4. Results
4.2 Chemical Compositions of Experimental Melt Inclusions .1 Major and Minor Element Compositions of Melt Inclusions .1 Major and Minor Element Compositions of Melt Inclusions
4.2.2 Compositional Zoning in Cooled Melt Inclusions and Host Olivines After the 24 hr homogenization step, three cooling regimes were employed: (1)
quenching directly from the hotspot at the T of homogenization, (2) displacement method cooling, and (3) controlled cooling via the control thermocouple and diffusion of heat out of the furnace hotspot. As shown in Chapter 1 and in Saper & Stolper (2020), crystallization of olivine on the melt inclusion walls during cooling induces chemical gradients which relax by diffusion. Given sufficiently low cooling rates or high temperatures, these gradients can modify the centers of melt inclusions – the degree to which they are modified depends on the relative diffusivities of the melt components with MgO being one of the most rapid diffusers (Guo & Zhang, 2018 note: Na2O has a
very high self-diffusivity but is a slave to SiO2 in basaltic melts, e.g., Watson 1982). To mitigate compositional variability beyond that which is present in the initially trapped melts in the starting materials, the experiments were designed to leave inclusion centers compositionally unchanged during cooling by picking large inclusions and employing high cooling rates. Experiments using cooling regime (1) have very narrow chemical boundary layers adjacent to the inclusion walls which were formed during quenching (Figure 7AB); for example, in inclusion SF1-1, MgO is 8.84±0.07 (1σ of distribution of 15 points) across the central compositional plateau, which extends from the inclusion center up to ~4 µm from the inclusion wall, where the MgO contents are slightly lower at ~8.65 wt%; CaO contents are high in this narrow boundary layer (11.64 wt% at wall) compared to the inclusion center (11.25 wt%), which is indicative of rapidly cooled melt inclusions (Saper & Stolper 2020). For regime (2) displacement method experiments, the boundary layer is wider than in regime (1): for example, in experiment SF7-2 (cooled for 46 seconds from 1225°C to 998 °C), the MgO boundary layer is ~20 µm wide (Figure 7C) and in experiment SF8-1 (cooled for 77 seconds from 1225 °C to 1000 °C) the boundary layer is ~35 µm wide (Figure 7D). Because these inclusions are well over 100 µm across each retains a wide compositional plateau that has not been modified by diffusion. Some of the largest melt inclusions found in the starting materials were reserved for the controlled cooling rate experiments, SF3 and SF4, which based on forward modeling were the most susceptible to having their central compositions modified by diffusion. SF3-2 (cooled at 2900 °C/hr from 1225°C to 1150°C has a broad
~100 µm wide central plateau (Figure 7E); SF4-3 (cooled at ~1900°C/hr from 1225°C to 1000 °C has central MgO contents that have been lowered relative to their initial value of ~9 wt% based on the central plateaus of regime (1) experiments conducted at the same T and ƒO2 and with similar host olivine
Figure 7. Chemical zoning across experimental olivine-hosted melt inclusions homogenized for 24 hr at 1225°C and FMQ+0.35 and subjected to different cooling rates. Vertical gray line indicates the olivine-glass boundary (inclusion wall). Al2O3 – red circles; CaO – blue triangles; FeO* – green diamonds; MgO – dark gray squares.
TQ indicates the T of the sample at the time of quenching. (A) SF1_1, homogenized and drop quenched from TQ = 1225°C, (B) SF11_5, homogenized and drop quenched from TQ = 1225°C, (C) SF7_2, displacement method cooling for 46s, TQ = 1000°C, (D) SF8_1, displacement method cooling for 77s, TQ = 1000°C, (E) SF3_2, controlled cooling for 100s, TQ = 1050°C, (F) SF4_3, controlled cooling for 420s, TQ = 1000°C.
A B
C D
E F
SF1_2 Homogenization TQ = 1225 °C
SF11_5 Homogenization TQ = 1225 °C
SF8_1 Displacement, 77s TQ = 1000 °C SF7_2
Displacement, 46s TQ = 1000 °C
SF4_3 Controlled, 420s TQ = 1000 °C SF3_2
Controlled, 100s TQ = 1050 °C
compositions (Figure 7F). This experiment can be clearly seen at low MgO for a given FeO* in Figure 6A, however the boundary layers of the other more slowly diffusing oxides (e.g., Al2O3) had not progressed into the inclusion interior by the T of quenching and maintain broad plateaus. Note that in this experiment the CaO profile has a local maximum in concentration at x = 13.5 µm from the inclusion wall, and a diffusion profile shape that is indicative of intermediate cooling rates (Saper & Stolper 2020). The two experiments shown in Figure 7 panels C and D also have local maxima in CaO at
~6-7 µm from the inclusion walls. With the exception of MgO in SF4-3, the compositions of the centers of the experimental melt inclusions represent that of the homogenized melt, and aside from the compositional variation imparted by heterogeneity of the initially trapped melts (which are minor, as shown by limited variability in the y-direction at a given FeO* and ƒO2, Figure 6B-E), differences in the Fe3+/Fe2+ and S6+/S2- for a set of experiments at a given ƒO2 are unlikely to be due to these minor perturbations of melt composition.
