Rituraj Borthakur posted an Question
July 13, 2022 • 12:41 pm 5 points
  • IIT JAM
  • Geology (GG)

36. the geochemical trends illustrated in the dia grams for a suite of cogenetic and coeval 1g neous rocks indicate: ww sio, a. olivine crystallization b. clino

36. The geochemical trends illustrated in the dia grams for a suite of cogenetic and coeval 1g neous rocks indicate: ww SIO, A. Olivine crystallization B. Clinopyroxene crystallization C. Plagioclase crystallization D. Alkali feldspar crystallization (11T-JAM 2015)

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    Sushmita

    This question is related to 'Harker diagram' and for more detail you can go through J.D. Winter chapter 8. Here, the illustrated geochemical trends suggests that increasing crystallization of minerals the MgO and CaO content of magma decrease, while the Al2O3 increases. So that means minerals crystallizing have MgO and CaO in their composition but no more alumina. Terms meaning__cogenetic(same genetic condition) and coeval (same age and origin). Thus, for olivine (ex__ fosterite: Mg2SiO4 and monticellite: CaMgSiO4) and cpx(diopside: CaMgSi2O6) have magnesium and calcium in their composition. So they must crystallize from the melt. And for plagioclase and alkali feldspar has alumina in its composition, hence as the alumina content rises in their residual melt with crystallization...so, neither plagioclase nor alkali feldspar crystallization from the melt is possible. So, correct option should be A,B.

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