Magmatic-hydrothermal molybdenum isotope fractionation and its relevance to the igneous crustal signature

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• 作者：Nicolas D. Greber ; ^{greber@geo.unibe.ch" class="auth_mail} ; Thomas Pettke¹ ; ^{pettke@geo.unibe.ch" class="auth_mail} ; Thomas F. Nä ; gler² ; ^{naegler@geo.unibe.ch" class="auth_mail}
• 关键词：Molybdenum ; Isotope ; Fractionation ; Molybdenite ; Hydrothermal
• 刊名：Lithos
• 出版年：March, 2014
• 年：2014
• 卷：190-191
• 期：Complete
• 页码：104-110
• 全文大小：2222 K

文摘

We analysed the Mo isotope composition of a comprehensive series of molybdenite samples from the porphyry-type Questa deposit (NM, USA), as well as one rhyolite and one granite sample, directly associated with the Mo mineralization. The 未⁹⁸Mo of the molybdenites ranges between 鈭?#xA0;0.48鈥?and + 0.40鈥? with a median at 鈭?#xA0;0.05鈥? The median Mo isotope composition increases from early magmatic (鈭?#xA0;0.29鈥? to hydrothermal (鈭?#xA0;0.05鈥? breccia mineralization (median bulk breccia = 鈭?#xA0;0.17鈥? to late stockwork veining (+ 0.22鈥?. Moreover, variations of up to 0.34鈥?are found between different molybdenite crystals within an individual hand specimen. The rhyolite sample with 0.12 渭g g^鈭?#xA0;1 Mo has 未⁹⁸Mo = 鈭?#xA0;0.57鈥?and is lighter than all molybdenites from the Questa deposit, interpreted to represent the igneous leftover after aqueous ore fluid exsolution. We recognize three Mo isotope fractionation processes that occur between about 700 and 350 掳C, affecting the Mo isotope composition of magmatic-hydrothermal molybdenites. 鈭?sub>1Mo: Minerals preferentially incorporate light Mo isotopes during progressive fractional crystallization in subvolcanic magma reservoirs, leaving behind a melt enriched in heavy Mo isotopes. 鈭?sub>2Mo: Magmatic-hydrothermal fluids preferentially incorporate heavy Mo isotopes upon fluid exsolution. 鈭?sub>3Mo: Light Mo isotopes get preferentially incorporated in molybdenite during crystallization from an aqueous fluid, leaving behind a hydrothermal fluid that gets heavier with progressive molybdenite crystallization. The sum of all three fractionation processes produces molybdenites that record heavier 未⁹⁸Mo compositions than their source magmas. This implies that the mean 未⁹⁸Mo of molybdenites published so far (~ 0.4鈥? likely represents a maximum value for the Mo isotope composition of Phanerozoic igneous upper crust.