Determination of Water D/H in Hydrated Chondrites using NanoSIMS Imaging
Authors: Vacher Lionel G. , Ogliore Ryan C. 
Published: March 2022

&lt;p&gt;&lt;strong&gt;Introduction: &lt;/strong&gt;Hydrogen isotopic compositions (D/H or &amp;#120575;D) in chondrites are a powerful tool for deciphering the source of water delivered to terrestrial planets (1). CM-type carbonaceous chondrites contain up to ~10wt.% H&lt;sub&gt;2&lt;/sub&gt;O, retained as OH in phyllosilicates. The D/H ratio of phyllosilicates (a direct proxy for water) in chondrites cannot be determined directly using whole rock measurements, because their matrices also accreted D-rich organics which are mixed with D-poor phyllosilicates at the sub-micrometer scale. To address this issue, water D/H has been estimated by in-situ measurements of both D/H and C/H in hydrated chondrites, which define a mixing line in a D/H vs. C/H plot. The intercept gives the isotopic composition of the phyllosilicate alone (1). However, SIMS measurements of water D/H using this method can be compromised by &lt;em&gt;(i&lt;/em&gt;&lt;em&gt;)&lt;/em&gt; contamination and &lt;em&gt;(ii&lt;/em&gt;&lt;em&gt;)&lt;/em&gt; limited dispersion of the phyllosilicates/organics ratio measured with a large primary beam.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Methods:&lt;/strong&gt; We addressed both issues using the Wash U NanoSIMS50 which allows us to obtain coordinated isotopic and elemental data with high-spatial resolution. H&lt;sup&gt;&amp;#8722;&lt;/sup&gt;,D&lt;sup&gt;&amp;#8722; &lt;/sup&gt;with &lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;&amp;#8722;&lt;/sup&gt;,&lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;14&lt;/sup&gt;N&lt;sup&gt;&amp;#8722;&lt;/sup&gt;,&lt;sup&gt;12&lt;/sup&gt;C&lt;sup&gt;15&lt;/sup&gt;N&lt;sup&gt;&amp;#8722;&lt;/sup&gt;,&lt;sup&gt;28&lt;/sup&gt;Si&lt;sup&gt;&amp;#8722;&lt;/sup&gt; are collected using magnetic-field peak-jumping in &amp;#8220;Combined Analysis&amp;#8221; mode. Centering of the secondary ions beam in Cy and P2/P3 planes of the secondary column changes between the low and high masses, resulting in misaligned ion images. So, we used AutoHotkey scripts to send a different Cy voltage for every B-field set up through the virtual keyboard of the NanoSIMS. To separate phyllosilicate-rich from organic-rich pixels, we assume that D/H is not simply a linear function of C/H, but in general D/H is approximated by a function using all measured species: &lt;img src=&quot;https://contentmanager.copernicus.org/fileStorageProxy.php?f=gnp.eadb5f8d60e163608902461/sdaolpUECMynit/22UGE&amp;app=m&amp;a=0&amp;c=7b6eafebc649b8fa9942e815659b307b&amp;ct=x&amp;pn=gnp.elif&amp;d=1&quot; alt=&quot;&quot; width=&quot;118&quot; height=&quot;25&quot;&gt;. The true phyllosilicate composition [C,N,Si,H] is estimated from the data and is then used to estimate the water D/H composition from the linear regression model. NanoSIMS isotopic analyses were carried out in a matrix area of the CM Maribo and our analytical conditions were the same as outlined in (2).&lt;/p&gt;&lt;p&gt;&lt;strong&gt;Results: &lt;/strong&gt;First,&lt;strong&gt; &lt;/strong&gt;we calculated a &amp;#120575;D value of &amp;#8722;178&amp;#177;46&amp;#8240; (2&amp;#963;) for the phyllosilicates in Maribo using the D/H vs. C/H correlation from the resized pixels. This value is higher than previous measurements using SIMS [&amp;#120575;D &amp;#8776; &amp;#8722;420 to &amp;#8722;270&amp;#8240;, (2, 3)], demonstrating that D/H ratio of phyllosilicate cannot be simply determined using the D/H &lt;em&gt;vs.&lt;/em&gt; C/H line in this matrix area. Second, we calculated the &amp;#120575;D value of the phyllosilicates in Maribo using all the measured species and the linear regression model described above. We found that the phyllosilicate D/H is best correlated for dominant contributions of N, Si and H (b=0.14, c=0.58 and d=&amp;#8722;0.86) and minor contributions of C (a=0.06). We calculated a &amp;#120575;D value of &amp;#8722;286+/-60&amp;#8240;. This value is consistent with those previously determined by SIMS, demonstrating that our method can be used to precisely determine the water D/H on very small areas.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;(1) Alexander C.M.O&amp;#8217;D. et al. (2012) &lt;em&gt;Science, 337&lt;/em&gt;, 721&amp;#8211;723.&lt;/p&gt;&lt;p&gt;(2) Vacher L.G. and Ogliore R.C. (2022) &lt;em&gt;53rd LPSC&lt;/em&gt;&lt;em&gt;, &lt;/em&gt;&lt;em&gt;2653&lt;/em&gt;.&lt;/p&gt;&lt;p&gt;(3) van Kooten E.M.M.E. et al. (2018) &lt;em&gt;GCA, 237&lt;/em&gt;, 79&amp;#8211;102.&lt;/p&gt;&lt;p&gt;(4) Piani L. et al. (2021) &lt;em&gt;EPSL, 567&lt;/em&gt;, 117008.&lt;/p&gt;