Evidence for Hadean mantle differentiation preserved in 142,143Nd isotope constraints of granitoid rocks from the western Dharwar Craton, India
Authors: Ravindran Arathy , Peters Bradley , Mezger Klaus , Srinivasan Balakrishnan , Schönbächler Maria 
Published: March 2022

&lt;p&gt;Evidence for Hadean mantle differentiation preserved in &lt;sup&gt;142,143&lt;/sup&gt;Nd isotope constraints of granitoid rocks from the western Dharwar Craton, India&lt;br&gt;Arathy Ravindran&lt;sup&gt;1&lt;/sup&gt;, Bradley J. Peters&lt;sup&gt;1&lt;/sup&gt;, Klaus Mezger&lt;sup&gt;2&lt;/sup&gt;, S. Balakrishnan&lt;sup&gt;3&lt;/sup&gt;, Maria Sch&amp;#246;nb&amp;#228;chler&lt;sup&gt;1&lt;/sup&gt;&lt;br&gt;&lt;sup&gt;1&lt;/sup&gt;Institut f&amp;#252;r Geochemie und Petrologie, ETH Z&amp;#252;rich, Switzerland&lt;br&gt;&lt;sup&gt;2&lt;/sup&gt;Institut f&amp;#252;r Geologie, Universit&amp;#228;t Bern, Switzerland&lt;br&gt;&lt;sup&gt;3&lt;/sup&gt;Department of Earth Sciences, Pondicherry University, India&lt;br&gt;Limited availability of exposed, ancient rocks and their widespread reworking by magmatism and metamorphism hampers the effort to obtain primary isotope compositions to constrain the early silicate differentiation of the Earth. However, the &lt;sup&gt;146&lt;/sup&gt;Sm-&lt;sup&gt;142&lt;/sup&gt;Nd chronometer is an efficient tracer of Hadean silicate differentiation and a powerful tool to identify enriched crustal and depleted mantle components even in the absence of well-preserved Hadean rocks [1] [2]. Positive and negative &lt;sup&gt;142&lt;/sup&gt;Nd anomalies have been recorded in samples from different Precambrian cratons around the world that are commonly interpreted to reflect ancient heterogeneity in early mantle composition [3].&lt;br&gt;Archean rocks from the well-preserved western Dharwar Craton in southern India have been previously well-characterized geochemically and petrologically [4], [5], however, they lack high-precision &lt;sup&gt;142&lt;/sup&gt;Nd isotope data. Here, we present coupled &lt;sup&gt;146,147&lt;/sup&gt;Sm-&lt;sup&gt;142,143&lt;/sup&gt;Nd isotope systematics of chemically diverse granitoids, including TTGs and granites. A significant positive &lt;sup&gt;142&lt;/sup&gt;Nd anomaly (+6.8&amp;#177;3.0 ppm) is revealed in the oldest (~3.4-3.3 Ga) crustal rocks in the craton whereas the composition in younger rocks is almost identical to that of the modern bulk mantle. Interestingly, all granitoids define a trend in &amp;#956;&lt;sup&gt;142&lt;/sup&gt;Nd vs &amp;#949;&lt;sup&gt;143&lt;/sup&gt;Nd isotope space that extends from the primitive mantle composition up to &amp;#956;&lt;sup&gt;142&lt;/sup&gt;Nd &amp;#8776; +10 ppm (&amp;#177;3 ppm, 2 S.D.) and &amp;#949;&lt;sup&gt;143&lt;/sup&gt;Nd = +5. The most primitive TTGs and early mantle-derived rocks of the craton show a clear distinction from the younger TTGs and granites in the &amp;#956;&lt;sup&gt;142&lt;/sup&gt;Nd vs. &lt;sup&gt;147&lt;/sup&gt;Sm/&lt;sup&gt;144&lt;/sup&gt;Nd diagram. It has been proposed that the oldest granitoids were derived by mixing of material from two sources- the depleted mantle and a slightly enriched, remelted mafic crustal source [6] that developed when &lt;sup&gt;146&lt;/sup&gt;Sm was still extant. The younger rocks formed by differentiation and reworking of the pre-existing mafic crustal reservoir. This diversity in the formation of crustal rocks in the Dharwar Craton is unique among Indian cratons and is related to heterogeneity of the mantle that developed during the Hadean.&lt;br&gt;[1] Peters et al. (2018). &lt;em&gt;Nature&lt;/em&gt; &lt;strong&gt;555&lt;/strong&gt;, 89-93&lt;br&gt;[2] O&amp;#8217;Neil et al. (2008). &lt;em&gt;Science&lt;/em&gt; &lt;strong&gt;321&lt;/strong&gt;, 1828-1831&lt;br&gt;[3] Hyung &amp; Jacobsen (2020). &lt;em&gt;PNAS&lt;/em&gt; &lt;strong&gt;117&lt;/strong&gt;, 14738-14744&lt;br&gt;[4] Ravindran et al. (2021). &lt;em&gt;Lithos&lt;/em&gt; &lt;strong&gt;404-405&lt;/strong&gt;, 106491&lt;br&gt;[5] Jayananda et al. (2015). &lt;em&gt;Precambrian Research&lt;/em&gt; &lt;strong&gt;268&lt;/strong&gt;, 295-322&lt;br&gt;[6] Ravindran et al. (2021). &lt;em&gt;Goldschmidt abstract&lt;/em&gt;&lt;/p&gt;