Indeed, Jesse!! Kudos! Woot! Woot!
Lin
On Sat, May 26, 2012 at 12:56 AM, Kim Noyes <kimnoyes@gmail.com> wrote:
That was great, Jesse... Thanks!
--On Fri, May 25, 2012 at 8:12 PM, Jesse person <geojesse@hotmail.com> wrote:
Hi all,
These articles are quite misleading by saying that these techniques are "new". Certainly the study has been refined greatly since 1980, but isn't something that just popped up over the past few years. I actually got to look at some thin sections recently from Mt. Saint Helens, and you could see the reaction rims around the orthopryoxene grains showing that the mineral was in disequilibrium right before eruption. Other signs such as Ca-Na zoning in plagioclase show different phases in the magma chamber and indicate magmatic recharge events which can be a precursor to an eruption. For example in some recent studies Dr. Adam Kent from Oregon State University found that a recharge event at Mt. Hood has historically resulted in an eruption within days. The magma that recharges the volcano is typically hot and has a basaltic composition, the magma chamber however might have a more silicic (in the case of the Cascade volcanoes) composition such as dacite or rhyolite. These more silicic magmas are much cooler, and usually have a higher concentration of crystals in them compared to the recharging melt (which may have some olivine or pyroxene). The recharge magma suddenly quenches against the cooler silicic magma and exsolves all its volatiles, these are less dense and rise up through the overlying dacitic or rhyolitic magma as a "mafic foam". This can cause an immediate eruption or if the chamber isn't breached can facilitate mixing, usually a bit of both happens. The mixing of the two end member types of magma is what causes the intermediate andesitic magma. Orthopyroxene and Olivine crystals in the mafic magma are out of their stability field when they come into contact with the dacitic or rhyolitic melt, so they develop reaction textures that can be visible under thin section. Plagioclase on the other hand is stable in both a highly silica rich magma and in a mafic magma, but the cations that bond to it's structure change. Calcium forms stronger/shorter bonds and requires higher temperatures of formation, and the plagioclase will begin to be more sodium rich gong down temperature. This can be used to tract the timing of recharge events and where the crystals originally began to cool. What is more recent is the methods used to quantify using geochemical data provided by an electron microprobe (which shoots an electron beam at the crystal, and spits out information on it's composition at a specific point). This way you can actually tract the exact composition of minerals over time. This information can be put into a program such as MELTS which can model relationships between the geochemical data. Computer modeling keeps getting better and better, as well as the analytical techniques. That combined can provide us with a better understanding of how magma chamber processes can be recorded in the rock record. But it is by no means a recent break through discovery. It's just taken the public a long time to become interested/convinced that the technique is useful.
Sorry if that was too much info, I just wanted to provide you guys with a bit more in depth explanation.
-Jesse Walters
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