1 Y 51.47 2.31 13.15 9.13 0.14 9.40 10.50 2.30 0.40 0.24 99.04 908 87.3 H3XC4 4.7 N 52.72 1.60 10.83 13.05 0.13 10.15 8.61 1.68 0.26 0.18 99.22 1121 90 H9X2_1 -1 Y 53.72 1.94 13.68 0.13 7.36 0.10 8.75 10.82 2.08 0.38 0.22 99.19 776 86.2 H9X2_2 -1 N 53.72 1.87 13.80 0.13 7.37 0.08 8.68 10.69 2.08 0.38 0.21 99.01 590 86.2 H2XC3 -1 N 52.59 2.02 13.59 0.13 7.45 0.08 9.07 10.59 2.36 0.43 0.22 98.54 488 88.2 H3XC3_1 4.7 N 51.34 1.53 10.49 13.60 0.15 9.80 8.36 1.76 0.23 0.21 97.48 1029 87.2 H2XC2 -1 N 53.40 2.10 14.19 6.94 0.12 9.02 10.77 2.24 0.30 0.26 99.34 602 87.2 H10XC1 -1 N 50.69 1.82 13.01 0.18 8.27 0.11 10.62 10.12 2.28 0.40 0.24 97.75 518 88.3 H10XC3 -1 N 51.97 1.83 13.00 0.19 7.29 0.12 10.78 9.97 2.35 0.44 0.24 98.17 148 89.7 H8XC2 -1 Y 52.94 2.04 13.87 0.09 7.77 0.08 8.88 10.37 2.44 0.39 0.22 99.10 664 88.3 H5XC2_1 -1 Y 53.57 2.07 13.61 8.05 0.14 8.72 10.45 2.27 0.35 0.21 99.44 777 87.5 H5XC1 -1 N 53.81 2.17 13.79 7.72 0.13 8.46 10.17 2.58 0.31 0.34 99.48 440 87.8 SF11_2 0.35 Y 52.36 1.86 13.56 0.09 7.92 0.10 8.93 10.61 2.34 0.41 0.26 98.44 1079 87.9 HTOX1 2.8 N 51.81 1.75 11.95 11.41 0.15 9.42 9.42 2.13 0.27 0.18 98.49 1353 89.8 SF1-1 0.35 Y 52.33 1.69 12.16 0.12 10.18 0.15 8.84 11.25 2.10 0.42 0.24 99.48 1125 85.6 H3XC3 4.7 N 51.36 1.54 10.49 13.61 0.15 9.80 8.37 1.76 0.23 0.21 97.52 1027 87.3
Experiments (con’t)
Name ΔFMQa Visible Sulfides SiO2 TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O K2O P2O5 Totals S
(ppm) XFo Olivineb SF7-2 0.35 Y 52.37 1.98 13.23 0.11 8.61 0.11 8.59 11.31 2.20 0.34 0.19 99.03 1065 88.4 SF8-1 0.35 Y 52.02 2.13 13.25 0.09 8.39 0.11 9.08 10.72 2.31 0.39 0.23 98.72 930 87.6 H2XC2_1 -1 Y 52.77 2.04 13.76 0.13 6.95 0.10 9.10 10.92 2.41 0.36 0.19 98.73 803 89.2 H2XC2_2 -1 N 52.70 2.08 13.96 0.13 6.86 0.08 9.22 10.73 2.25 0.36 0.30 98.65 633 89.2 SF11-3_1 0.35 Y 53.21 2.15 13.61 0.08 7.92 0.11 9.11 10.74 2.30 0.43 0.24 99.91 754 88 SF11-3_5 0.35 Y 53.29 2.11 13.66 0.08 7.95 0.11 9.12 10.84 2.30 0.43 0.25 100.14 775 88 SF11-3_6 0.35 Y 53.04 2.18 13.75 0.08 7.90 0.12 9.18 10.80 2.27 0.42 0.31 100.05 710 88 SF11-3_15 0.35 Y 53.66 2.09 13.72 0.09 7.88 0.12 9.12 10.71 2.28 0.43 0.24 100.33 718 88 SF11-3_16 0.35 N 53.23 2.09 13.71 0.10 7.71 0.11 9.15 10.61 2.31 0.47 0.35 99.84 254 88 SF11-3_2 0.35 N 53.29 2.16 13.72 0.10 7.54 0.12 9.06 10.65 2.30 0.46 0.31 99.71 126 88 SF11-3_3 0.35 N 53.40 2.12 13.71 0.09 7.71 0.12 9.09 10.65 2.37 0.42 0.29 99.96 514 88 SF11-3_4 0.35 N 54.00 2.04 13.87 0.08 7.56 0.11 9.05 10.60 2.29 0.47 0.31 100.38 53 88 SF11-3_7 0.35 N 53.61 2.11 13.88 0.08 7.60 0.11 9.14 10.70 2.27 0.47 0.39 100.35 181 88 SF11-3_8 0.35 N 53.52 2.12 13.90 0.09 7.74 0.12 9.10 10.67 2.29 0.50 0.34 100.40 196 88 SF11-3_9 0.35 N 53.58 2.14 13.82 0.07 7.70 0.11 9.10 10.58 2.31 0.48 0.32 100.20 120 88 SF11-3_10 0.35 N 53.57 1.97 13.94 0.10 7.70 0.12 9.20 10.50 2.25 0.39 0.45 100.19 124 88 SF11-3_11 0.35 N 53.36 2.16 13.71 0.08 7.96 0.12 9.15 10.67 2.31 0.48 0.34 100.34 521 88 SF11-3_12 0.35 N 53.69 2.08 13.72 0.08 7.67 0.11 9.09 10.65 2.27 0.46 0.32 100.15 46 88 SF11-3_13 0.35 N 53.83 2.05 13.65 0.07 7.72 0.10 9.17 10.69 2.27 0.46 0.35 100.37 142 88 SF11-3_14 0.35 N 53.50 2.08 13.63 0.08 7.78 0.10 9.02 10.61 2.31 0.46 0.35 99.92 73 88
Unheated melt inclusions from Papakōlea, Mauna Loa, Hawai‘i
HIGS11 N 53.73 2.17 13.63 8.96 0.14 7.28 11.05 2.11 0.33 0.05 99.46 88.8 HIGS14 N 55.55 2.41 15.17 6.80 0.12 4.04 12.69 2.18 0.34 0.05 99.34 154 88.5 HIGS20 Y 54.34 2.17 14.48 8.14 0.13 5.95 11.44 2.21 0.35 0.17 99.38 1035
HIGS21 N 53.73 2.17 13.63 8.96 0.14 7.28 11.05 2.11 0.33 0.05 99.46 129 88.6
4.2 Chemical Compositions of Experimental Melt Inclusions