Maruyamaite, a new K-dominant tourmaline coexisting with diamond –an important accessory mineral in UHP rocks

Recently the K-dominant tourmaline with microdiamond inclusions from the Kokchetav Massif, Kazakhstan (Shimizu & Ogasawara, 2005), has been approved by IMA-CNMNC as a new end-member species of the tourmaline supergroup, maruyamaite (IMA 2013-123, Lussier et al., 2014: Min. Mag. V. 78, p. 550). Maruyamaite has an ideal chemical formula of K(MgAl₂)(Al₅Mg)(BO₃)₃(Si₆O₁₈)(OH)₃O and is a unique K-dominant tourmaline species. Currently the occurrence of maruyamaite is limited from the Kokchetav Massif. Moreover, no K-rich dravitic tourmaline has been reported from other UHP terranes. This rarelity is probably due to the large ionic radius of K with respect to the X-site of tourmaline. The first synthesis of K-dominant tourmaline by Berryman et al. (2014) indicated that K-incorporation in tourmaline structure requires high-pressure and high K/Na conditions. A number of studies (e.g., Hwang et al., 2005) have shown that K-rich fluid had been present at the Kokchetav UHP stages and played an important role for the formation of metamorphic diamond and other UHP minerals such as K-rich clinopyroxene.

The systematic chemical analysis combined with detailed inclusion study (Shimizu & Ogasawara, 2013) demonstrated that K in tourmaline decrease with pressure decrease in the diamond-bearing Kokchetav UHP rocks and the maruyamaite component can form continuous solid solutions with dravite–oxy-dravite–uvite tourmalines. The figure shows that microdiamond inclusions occur only in K-dominant core (maruyamaite) and the surrounding parts contain low-pressure minerals such as graphite and quartz. UHP partial melting was deduced for the genesis of the maruyamaite-bearing tourmaline-rich quartzofeldspathic rock from the heavy B-isotope composition of maruyamaite (Ota et al., 2008). Contrarily, Marschall et al. (2009) and Korsakov et al. (2009) insisted that K-rich tourmaline was not UHP phase because of the significantly young ⁴⁰Ar/³⁹Ar age compared to the peak metamorphism. However, the K-content of tourmaline used for their dating is not more than 1.6 wt% (i.e., the tourmaline was not maruyamaite but K-rich (oxy-) dravite), suggesting the age may represent a later event during exhumation.

K-bearing tourmalines including maruyamaite could be a UHP indicator.