High-pressure homogenization of olivine-hosted CO<sub>2</sub>-rich melt inclusions in a piston cylinder: insight into the volatile content of primary mantle melts
<p>Experimental homogenization of olivine-hosted melt inclusions representative of near-primary basic and ultrabasic magmas is a powerful approach to investigate the nature of their source regions and the melting conditions in Earth's mantle. There is growing evidence that the total CO<...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2022-05-01
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| Series: | European Journal of Mineralogy |
| Online Access: | https://ejm.copernicus.org/articles/34/325/2022/ejm-34-325-2022.pdf |
| Summary: | <p>Experimental homogenization of olivine-hosted melt inclusions
representative of near-primary basic and ultrabasic magmas is a powerful
approach to investigate the nature of their source regions and the melting
conditions in Earth's mantle. There is growing evidence that the total
CO<span class="inline-formula"><sub>2</sub></span> contents of olivine-hosted melt inclusions may reach values of the
order of a single to several weight percent, especially in intraplate continental
basalts. To be able to homogenize melt inclusions with such high CO<span class="inline-formula"><sub>2</sub></span>
contents, we developed a technique allowing for heat treating of the melt
inclusions under hydrostatic pressures up to 3–4 GPa in a piston cylinder,
using thick-walled Au<span class="inline-formula"><sub>80</sub></span>–Pd<span class="inline-formula"><sub>20</sub></span> containers and molten NaCl as
the surrounding medium for the inclusion-bearing olivines. We applied this
technique to olivine phenocrysts from Thueyts basanite, Bas-Vivarais
volcanic province, French Massif Central. Thueyts melt inclusions were
chosen because of their high CO<span class="inline-formula"><sub>2</sub></span> contents, as indicated by up to
1.19 wt % dissolved CO<span class="inline-formula"><sub>2</sub></span> in the glasses and by the presence of
shrinkage bubbles containing abundant carbonate microcrystals in addition to
a CO<span class="inline-formula"><sub>2</sub></span> fluid phase. The homogenization experiments were conducted at
pressures of 1.5 to 2.5 GPa, temperatures of 1275 and 1300 <span class="inline-formula"><sup>∘</sup></span>C,
and run durations of 30 min.</p>
<p>In all the melt inclusions treated at 2.5 GPa–1300 <span class="inline-formula"><sup>∘</sup></span>C and half of
those treated at 2 GPa–1300 <span class="inline-formula"><sup>∘</sup></span>C, we were able to completely
homogenize the inclusions, as indicated by the disappearance of the starting
bubbles, and we obtained total CO<span class="inline-formula"><sub>2</sub></span> contents ranging from 3.2 wt % to
4.3 wt % (3.7 wt % on average). In all the other melt inclusions
(equilibrated at 1.5 or 2 GPa and 1300 <span class="inline-formula"><sup>∘</sup></span>C or at
2.5 GPa–1275 <span class="inline-formula"><sup>∘</sup></span>C), we obtained lower and more variable total
CO<span class="inline-formula"><sub>2</sub></span> contents (1.4 wt % to 2.9 wt %). In the inclusions with the highest
total CO<span class="inline-formula"><sub>2</sub></span> contents, the size of the shrinkage bubble was in most cases
small (<span class="inline-formula"><5</span> vol %) to medium (<span class="inline-formula"><10</span> vol %): this is a
strong argument in favor of an origin of these melt inclusions by
homogeneous entrapment of very CO<span class="inline-formula"><sub>2</sub></span>-rich basanitic liquids
(<span class="inline-formula">∼</span> 4 wt %) at pressures of 2 to 2.5 GPa. The lower total
CO<span class="inline-formula"><sub>2</sub></span> contents measured in some inclusions could reflect a natural
variability in the initial CO<span class="inline-formula"><sub>2</sub></span> contents, due for instance to melt
entrapment at different pressures, or CO<span class="inline-formula"><sub>2</sub></span> loss by decrepitation. An
alternative scenario is heterogeneous entrapment of basanitic liquid plus
dense CO<span class="inline-formula"><sub>2</sub></span> fluid at lower pressures but still at least on the order of
1 GPa as indicated by dissolved CO<span class="inline-formula"><sub>2</sub></span> contents up to 1.19 wt % in the
glasses of unheated melt inclusions. Whatever the scenario, the basanites
from the Bas-Vivarais volcanic province were generated in a mantle
environment extremely rich in carbon dioxide.</p> |
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| ISSN: | 0935-1221 1617-4011 |