As discussed in Chapter 1, the Fe/Mg zoning in olivines around natural melt inclusions is typically characterized by two to three zones: a steeply zoned narrow band of Fe-rich olivine a few micrometers wide directly adjacent to the inclusion, which is formed during syneruptive cooling; a broad zone that is sometimes present and can extend tens of micrometers from the narrow band into the olivine interior; and a zone in the ‘far-field’ that is either a compositional plateau or is continuous with large-scale growth zoning in the host olivine. Experimental homogenization of melt inclusions at any T and ƒO2 condition that differs from that in which the natural melt inclusion was trapped and equilibrated will lead to disruption of the narrow zone, by dissolution and/or precipitation of new olivine on the inclusion walls and/or by diffusive exchange with the far-field olivine because of the different Fe/Mg imposed by local equilibrium at the
olivine-liquid interface (Gaetani 2000; Gaetani 2002; Danyushevsky 2002; Saper &
Stolper 2020). Figure 8 shows FeO* wt% in four experimental melt inclusions and surrounding olivines that were homogenized at 1225 °C for 24 hr, but at different ƒO2. Despite some uncertainty as to how much of the Fe/Mg zoning in the surrounding olivines was inherited from the natural samples, the trends observed over >5 orders of magnitude range of experimental ƒO2 are strong enough to warrant comment.
The average FeO* of the inclusion glasses increases with increasing ƒO2 (Figure 6A). Between FMQ+2.8 and FMQ+4.7 the FeO* contents of the liquids become greater than that in the coexisting olivine; the crossover is bracketed to be close to the upper ƒO2 stability limit of Fo88 olivine at 1225 °C, which is approximately ~FMQ+4.2 (Nitsan 1974). Recall that the initial FeO contents of the olivine starting materials were likely somewhere between 11-12 wt%, or Fo87.5 to Fo88.5, although in rare cases a more Fe- poor host olivine phenocryst was used, e.g., in experiment HTOX4 shown in Fig. 8C (~10 wt% FeO). The low FeO contents of the olivines in contact with, and presumably in local equilibrium with, the melt inclusions in the more oxidizing experiments (Fo89.6
at FMQ+2.8 and Fo90.1 at FMQ+4.7) reflect the lower solubility of fayalite in olivine at these ƒO2, and the corresponding high FeO* contents of the coexisting liquids are a consequence of dumping Fe from the olivine into the melt inclusion. In this case, Fe is transported to the melt inclusions by diffusion through the olivine volume (e.g. Wu &
Kohlstedt 1988). Note that in the most oxidizing experiments magnetite-pyroxene symplectites formed within the olivine (Figure 5), via the reaction: 6Fe1/2SiO2 + ½ O2
⇌ Fe3O4 + 3FeSiO3, which leads to locally magnesian olivines in the FMQ+4.7 experiments. It cannot be ruled out whether the symplectite-forming reaction occurred first, and that the Fe that was added to the melt inclusions was scavenged from the products of this reaction.
Figure 8. FeO* wt% traverses across experimental melt inclusions (red squares) and the surrounding host olivine (green squares). The vertical line corresponds to the interface between the glass and olivine. Olivine and inclusions from experiments conducted at (A) FMQ-1, (B) FMQ+0.35, (C), FMQ+2.8, (D) FMQ+4.8. Listed in each panel is the Fo contents of the olivines adjacent to the inclusions and in the far- field 75 µm from the glass-olivine interface. All of the experiments shown were run for 24 hr at 1225°C and then drop-quenched directly from the hotspot.
slight normal growth zoning
It is worth pointing out that the length-scale of the Fe depleted zone of olivine adjacent to the melt inclusion in the FMQ+4.7 experiment is ~40 µm which is about 2x longer than that the diffusion distance (i.e., x = (D∙t)1/2) estimated for Fe-Mg interdiffusion at the experimental condition (x ≈ 16 µm, extrapolated to high ƒO2 from Dohmen & Chakraborty 2007). It is possible that an Fe3+-defect-rich olivine increases bulk diffusivities, especially of charged defect species, making it easier to transmit ƒO2
information across the crystal volume at high ƒO2; the fact that dislocations in olivine are rapidly decorated at oxidizing conditions (e.g., Wu and Kohlstedt 1988), and symplectites are found to follow traces of cracks in the experimental olivines (Figure 5A), indicates that fast paths for diffusion of oxygen or oxidant-bearing defects were likely present (e.g., Spandler and O’Neill 2010, Burgess and Cooper 2013).