On the occurrence and boron isotopic composition of tourmaline in (ultra)high-pressure metamorphic rocks

The extensive P–T stability and the high chemical variability of tourmaline (Tur) together with its common occurrence in metasediments proves its high potential for petrological and (isotope) geochemical studies on fluid–rock interaction in subduction- and collision-related rocks. This paper reviews the occurrence, major element chemistry and boron isotopic composition of Tur in high- and ultrahigh-pressure metamorphic (UHPM) rocks. In addition, it presents a new discovery of coesite-bearing Tur (schorl) from the Erzgebirge (Germany), as well as Tur (dravite) related to the retrograde history of coesite- and diamond-bearing rocks from the Erzgebirge and the Kokchetav Massif (Kazakhstan). The scarce data on worldwide occurrences of (U)HPM Tur reveal a high occupation of the crystallographic X-site (dominated by Na) and the possible presence of excess B, with little further distinctiveness in its major element composition when compared with Tur from medium-grade rocks. High K2O contents in Tur are probably not related to UHP growth or equilibration. The B isotopic composition of (U)HPM Tur ranges in δ11B from −16 to +1‰, with many samples in or below the range of continental crust. In contrast, Tur formed during retrograde fluid influx typically shows high δ11B values (up to +28‰), suggesting heavy-B fluids infiltrating the exhuming (U)HPM units. Coesite inclusions in Tur, characterized by Raman spectroscopy, are regarded as the best indicator for its UHP stability.电气石（tourmaline, Tur）广泛的温压稳定性与显著的化学多变性，加之其在变沉积岩中的普遍产出，使其在俯冲及碰撞相关岩石的流体-岩石相互作用岩石学与（同位素）地球化学研究中具备极高应用潜力。本文综述了高压-超高压变质（ultrahigh-pressure metamorphic, UHPM）岩石中电气石的产出特征、主量元素化学组成与硼同位素组成。此外，本文报道了来自德国厄尔士山脉（Erzgebirge）的含柯石英（coesite）电气石（黑电气石，schorl）的新发现，以及与厄尔士山脉和哈萨克斯坦科克切塔夫地块（Kokchetav Massif）含柯石英、金刚石岩石退变质历史相关的电气石（镁电气石，dravite）。全球范围内（超）高压变质带中电气石的有限数据显示，其晶体学X位点（以钠为主）占据比例较高，且可能存在过剩硼；与中级变质岩中的电气石相比，其主量元素组成无显著差异。电气石中较高的K₂O含量可能与超高压生长或平衡过程无关。（超）高压变质电气石的硼同位素δ¹¹B值范围为-16‰至+1‰，多数样品的δ¹¹B值处于大陆地壳范围之内或低于该范围。与之相反，退变质流体注入过程中形成的电气石通常表现出较高的δ¹¹B值（最高可达+28‰），这表明重硼流体渗入了折返的（超）高压变质单元。经拉曼光谱（Raman spectroscopy）表征的电气石内含柯石英包裹体，被认为是其超高压稳定性的最佳指示标志。