(2020) A Smoking gun for Copper Enrichment in Arc Volcanoes?
Ubide T, Costa F, Kendrick MA & Berry AJ
05i: Room 2, Friday 26th June 00:48 - 00:51
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Thank you for an excellent presentation. Beautiful data! I am curious about the Cl 'hotspots' you discovered in the melt inclusions that also contain the Cu-S phase and I have several questions. 1. Are the Cl concentrations spatially correlated with the Cu-S crystals? If not, do you have an idea how the glass could be heterogeneous with respect to Cl? 2. What are your thoughts about why the plagioclase of the 2009 andesite contains these Cu-concentration zones in the MI whereas the 2000 plagioclase crystals do not. Relatedly, the 2009 crystals appear to be repeatedly-zoned with respect to growth of MI-rich, Ca-rich plagioclase. Do you have an idea about what magmatic process causes these differences? 3. You suggest a link between primary magmatic Cu-rich, hydrothermal processes operating (in some, but not all, Mayon andesite lavas), culminating in possible formation of porphyry copper deposits. Do you have in mind a conceptual model or sequence of events, say, in cartoon fashion? 4. Last, your plot of the Mayon WR and reconstructed glass compositions overlayed on literature data (Cao-Al2O3-SiO2 systematics) shows coincidence and you infer cpx control for all, while noting that 'something is different' at Mayon. Without these amazing Cu maps and other trace element (volatile element) analytical data, can you be sure that all the other samples aren't similarly enriched?
Thank you very much for your feedback and questions Julia!! 1. We observe Cl enrichments in the same melt inclusions that host the Cu-S droplets, in the glass that surrounds the microcrystals. In the cases of maximum Cl, the glass is homogeneously enriched, whereas lower Cl-enrichments occur as hotspots within the glass. It seems that Cl is related to extensive fractionation (the melt within the melt inclusions is dacitic in composition), although we are also investigating the potential development of brines. 2. Texturally the plagioclases from 2000 and 2009 look identical, with rhythmic An-rich sieve-textured zones. However only 2009 plagioclases contain Cu-S droplets appreciable in entire thin section XFM elemental maps. This is very interesting as SO2 monitoring shows prolonged SO2-degassing at Mayon between 2000 and 2009. We interpret that the Cu-S precipitates could be related to repeated influxes of mafic, hot, volatile-rich and metal-rich magma recharging the shallow plagioclase mush. 3. We do not have a cartoon-style genetic model yet (these are preliminary data), but we think that hydrothermal systems may be developing in relation to recharge and flushing of shallow mushes. 4. I totally agree that other samples from the volcano and the Luzon arc could return similar results when investigated at high resolution. Literature data on metal concentrations is scarce, but the observation so far is that none of the available compositions in the region show such enrichments. Thank you very much indeed and I am looking forward to discussing further!
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