Tetragonal Almandine-pyrope Phase Research Articles

Tetragonal Almandine-Pyrope Phase, TAPP: finally a name for it, the new mineral jeffbenite

AbstractJeffbenite, ideally Mg3Al2Si3O8, previously known as tetragonal-almandine-pyrope-phase ('TAPP’), has been characterized as a new mineral from an inclusion in an alluvial diamond from São Luiz river, Juina district of Mato Grosso, Brazil. Its density is 3.576 g/cm3 and its microhardness is ∼7. Jeffbenite is uniaxial (-) with refractive indexes ω = 1.733(5) and ε = 1.721 (5). The crystals are in general transparent emerald green.Its approximate chemical formula is (Mg262Fe2+0.27)(Al186Cr016)(Si2 g2Al018)O12 with very minor amounts of Mn, Na and Ca. Laser ablation ICP-MS showed that jeffbenite has a very low concentration of trace elements. Jeffbenite is tetragonal with space group I42d, cell edges being a = 6.5231(1) and c = 18.1756(3) Å. The main diffraction lines of the powder diagram are [d (in Å), intensity, hkl]: 2.647, 100, 2 0 4; 1.625, 44, 3 2 5; 2.881, 24, 2 1 1; 2.220, 19, 2 0 6; 1.390, 13, 4 2 4; 3.069, 11,2 0 2; 2.056, 11,2 2 4; 1.372, 11,2 0 12.The structural formula of jeffbenite can be written as (M1)(M2)2(M3)2(T1)(T2)2O12 with M1 dominated by Mg, M2 dominated by Al, M3 dominated again by Mg and both T1 and T2 almost fully occupied by Si. The two tetrahedra do not share any oxygen with each other (i.e. jeffbenite is classified as an orthosilicate).Jeffbenite was approved as a new mineral by the IMA Commission on New Minerals and Mineral Names with the code IMA 2014-097. Its name is after Jeffrey W. Harris and Ben Harte, two world-leading scientists in diamond research. The petrological importance of jeffbenite is related to its very deep origin, which may allow its use as a pressure marker for detecting super-deep diamonds. Previous experimental work carried out on a Ti-rich jeffbenite establishes that it can be formed at 13 GPa and 1700 K as maximum P-T conditions.

Tetragonal almandine pyrope phase (TAPP): retrograde Mg-perovskite from subducted oceanic crust?

Laser heated diamond-anvil-cell experiments performed on a “tetragonal almandine pyrope phase” (TAPP) bulk composition constrain high-temperature phase relations between 6 and 48 GPa. Phase assemblages determined by synchrotron X-ray diffraction consist of TAPP + garnet ± pseudobrookite ± enstatite from 6 to 10 GPa, garnet from 13 to 22 GPa, Mg-perovskite + garnet from 23 to 30 GPa, and Mg-perovskite above 30 GPa. TAPP has the same structure at 1 atm and at high pressure based on in situ X-ray diffraction data, and is stable at a maximum pressure of 10–13 GPa at 1300–1700 K. These results rule out direct incorporation of TAPP in diamond at transition-zone or lower-mantle pressures. Possible origins of TAPP are (1) entrapment by diamonds in the upper mantle, and (2) retrograde formation from a high-pressure garnet or Mg-perovskite precursor. We suggest that single-phase TAPP inclusions and possibly composite TAPP-bearing inclusions originated as Mg-perovskite in the lower mantle in mafic protoliths, and that TAPP forms upon retrograde conversion at pressures below 13 GPa upon exhumation of the diamonds from the lower mantle to shallower depths of less than 400 km.

Diamond formation in the deep mantle: the record of mineral inclusions and their distribution in relation to mantle dehydration zones

AbstractStudies of the inclusions contained in natural diamonds have shown the occurrence of minerals which must have formed at depths below the lithosphere and which may be closely matched with the silicate mineral assemblages determined by high pressure and temperature experimental studies for depths of 300 to 800 km in the Earth's mantle. The inclusions come principally from two main depth zones: (1) the lower asthenosphere and upper transition zone; (2) the Upper Mantle/Lower Mantle (UM/LM) boundary region and the uppermost LM. The inclusions from zone 1 are very largely majoritic garnets (with or without clinopyroxene) which indicate bulk compositions of eclogitic/metabasic affinity. The minerals from zone 2 include Ca-Si and Mg-Si perovskites and ferropericlase and are dominantly of metaperidotitic bulk composition, but include some possible metabasite assemblages. In many of these natural assemblages, the tetragonal almandine pyrope phase occurs rather than the garnet found in experiments.As natural diamonds are believed to crystallize in fluids/melts, the hypothesis is developed that the restriction of diamonds and inclusions of particular compositions to the above two depth intervals is because they are controlled by loci of fluid/melt occurrence. Attention is focused on subduction zones because both suites of inclusions show some evidence of subducted protoliths. The lower zone (600–800 km) coincides with the region where dehydration may be expected for hydrous ringwoodite and dense hydrous Mg-silicates formed in subducted peridotites. The dehydration of lawsonite in subducted metabasites provides a particular location for melt formation and the inclusion of the shallower (~ 300 km) majoritic inclusions. For the deeper majoritic inclusions in the region of the upper transition zone, melt development may occur as a consequence of the hydrous wadsleyite-to-olivine transformation, and such melt may then interact with the upper crustal portion of a subducting slab. These suggestions offer an explanation of the depth restrictions and the compositional restrictions of the inclusions. The differences in δ13C values in the host diamonds for the two suites of inclusions may also be explained on this basis.

