As the volatile fluids moved through the passageways in the rhyolite, could they chemically evolve and pick up a wider variety of elements? Since the coated garnets (Pop. 2) occupy the upper margins of the outcrop, it leads me to speculate that the initial volatile fluid pulse was simpler, depositing the Pop. 1 garnets. As the system expanded outward and upward, the fluid chemistry changed through prolonged interaction with the host rock.
Furthermore, the connection of Pop. 2 to a more mafic signature suggests that the initial phase of the eruption was purely felsic. As the eruption sequence progressed, it may have tapped deeper, more mafic layers of a zoned magma chamber (or experienced a late-stage mafic injection). Highly mobile fluids moving through this later cycle would then be naturally enriched in Scandium and Chromium, capturing that distinct signature in the Pop. 2 garnets.
You're not wrong, and the two-pulse model is exactly where the geochemistry points โ I just didn't want to stake a claim about deposition mechanisms for a field area I've only seen from the talus pile.
The cliff face would tell us whether the spatial distribution you're describing is real, and it would also require a rope and more rappelling skills than I currently have. So until someone more athletic than me wants to rappel it with a rock hammer and a sample bag, 'upper margins' remains a hypothesis rather than a dataset.
That said โ if your memory of the spatial distribution holds up, it's actually the cleanest possible field test of the two-pulse model. A vertical transect would either confirm it or kill it in an afternoon. All we need is cliff access :)
Correction, May 31, 2026: This post and Part 1 refer to these garnets as andradite throughout. That identification was based on field observation โ black, dodecahedral, volcanic context โ not chemistry. The XRF data in this post, and Mindat's listing for this locality, are both consistent with almandine rather than andradite. The garnet identity question is now part of the open investigation. Relevant passages have been updated.
As the volatile fluids moved through the passageways in the rhyolite, could they chemically evolve and pick up a wider variety of elements? Since the coated garnets (Pop. 2) occupy the upper margins of the outcrop, it leads me to speculate that the initial volatile fluid pulse was simpler, depositing the Pop. 1 garnets. As the system expanded outward and upward, the fluid chemistry changed through prolonged interaction with the host rock.
Furthermore, the connection of Pop. 2 to a more mafic signature suggests that the initial phase of the eruption was purely felsic. As the eruption sequence progressed, it may have tapped deeper, more mafic layers of a zoned magma chamber (or experienced a late-stage mafic injection). Highly mobile fluids moving through this later cycle would then be naturally enriched in Scandium and Chromium, capturing that distinct signature in the Pop. 2 garnets.
You're not wrong, and the two-pulse model is exactly where the geochemistry points โ I just didn't want to stake a claim about deposition mechanisms for a field area I've only seen from the talus pile.
The cliff face would tell us whether the spatial distribution you're describing is real, and it would also require a rope and more rappelling skills than I currently have. So until someone more athletic than me wants to rappel it with a rock hammer and a sample bag, 'upper margins' remains a hypothesis rather than a dataset.
That said โ if your memory of the spatial distribution holds up, it's actually the cleanest possible field test of the two-pulse model. A vertical transect would either confirm it or kill it in an afternoon. All we need is cliff access :)
Correction, May 31, 2026: This post and Part 1 refer to these garnets as andradite throughout. That identification was based on field observation โ black, dodecahedral, volcanic context โ not chemistry. The XRF data in this post, and Mindat's listing for this locality, are both consistent with almandine rather than andradite. The garnet identity question is now part of the open investigation. Relevant passages have been updated.