Trace elemental nano-imaging of monazite and zircon using synchrotron radiation induced XRF: new insights into polyphase evolution

Trace elements in accessory minerals provide unique information on rock formation: while Rare Earth Elements (REE) offer the possibility to derive conditions during continental crust formation, U, Th and Pb enable to establish age constraints for the different evolutionary stages. Accessory minerals are well known to preserve signatures of polyphase rock formation. Here, we aim at deciphering the different evolutionary stages at the best available spatial resolution for combined age and trace element determinations. We performed nano-imaging of the accessory minerals monazite and zircon with a spatial resolution better than 250 nm using synchrotron radiation induced XRF. Monazite and zircon were selected as both minerals are stable over a wide P-T range, common in various rock types and incorporate REE and U and Th during formation. The study was performed on three different polyphase rock types from well-studied localities: 1) magmatism followed by ultra-high temperature metamorphism (Rogaland), 2) polymetamorphism (Zambia), 3) metamorphism followed by tectonic activity (Red Bank sheer zone). Trace element measurements were performed at the nano-imaging beamline ID22NI at ESRF, Grenoble. The excitation energy was set to 17.6 keV in order to allow simultaneous detection of REE and Pb, Th and U via L-shell excitation and elements with Z between 14 and 39 via K-shell excitation, but exclude the excitation of the Zr K-edge. The incoming beam was focused to a spot size of 190 nm horizontally and 164 nm vertically. Fluorescence signals were recorded with an energy-dispersive SDD detector in confocal geometry using a polycapillary half-lens on the detection side. The confocal geometry resulted in a detector acceptance of less than 15 μm in the energy range with the emission lines of interest for chemical age dating and between 20 and 26 μm for the REE. 2D elemental maps were performed in the layer of the sample yielding maximum fluorescence signal. Sample times per point varied between 0.4 and 50 sec and maximum step sizes were 250 nm. Chemical age dating was performed with confocal fluorescence intensities of the Pb, Th and U peak areas using the Ranchin formula. Six known age reference samples for monazite ranging in age between 101 Ma and 1821 Ma were measured at a sub-micron resolution. The obtained chemical ages at the nano-scale showed a precision of 10 % with an accuracy better than 7 %. Elemental nano-imaging reveals different states of mineral growth and alteration in the selected sample set: while the example from Rogaland shows that the boundary between the inherited core in zircon and the overgrowth at ultra-high temperature was sharp on the 250 nm scale, indicating this to be a true overgrowth with minimal or no recrystallisation and scavenging of the core, the two monazite examples from Zambia and the Red Bank sheer zone document polymetamorphic growth with diffuse boundaries and later changes due to fluid activity, respectively.