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Preservation of Mantle-Derived Recharge Signatures in Olivine during Protracted Magma Storage
Mourey A, Shea T, Costa F, Shiro B, Oalmann J, Lee L & Gansecki C
Mourey A, Shea T, Costa F, Shiro B, Oalmann J, Lee L & Gansecki C (2020) Goldschmidt Abstracts, 2020 1858
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05c: Plenary Hall, View in program
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Hi Adrian, nice presentation. I'm wondering if you have any sense of where the resorption occurred (your step 2, original olivine + recharge = resorption and homogenization to Fo89)? Did this happen in the SC reservoir? Or deeper...? I note you assume in Step 1 an initial Fo content also of 89 - which suggests the crystal grew quite deep, and then was intruded by a deeper more magic melt, all before traveling down the ERZ? Thanks!
Hi Kendra, thanks for your question. Dissolving a Fo89.3 olivine is probably not an easy thing to do, may require something pretty darn hot (!). [i.e., whatever Fo89 olivine grew/was floating in prior to eruption, it may have encountered something even more primitive]. In this work, we are contemplating alternatives to heating and suggest that the resorption occurred during magma storage in the SC reservoir (the crystallization of the Fo89 olivine could occur as shallow as 3km deep during magma ascent. The question was whether it's possible that dissolution occurs instead of reversely-zoned growth, i.e., dissolution-reprecipitation rather than burial-by-normal-zoning. If the chemical potential of Fo was reduced in the liquid because it's Mg# went down suddenly, for example, it should destabilize high-Mg olivine relative to lower-Mg olivine. An analogy with feldspar (see Tsuchiyama et al 1983) showed that dissolution of the feldspar occurs at the interface for all experiments above the solidus (not just ones above the liquids). This result (besides obvious rounding when the crystal is less refractory than the equilibrium solid) is that in all cases, the liquid near the crystal migrates toward the composition of the plagioclase. If olivine is like plagioclase in this regard, then perhaps magma mixing in the SC reservoir could erode the P-rich apices. However, the fact that this dissolution front occurred in the high Fo part suggest that at least a second intrusion occurred and equilibrated the olivine to a high Fo (before the eruption). Then, the final mixing in the ERZ created the final rim with lower Fo.
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