Crystal growth versus diffusional re-equilibration in k-feldspar
Authors: Toussaint Aurore , Grocolas Thomas , Müntener Othmar 
Published: March 2022

&lt;p&gt;&amp;#160; &amp;#160; Diffusion and crystal growth are key processes to understand magma dynamics, and thus the evolution of igneous systems to extract crucial information on residence times, crystal growth timescales or cooling rates (e.g., Costa et al., 2020, Costa 2021). As k-feldspar is a major mineral in felsic plutonic rocks and sometimes exceeds 30 vol%, investigating k-feldspar provides valuable information on magma dynamics. K-feldspar has been used to determine timescales of crystal growth or residence times for volcanic and plutonic systems, although to a lesser extent for the latter (e.g., Moore &amp; Sisson 2008, Rout et al., 2021).&lt;/p&gt;&lt;p&gt;&amp;#160; &amp;#160; As chemical zoning (e.g., Ba zoning, often Ba-rich cores, but not always) is commonly observed in k-feldspar megacrysts, this makes them good targets for studying and developing methods to distinguish crystal growth from diffusional re-equilibration. Therefore, our study focuses on the investigation of trace element chemistry and zoning patterns (EPMA, LA-ICP-MS analyses and high-resolution imaging) of k-feldspar megacrysts using samples from Sierra Nevada (United States), Bergell (Italy/Switzerland), and Sardinia (Italy) to unravel coupled growth and/or diffusion patterns. The preliminary results on k-feldspar zoning from the different samples show Ba zoning preserved despite perthite exsolutions (Sierra Nevada, Bergell), with an average of 7 to 10 zones of random spacing and thickness (80 to 700&amp;#181;m) per crystal. Petrography and EPMA data highlight three different features. (1) Sawtooth chemical profiles encompassing textural growth patterns combined with Ba zoning and a more evolved composition towards the borders (more K-rich zones, as commonly observed with fractional crystallization) correlated with the zones of higher Ba composition (Sierra Nevada, Bergell). (2) Sawtooth and relaxed step-function profiles (potentially related to diffusion) when megacrysts display textural growth patterns associated to Ba zoning essentially (Sierra Nevada, Bergell). (3) No obvious zoning on high-resolution maps but relaxed step-function profiles within single crystals (Sardinia). (1) and (2) could be observed within a single rock sample. In addition to Ba zoning, there are various inclusions related to Ba zoning (zoned plagioclase, biotite &amp;#8211; Sierra Nevada, Bergell &amp;#8211; amphibole and titanite &amp;#8211; Sierra Nevada) that are located on and parallel to the growth planes, correlated to Ba-rich zones. On the contrary, inclusions (mostly zoned plagioclase and biotite) are &amp;#177;randomly distributed within k-feldspar from Sardinia. Ultimately, we aim to quantify k-feldspar growth times by coupled-growth and diffusion and comparing the results to absolute growth times derived from U-Pb dating of zircons (e.g., Barboni and Schoene 2014).&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Barboni, M., &amp; Schoene, B. (2014) &lt;em&gt;Nat. Geosci. 7&lt;/em&gt;, 524&amp;#8211;528; Costa, F. (2021) &lt;em&gt;Annual Review of Earth and Planetary Sciences&lt;/em&gt;,&amp;#160;&lt;em&gt;49&lt;/em&gt;, 231-252; Costa, F., Shea, T., &amp; Ubide, T. (2020) &lt;em&gt;Nature Reviews Earth &amp; Environment&lt;/em&gt;,&amp;#160;&lt;em&gt;1&lt;/em&gt;(4), 201-214; Moore, J. G., &amp; Sisson, T. W. (2008) &lt;em&gt;Geosphere&lt;/em&gt;,&amp;#160;&lt;em&gt;4&lt;/em&gt;(2), 387-400; Rout, S. S., Blum-Oeste, M., &amp; W&amp;#246;rner, G. (2021) &lt;em&gt;Journal of Petrology&lt;/em&gt;, &lt;em&gt;62&lt;/em&gt;:#egab010&lt;/p&gt;