A CTIVITY 5.7 Thin Section Analysis and Bowen\'s Reaction Series Course/Section:
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A CTIVITY 5.7 Thin Section Analysis and Bowen's Reaction Series Course/Section: Date: Name: A thin section of a rock can be made by grinding one side of it flat, gluing the flat side to a glass slide, and then grinding the rock so thin that light passes through it. The thin section is then viewed with a polarizing microscope. The view in plane polarized light is the same as looking at the thin section through a pair of sunglasses. If you place a second pair of sunglasses behind the thin section and hold it perpendicular to the first pair, then you are viewing the thin section through cross polarized light. These images of thin sections were made by geologist LecAnn Srogi. West Chester University of Pennsylvania. plag Ca-Na edge (gray) Ca-rich center (white) plag pX ol px Thin section 2: This thin section is shown in cross polarized light. There green color) and plagioclase (white to "zoned." It has a calcium rich (sodium splag pl Thin section 1: This thin section has a crystal of olivine (ol) that is medium gray ray color). The large plag had a rectangular outline, but is now surrounded and partly replaced by the "Zon Pyroxene is pale brown in plane polarized light, but yellow. to orange-brown in also have progressively more and more Cross polarized light are crystals of amphibole (brown to Plane polarized in plane polarized light and black in cross polarized light. The olivine crystal oncecrystal in the center of the image is mineral ovroxene (ox), which has a different composition and crystal structure. poor cended y znes cross polarized light. The other white to light gray crystals in this rock are plagioclase (plag). sodium. The zone at its outer edge is equally rich in both calcium and sodium. 1 mm A. Based on Bowen's Reaction Series (FIGURE S.3), explain as exactly as you can what may have caused the relationship between olivine and pyroxene observed in thin section 1. B. Based on Bowens Reaction Series (FIGURE 3.3), explain as exactly as you can what may have caused the large plagioclase crystal in thin section 2 to be zoned as it is.Explanation / Answer
A) Bowen's reaction series idealized the general model for the evolution of magmas during the cooling process. Bowen determined that specific minerals form at specific temperatures during cooling of magma. Higher temperature associated magma is rich in mafic minerals and lower temperature magma at the lower part of Bowen's reaction series is enriched in felsic minerals. Bowen's reaction series can be separated into two branches. The continuous branch describes the evolution of the plagioclase feldspars from calcium-rich to more sodium-rich and the discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. At a certain temperature during cooling a magma might produce olivine in the discontinuous branch of Bowen's reaction series. If that same magma was further allowed to cool, then the olivine would react with the residual magma, and change to the next mineral on the series. Such as olivine reacts with the residual magma and changes to pyroxene. Continue cooling and the pyroxene would convert to amphibole, and then to biotite.
In the picture Olivine is formed first which reacts with residual magma to form pyroxene. The olivine core indicates that the surrounding part of olivine reacts with residual magma and changes to pyroxene. The surrounding pyroxene grain acts as shield which prevents further reaction with residual magma and olivine grains. So, olivine is present as core, surrounded by pyroxene. In the picture Olivine is formed first which reacts with residual magma to form pyroxene. The earlier formed crystals will not be in equilibrium with this melt and will be dissolved again to form new minerals. The olivine core indicates that the surrounding part of olivine reacts with residual magma and changes to pyroxene. The surrounding pyroxene grain acts as shield which prevents further reaction with residual magma and olivine grains. So, olivine is present as core, surrounded by pyroxene.
B. The thin section showing normal zoning where Ca-plagioclase is surrounded by Na-plagioclase. Continuous series starts with the highest temperature mineral, which is calcium-rich plagioclase. The calcium is replaced with sodium with cooling down magma temperture. So, Plagioclase in the middle of the Bowen's series could be thought to have about 50% calcium and 50% sodium. At the bottom of the series, sodium-rich plagioclase is formed. So, the inner part of feldsper is Ca-rich whereas the outer part is Na-rich. It is zoned feldsper. Accroding to Bowen's reaction rule, it is formed by reaction with residual magma and outer rim changing into Na-plagioclase. The zoning of feldsper is formed due to intracrystalline failure of reaction. If Ca-plagioclase is surrounded by Na-plagioclase then it is called normal zoning. This is the effect of fractional crystallization. Water pressure may also facilitate the zoning.
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