Standardizing high precision &#916;&#697;17O data from silicate rocks and minerals
Authors: Miller Martin , Pack Andreas , Bindeman Ilya , Greenwood Richard 
Published: March 2020

&lt;p&gt;The &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O value of O&lt;sub&gt;2&lt;/sub&gt; gas (the analyte for nearly all oxygen triple isotope measurements) can currently be measured to a precision of about 6 ppm; significantly better than the precision of the corresponding &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O data. However, reporting &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O measurements of silicate rocks to this degree of accuracy, relative to (for example) the VSMOW-SLAP line on the ln(1 + &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O) versus ln(1 + &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O) plot, poses practical challenges. Regardless of the reference line assigned, &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O values are still inextricably linked to the &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O calibration of the &amp;#8216;working standard&amp;#8217; O&lt;sub&gt;2&lt;/sub&gt; on the VSMOW&amp;#8211;SLAP scale. Yet few laboratories have the capability to make such measurements on waters and on silicates. Even when direct calibration to VSMOW and SLAP is possible, there is not yet consensus on the &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O values of widely used silicate standards such as UWG-2 garnet or San Carlos olivine, when reported to a common reference line.&lt;/p&gt;&lt;p&gt;Fluorination of silicate rocks and minerals, to produce O&lt;sub&gt;2&lt;/sub&gt; for isotope ratio measurements, requires a different procedure from that used for the fluorination of waters. Thus, there is the possibility of systematic errors being introduced by using a water reference material for reporting &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O data of silicates. Furthermore, fractionation arrays of natural silicates on the three-isotope plot are generally offset from VSMOW, which introduces an additional complication. To eliminate such potential sources of error, some authors have chosen to report &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O data relative to San Carlos olivine (as representative of Earth&amp;#8217;s mantle) rather than to VSMOW, in conjunction with a reference line of assigned slope, for characterizing &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O values. However, a two-point scale, such as VSMOW&amp;#8211;SLAP for waters, is preferable to a single point calibration&lt;/p&gt;&lt;p&gt;We have therefore characterized &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O values (and with inter-laboratory comparison) of two silicates spanning a greater &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O range than VSMOW&amp;#8211;SLAP and suggest that these materials may be used for accurate determinations of silicate &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O values. Our high-&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O standard is a flint, designated SKFS, with &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O = 33.93 &amp;#177; 0.08 &amp;#8240; (standard error) and &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O = &amp;#8211;69 &amp;#177; 3 ppm relative to the VSMOW-SLAP reference line. This material can therefore be used to calibrate the position of an assigned reference line such that it passes through VSMOW. Alternatively, in combination with our low-&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O silicate standard, designated as KRS (&amp;#948;&lt;sup&gt;18&lt;/sup&gt;O = &amp;#8211;25.20 &amp;#177; 0.03, &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O = &amp;#8211;114 &amp;#177; 2 ppm relative to the VSMOW-SLAP reference line), an empirical two-point silicate reference line may be defined from high precision &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O measurements of these proposed standards. &amp;#916;&amp;#697;&lt;sup&gt;17&lt;/sup&gt;O data of silicate rock and mineral samples reported relative to this reference line are independent of whether the &amp;#948;&lt;sup&gt;17&lt;/sup&gt;O and &amp;#948;&lt;sup&gt;18&lt;/sup&gt;O measurements are reported relative to VSMOW or to the &amp;#8216;working standard&amp;#8217; O&lt;sub&gt;2&lt;/sub&gt;, of any isotopic composition. This confers significant advantages for inter-laboratory comparisons.&lt;/p&gt;