Jeffbenite (ex-“TAPP”): a high-pressure marker in diamonds

Super-deep diamonds are thought to crystallize between about 300 and 800 km (Harte, 2010) on the basis of the inclusions trapped within them. Many of the inclusions are composites of multiple minerals and show evidence of retrograde transformation from lower-mantle (LM) or transition-zone (TZ) precursors, and their depth of origin is inferred. However, due to the lack of experimental evidence relating composite inclusions directly to high-pressure precursors, their actual depth of origin has never been proven. Jeffbenite is a new official mineral (Nestola et al., 2016), previously referred to as TAPP (Tetragonal Almandine-Pyrope Phase), which occurs both in composite inclusions and as homogeneous inclusions, and may provide key information about the depth of formation of super-deep diamonds. Jeffbenite is a tetragonal phase with garnet-like stoichiometry discovered in Brazilian diamonds (Harris et al., 1997) and found exclusively in nature as inclusions in super-deep diamonds. The main argument in favour of a super-deep origin for jeffbenite is its coexistence in the same diamond with minerals, which are interpreted to have crystallized in the TZ or LM (Harris et al., 1997; Harte et al., 1999; Hutchison et al., 2001; Brenker et al., 2002; Hayman et al., 2005; Kaminsky, 2012). An origin for jeffbenite in the LM is also supported by its capacity to hold ferric iron, as it was demonstrated that in deep mantle silicates Fe3+ is significantly abundant (McCammon et al., 1997; Harte, 2010). By contrast, the absence of octahedral silicon in the crystal structure, which is typical of high-pressure silicates below 200 km depth, makes a LM origin difficult to explain if the jeffbenite structure is primary (Harris et al., 1997; Finger & Conrad, 2000). Thus, whether jeffbenite forms as a primary phase in the LM or is the product of retrogression from high-pressure mantle phases has been a matter of debate and is still controversial. The only experimentally determined stability field for jeffbenite is that of Armstrong & Walter (2012), which provides a maximum pressure for jeffbenite stability of ~13 GPa (~390 km) at 1700 K. This confirms that jeffbenite is a sub- lithospheric mineral, but rules out direct incorporation of jeffbenite in diamond at the TZ-LM boundary. However, these results were obtained on a Ti-rich jeffbenite, which is usually found as part of composite inclusions, and not on a Ti-free jeffbenite, which occurs as single-phase inclusions in diamonds (Nestola et al., 2016). We therefore report here new laser heated diamond-anvil cell experiments from 5 to 30 GPa on a Ti-free jeffbenite, in order to determine the role that TiO2 plays in the stability field of jeffbenite and to figure out if the latter can be directly incorporated into diamond in the TZ or LM. Our preliminary results show that the absence of TiO2 extends the stability field of jeffbenite to higher pressures than previously determined.