Other| June 01, 1996 Diffusion of the hydrous component in pyrope Liping Wang; Liping Wang University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States Search for other works by this author on: GSW Google Scholar Youxue Zhang; Youxue Zhang Search for other works by this author on: GSW Google Scholar Eric J. Essene Eric J. Essene Search for other works by this author on: GSW Google Scholar Author and Article Information Liping Wang University of Michigan, Department of Geological Sciences, Ann Arbor, MI, United States Youxue Zhang Eric J. Essene Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1996 by the Mineralogical Society of America American Mineralogist (1996) 81 (5-6): 706–718. https://doi.org/10.2138/am-1996-5-618 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Liping Wang, Youxue Zhang, Eric J. Essene; Diffusion of the hydrous component in pyrope. American Mineralogist 1996;; 81 (5-6): 706–718. doi: https://doi.org/10.2138/am-1996-5-618 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Dehydrogenation experiments have been performed on natural pyrope megacrysts (Py70Alm16Gr14) containing 22-112 ppm total H20 by weight (H/Si is from 0.00035 to 0.0018). The concentrations of OH in pyrope crystals were determined by Fourier-trans-form infrared spectroscopy. Two methods were used to obtain diffusivities. One method involved measurement of OH concentration profiles after a heating experiment. The dif-fusivity of the hydrous component was found to be proportional to the OH concentration along each profile, indicating that the diffusing species is not OH and is probably a minor or trace species with a concentration proportional to the square of the OH concentration (such as H2). Hence, the diffusivities are referred to as the apparent diffusivities (D*, which equals D*0C/C0, where C0 is the initial OH concentration and D*0 is the D* when C = C0). The other method involved measurement of the overall decrease of OH content across a wafer as a function of cumulative heating duration. From the overall decrease of OH content, the apparent bulk diffusion-out diffusivities (D*out) were obtained. (D*out,D*in, and D*0 are different if D* depends on concentration.) The diffusivities from the two methods are consistent for a given pyrope crystal. However, the D*out values in different pyrope crystals are roughly inversely proportional to the initial OH content in the crystal, opposite the concentration dependence of D* along a diffusion profile. This apparent paradox places strong constraints on the diffusion and incorporation mechanism of the hydrous component in pyrope and indicates a control on the concentration of the diffusing species by factors other than OH content (such as Fe3+/Fe2+). Any proposed diffusion mechanism must be able to explain this apparent paradox (both the proportionality and the inverse proportionality).The D*out values (in squared micrometers per second) in a crystal with 82–90 ppm initial total H2O can be described by In D*out = [(28.20 ± 1.30) − (30580 ± 1450)]/T (where Tis temperature in kelvins and errors are at the 2σ level). The D*out values are greater by a factor of ∼ 3 in a crystal with 22–23 ppm initial total H2O. The D*0 values can be determined by dividing the D*out values by 0.347, and D* at each concentration can be determined from D* = D*0C/Co. The average activation energy for hydrous component diffusion in the two crystals is 253 ± 13 (2σ) kJ/mol. The diffusion-out diffusivities of the hydrous component are seven to eight orders of magnitude greater than the Fe-Mg interdiffusivities. They are so large that the OH content of a pyrope crystal can adjust to changing environmental conditions on a time scale of hours at temperatures as low as 800 °C. Pyrope crystals from the mantle may dehydrogenate during ascent. Caution should be exercised in using OH content in natural pyrope crystals to infer conditions of the source region. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.