Do Pumice and obsidian pyroclasts from a single explosion have the same chemical composition?
For same vent, same eruption. Obsidian is a glass and pumice is a glass too. They must have the same chemical composition. Have they?
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Same remark that your other question: are you sure this is an explosive event? I've never heard of obsidian pyroclasts. Generally obsidian is found in rhyolite lava flows (i.e., non-explosive volcanism), although it is true that the effusion of silicic magma is often preceded by an explosion of a first batch of the same magma, non-degassed, while the rest of the degassed magma flows gently over it. But this basal, explosive layer is made of tephra, not obsidian. You can still have obsidian clasts, for example at the bottom of the lava flow unit, where it has broken into a basal breccia. – Jean-Marie Prival Feb 22 '24 at 13:35
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Another possibility is an explosive event through a pre-existing obsidian layer. In such case, the obsidian clasts would be considered as lithics (in opposition to juvenile magma) and could well have a different composition, being from another, older eruption. – Jean-Marie Prival Feb 22 '24 at 13:38
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Why do you think obsidian pyroclasts can not occur from the juvenile material ? If Juvenile material cooling slower, lithic pyroclasts will be generated, If this material cools rather faster it will be an obsidian pyroclast. I myself once collected obsidian pyroclasts for chemical analysis from an ignimbrite. It can be seen as ash or lapilli or block size. Pumice can be regarded as a vesicular kind of obsidian. – Muharrem Yavuz Feb 28 '24 at 13:07