I previously believed that continental crust owes its lower density to the partial melting of oceanic crust; the mantle would partially melt at mid-ocean ridges to produce basaltic crust, and when this oceanic crust eventually subducted, the basalt would then partially melt to produce an even lig...
@Fred I think the poster wanted to include the oceanic-crust tag and used continental-crust instead, but the tag is not created. I cannot create it and neither him as it needs 150 points, but the question is about oceanic-crust, not continental-crust. I would remove here crust and continental-crust tags and I would create island-arc and oceanic-crust here. This migth be also a question for petrology tag, but only five can be applied.
@richard I think your question title is ambiguous as by subduction zones one think in volcanic arcs as Andes, where the melt crosses continental crust, and not island arcs. I think the question would be more clear if you change the title for "How is felsic magma produced at island arcs?"
@Jean-Marie Prival Isn't this also a volcanology question? The poster wants to know about island arcs and stratovolcanoes, so I ping you as our volcanologist :) I hoppe Gimelist is simply bussy with his home and we haven't loose another expert :(
@Universal_learner It's a question about processes at depth, while I focus more on surface processes (when magma becomes lava and/or tephra). I have some igneous petrology notions of course, but I'd rather let a true specialist answer. I hope he's still around, but I have since learned that pings doesn't work if the user has not been active on the question...
@Jean-Marie Prival Have you taken a look to the site graphs The questions behaviour is normal, slowly, but increasing because of the site page rank. But the loose in votes means loose in experts, the site is loosing prestige. We need to adapt. You cannot expect David Hammen has time to answer all the geology questions (he is a rocket-scientist, neither a geology expert). And Fred is also in an overbooking.
@Universal_learner Your source seems wrong to me. As far as I know, like OP said, it is the mantle wedge above the subducted plate that partially melts. The plate and sediments just get dehydrated, and it is this release of fluids that lower the melting point of the mantle. Oceanic crust does melt in some rare cases making rocks called adakites, not andesite.
@Universal_learner I saw your meta posts but didn't have time to answer them. Answering questions on the main site is already quite time consuming, and although I enjoy it a lot I can't really commit more time on the site...
@richard Anyhow the slab alters the proccess. Maybe it does not add Ca, Si, or Al to the magma bulk, but water affects the magma diferenciation proccess and that is the difference with MORB basalts on hotspots and mid-oceanic ridges.
@richard An isotopic analysis of andesites Si ,Al and Ca on volcanic arcs would estimate if the source comes partially from organism shells or just the water differenciates the mantle magma, enriching it in alkali. I have recently adquired a ScienceDirect license but I don't find any article about isotopes of andesites in volcanic arcs. I will delete my answer if it is clarified I am wrong.
@Jean-Marie Prival I have learned rarelly the slab melts to form adakites or melts only very partially, thanks. But do you think is possible that the sediments stack melts in higher proportions? Maybe CaCo3 melting point is 825 °C while SiO2 melting point is higher, 1.710 °C. Maybe the origin of Ca in calc-alkaline series is foraminifera shells but the origin of Si enrichment is not radiolaria shells?
@Universal_learner I guess, I've seen some papers mentioning a sediment signature in the trace elements composition of magmas, but that's geochemistry, not volcanology... Things are really complicated in the subduction zone, to be honest I don't fully grasp what is happening down there, and that's why I didn't want to answer this question in the first place! But kudos to you for trying, you got my +1 for your effort! :)
@Jean-Marie Prival thanks. It doesn't wonder to me a lot points, but I am gonna use my profile to acreditate my geology in my apps. I am stupid I closed my account with 2k and some upvoted questions. Handicaps of having a menthal illness.
Discussion on question by richard: Ho…
Discussion on question by richard: Ho…