A combined diffusion and thermal modeling approach to determine peak temperatures of thermal metamorphism experienced by meteorites

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• 作者：Sabrina Schwinger ; ^{sabrina.schwinger@rub.de" class="auth_mail" title="E-mail the corresponding author} ; Ralf Dohmen ; Hans-Peter Schertl
• 关键词：Chondrites ; Thermal metamorphism ; Diffusion ; Olivine ; Type II chondrules
• 刊名：Geochimica et Cosmochimica Acta
• 出版年：2016
• 出版时间：15 October 2016
• 年：2016
• 卷：191
• 期：Complete
• 页码：255-276
• 全文大小：2301 K

文摘

Carbonaceous chondrites are affected to different degrees by thermal and aqueous metamorphism on their parent bodies. However, the degree of alteration has been categorized mainly by relative scales and achieving quantitative information about metamorphic temperature by conventional mineral thermometry is problematic for low petrologic types. We have developed a general approach to estimate the metamorphic peak temperature experienced by type 3 chondrites from diffusion zoning in minerals, and have applied this approach to olivine in type I and type II chondrules of CO3 chondrites.

To obtain metamorphic temperatures from diffusion zoning, we have combined diffusion modeling with thermal modeling of the meteorite parent body. The integrated diffusion coefficient over time (class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$) was identified as a useful parameter to quantify the extent of chemical change by diffusion occurring in a mineral during a given thermal history. Knowing the temperature dependence of the diffusion coefficient, class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ values can be calculated for each thermal history and be compared to the class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ values obtained from diffusion modeling. For thermal histories realistic for the parent body, class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ depends primarily on the metamorphic peak temperature, so that class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ values determined from diffusion profiles in meteorite minerals can be directly related to the metamorphic peak temperature. This general approach is relatively insensitive to uncertainties in the input parameters for the thermal model.

We found that chemical zoning in type I and type II chondrule olivine of the CO chondrites Kainsaz and Lancé was largely influenced by solid state diffusion, which is evident from the observed correlation of zoning anisotropy with the crystallographic orientation. Chemical zoning in type II chondrule olivine is mainly igneous for CO chondrites of petrologic types up to at least 3.2 (Kainsaz) and was influenced only minor by diffusion during parent body metamorphism. Fe–Mg zoning in type II chondrule olivine and around sealed cracks in type I chondrule olivine yields similar class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ values, indicating a formation of both zoning features during a common thermal history on the parent body. In addition, class="mathmlsrc">class="formulatext stixSupport mathImg" data-mathURL="/science?_ob=MathURL&_method=retrieve&_eid=1-s2.0-S0016703716303453&_mathId=si1.gif&_user=111111111&_pii=S0016703716303453&_rdoc=1&_issn=00167037&md5=b544f704e3378f4cc18c5edd9e5dac91" title="Click to view the MathML source">Γclass="mathContainer hidden">class="mathCode">$\mathrm{\Gamma }$ values for type II chondrule olivine correlate with metamorphic grade. The application of this approach on Fe–Mg zoning in type II chondrule olivine of CO3 chondrites yields estimates of maximum metamorphic peak temperatures ranging from 653 to 849 K for different petrologic subtypes. The Fe–Mg zoning of type I chondrule olivine is not consistent with the peak temperature estimates from type II chondrule olivine, suggesting an additional contribution of solar nebular processes to type I chondrule olivine zoning prior to accretion into the parent body.