NanoSIMS mapping and LAICPMS chemical and U–Th–P

Authors: A Didier V Bosse J Bouloton S Mostefaoui M Viala J L Paquette J L Devidal R Duhamel

Publish Date: 2015/10/31

Volume: 170, Issue: 5-6, Pages: 45-

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Abstract

In this study we use NanoSIMS element and isotope ratio mapping and LAICPMS trace element measurements to elucidate the origins of monazites from a restitic xenolith enclosed in a 135 ± 03 Ma andesitic lava Slovakia The xenolith/lava interaction is mainly characterized by the growth of a plagioclasebearing corona around the xenolith and magmatic garnet overgrowths on primary metamorphic garnets within the xenolith NanoSIMS images 89Y 139La 208Pb 232Th and 238U and trace element analyses indicate that variations of HREE Y and Eu contents in the monazite are correlated with the resorption and the following overgrowth of garnet and plagioclase in the xenolith Three domains are distinguished in the monazite grains the inherited Variscan core at ca 310 Ma M1 domain characterized by low Y and HREE contents and a weak negative Eu anomaly the inner rim M2 domain crystallized during the growth of the plagioclase magmatic corona large negative Eu anomaly and the resorption of metamorphic garnet high HREE and Y contents and the external rim M3 domain crystallized during the growth of the plagioclase corona large negative Eu anomaly and during the crystallization of magmatic garnet low Y HREE contents at ~13 Ma ie the age of the andesitic lava The age and chemical zonation of the monazites attest to the preservation of primary monazite in the xenolith despite the interaction with the andesite lava NanoSIMS imaging provides highquality subµm scale images of the monazite that reveals chemical domains that were not distinguishable on WDS Xray maps especially for depleted elements such as U and Pb Owing to its small size the M2 domain could not be accurately dated by the LAICPMS method However NanoSIMS isotopic maps reveal that the M2 domain has similar 208Pb/232Th isotope ratios to the M3 domain and thus similar ages These results support the hypothesis that meltassisted partial dissolution–precipitation in monazite efficiently records chemical and mineralogical changes during xenolith/lava interactionThanks are due to JeanMarc Hénot for his help in the use of SEM Emily Mullen for English corrections and O Laurent for fruitful discussion The paper has been greatly improved by the critical and constructive comments of Alexander Stepanov and an anonymous reviewer This work is supported a posteriori by CNRSINSU

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