NDLI: Fluid-mediated partial alteration in monazite: the role of coupled dissolution–reprecipitation in element redistribution and mass transfer

Content Provider	Springer Nature Link
Author	Harlov, Daniel E. Wirth, Richard Hetherington, Callum J.
Copyright Year	2010
Abstract	Monazite [(Ce,LREE,Th,U,Ca)(P,Si)O4], with complex zoning in Th and other elements, is commonly observed in metamorphic and igneous rocks. The hypothesis that this alteration is a product of fluid-mediated element mass transfer has been tested in the piston-cylinder press (CaF2 assembly, cylindrical graphite oven) at 1,000 MPa and 900°C and in cold seal autoclaves on a hydrothermal line at 500 MPa and 600°C. Experiments included a relatively homogeneous monazite-(Ce) (7–8 wt% ThO2) from a heavy mineral sand plus a series of alkali-bearing fluids including 2N NaOH, 2N KOH, and Na2Si2O5 + H2O. Experiments were conducted using BSE imaging, EMP analysis, and both TEM and HRTEM. A subset of monazite grains from each experiment show evidence of partial alteration in the form of areas enriched in Th + Si with sharp curvilinear compositional boundaries extending from the grain rim into the monazite interior. These ThSiO4-enriched textures are similar to those commonly seen in natural examples of metasomatised monazite in both magmatic and metamorphic rocks. In the Na2Si2O5 + H2O experiments, scarce inclusions of britholite formed in the altered monazite. The altered monazite is also characterised by strong depletion in Pb, Ca, and Y. Thorium and Si mobility, coupled with the formation of britholite inclusions, during partial alteration in the monazite grain is considered to be the product of fluid-aided coupled dissolution–reprecipitation as opposed to solid-state diffusion. Since other fluids, including NaCl and KCl brines, do not result in the formation of these textures, the experimental replication of ThSiO4-enriched areas in the monazite strongly suggests that similar textures in monazite observed in nature are fluid induced, specifically by alkali-bearing fluids. If true, complex metasomatically induced textures in monazite could yield information concerning the nature of the fluid responsible for their formation as well as allow for the dating of the metasomatic event, presuming that all the original radiogenic Pb has been removed.
Starting Page	329
Ending Page	348
Page Count	20
File Format	PDF
ISSN	00107999
Journal	Contributions to Mineralogy and Petrology
Volume Number	162
Issue Number	2
e-ISSN	14320967
Language	English
Publisher	Springer-Verlag
Publisher Date	2010-12-09
Publisher Place	Berlin/Heidelberg
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	Monazite Huttonite Cheralite Thorite Britholite Alkali-bearing fluids Metasomatism Dissolution–reprecipitation Experimental petrology Geology Mineralogy Mineral Resources
Content Type	Text
Resource Type	Article
Subject	Geochemistry and Petrology Geophysics

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