Discovery of `K-tourmaline' in Diamond-Bearing Tourmaline-K-Feldspar-Quartz Rock From the Kokchetav Massif, Kazakhstan

`Potassium tourmaline' coexisting with microdiamond was newly discovered in tourmaline-K-feldspar-quartz rock at Kumdy-kol area in the Kokchetav UHP Massif, northern Kazakhstan. Tourmaline is generally expected to retain information of each metamorphic stage because of its refractory nature. Tourmalines in gneisses from the Kokchetav Massif, however, have been regarded as a retrograde mineral until now. Occurrence of diamond inclusion in K-tourmaline, which will be discussed here, indicates that K-tourmaline is stable in the diamond stability field. The rock sample containing K-tourmaline consists mainly of quartz, K-feldspar, and tourmaline, with small amounts of titanite, phengite, phlogopite, pumpellyite, chlorite, zircon, apatite and goethite (after magnetite?). The modal composition of tourmaline is up to 25%. Tourmaline is euhedral to subhedral coarse grain (ca. 1mm). Microdiamond is included only in tourmaline and in zircon. No UHP evidence is observed in matrix. All tourmaline grains display clear chemical zonation of core, mantle and rim by K2O contents. The core part is a potassium-analogue of dravite (K > Na, Ca), and the K2O content reaches 2.94 wt%. Such K2O-rich compositions have never been reported. The representative microprobe analysis of the core shows K2O: 2.94 wt%, Na2O: 0.49 wt%, CaO: 1.37 wt%, MgO: 8.74 wt%, FeO: 3.76 wt%, TiO2: 0.99 wt%, Al2O3: 30.78 wt%, and SiO2: 36.23 wt%, and the chemical formula is written as (K0.62Ca0.24Na0.16)1.02(Mg2.15Fe0.52Ti0.12)2.79Al5.99Si5.98O18(BO3} )3(OH) 4. The core is enriched in TiO2 than the mantle and the rim. The K2O content decreases from the core through the mantle to the rim. That in the mantle is 0.54 wt% in average, and 0.17 wt% at the rim. The Na2O content ranges from 0.43 wt% (core) through 1.70 wt% (mantle) to 1.54 wt% (rim); CaO ranges from 1.24 wt% (core) to 2.35 wt% (rim). The X-site vacancy increases at the mantle toward the rim (0.14 apfu in maximum), whereas the core has no X-site vacancy. Twenty seven microdiamond grains in tourmaline were confirmed in three thin sections with laser Raman spectroscopy. Diamond is included only in the K2O-rich part (core) of tourmaline, and flaky euhedral graphite sometimes occurs in the mantle and the rim as in matrix. Quartz, K-feldspar, phengite, zircon, calcite, and aggregates of fine-grained anhedral graphite (pseudomorph after microdiamond) also occur in the core of tourmaline. Relation between the occurrence of microdiamond inclusion and the chemical zonation of tourmaline suggests that the core part of tourmaline formed under UHP conditions. Flaky graphite in the mantle and the rim demonstrates crystallization out of the diamond stability. The chemical zonation from the mantle to the rim is corresponding to retrograde pattern, and it is consistent with the occurrence of graphite. The UHP evidence is only in refractory minerals such as tourmaline and zircon. Matrix minerals were strongly affected by the hydration during exhumation like other Kokchetav UHP rocks. The K-rich tourmaline in this study can be a new mineral and may give unique UHP evidence. Moreover, it is the first example of boron-bearing mineral formed at diamond stability. Formation of diamond-bearing tourmaline-rich rock requires boron enrichment under UHP conditions, but its mechanism is still controversial. Boron may be carried by aqueous fluid of dehydration origin of phyllosilicates through UHP metamorphism. Thus, tourmaline is expected to retain information about fluid behavior in crustal rocks subducted to great depths.