Lower mantle diamonds from Rio Soriso (Juina area, Mato Grosso, Brazil)

The morphology, colour, fluorescence, cathodoluminescence, nitrogen content and aggregation state, internal structure and mineral inclusions have been studied for 69 alluvial diamonds from the Rio Soriso (Juina area, Mato Grosso State, Brazil). Nitrogen in most diamonds (53%) is fully aggregated as B centres, but there is also a large proportion of N-free stones (38%). A strong positive correlation between nitrogen and IR-active hydrogen concentrations is observed. The diamonds contain (in order of decreasing abundance) ferropericlase, CaSi-perovskite, magnetite, MgSi-perovskite, pyrrhotite, ‘olivine’, SiO2, perovskite, tetragonal almandine-pyrope phase and some other minerals represented by single grains. The Rio Soriso diamond suite is subdivided into several subpopulations that originated in upper and lower mantle of ultramafic and mafic compositions, with the largest subgroup forming in the ultramafic lower mantle. Analysed ferropericlase grains are enriched in Fe (Mg#=0.43–0.89), which is ascribed to their origin in the lowermost mantle. The Juina kimberlites may be unique in sampling the material from depths below 1,700 km that ascended in a plume formed at the core–mantle boundary.

Exhumation of lower mantle inclusions in diamond: ATEM investigation of retrograde phase transitions, reactions and exsolution

A multiphase inclusion (KK-83) in a diamond from the Kankan deposits in Guinea, resulting from complex reactions between primary lower mantle phases, was examined in detail using analytical transmission electron microscopy. The inclusion consists of a large (300 μm) diopside crystal with a small symplectitic intergrowth in one corner, comprising olivine and tetragonal almandine pyrope phase (TAPP). The olivine part of the symplectite contains small exsolutions of Mg-Al-chromite and rare Ca-carbonate. A second inclusion of ferropericlase observed in the same diamond indicates a primary origin within the lower mantle. The clinopyroxene formed through a series of reactions at the expense of touching inclusions of CaSi- and MgSi-perovskite during convective ascent of the diamond through the transition zone. The proposed reaction sequence is: MgSiPvk+CaSiPvk→MgSiIlm+CaSiPvk→ringwoodite+stishovite+CaSiPvk→clinopyroxene+relict ringwoodite. The high Al-content of the bulk symplectite indicates ringwoodite as precursor phase coexisting with clinopyroxene. A slight deficiency of SiO 2 during the diopside forming reaction leads to a small amount of relict ringwoodite (<0.1 vol% of the total inclusion is occupied by ringwoodite) coexisting with diopside. The ringwoodite captures the observed high amount of Al. Subsequent breakdown of ringwoodite to wadsleyite+TAPP produces the observed symplectitic intergrowth. During the phase transformation of wadsleyite to olivine, exsolution of Mg-Al-chromite occurs further decreasing the original solubility of Al, Cr and Ti, in accordance with experimental data on the element partitioning between wadsleyite and olivine. Based on these observations, TAPP can form as a retrograde phase within the transition zone of the Earth’s mantle and is not restricted to the upper part of the lower mantle. High Fe 3+-contents may favour its formation.

Mineral inclusions in diamonds: associations and chemical distinctions around the 670-km discontinuity

Three new mineral associations have been discovered within diamonds from the Juina district of Brazil. These include a previously unrecorded Na-Al-(Mg, Fe)SiO3 phase associated with ferropericlase and the tetragonal almandine pyrope phase, TAPP. Also reported are an association of corundum with aluminous-pyroxene and an olivine composition phase associated with ferropericlase. The minerals in each association often occurred within the same diamonds in addition to being recovered from individual diamonds in different combinations. High-pressure experimental data indicate that these associations formed at different depths within a region ca. 60 km on either side of the upper-mantle/lower-mantle boundary. Mineral compositions show that for the regions sampled, the deep transition zone and lower mantle are chemically distinct and inhomogeneous. Importantly, in the shallow lower mantle, Al is not solely accommodated within perovskite-structured (Mg, Fe)SiO3 as some recent experimental studies have suggested.

Superdeep diamonds from the Juina area, Mato Grosso State, Brazil

Alluvial diamonds from the Juina area in Mato Grosso, Brazil, have been characterized in terms of their morphology, syngenetic mineral inclusions, carbon isotopes and nitrogen contents. Morphologically, they are similar to other Brazilian diamonds, showing a strong predominance of rounded dodecahedral crystals. However, other characteristics of the Juina diamonds make them unique. The inclusion parageneses of Juina diamonds are dominated by ultra-high-pressure ("superdeep") phases that differ both from "traditional" syngenetic minerals associated with diamonds and, in detail, from most other superdeep assemblages. Ferropericlase is the dominant inclusion in the Juina diamonds. It coexists with ilmenite, Cr-Ti spinel, a phase with the major-element composition of olivine, and SiO2. CaSi-perovskite inclusions coexist with titanite (sphene), "olivine" and native Ni. MgSi-perovskite coexists with TAPP (tetragonal almandine-pyrope phase). Majoritic garnet occurs in one diamond, associated with CaTi-perovskite, Mn-ilmenite and an unidentified Si-Mg phase. Neither Cr-pyrope nor Mg-chromite was found as inclusions. The spinel inclusions are low in Cr and Mg, and high in Ti (Cr2O3 10 wt%). Most ilmenite inclusions have low MgO contents, and some have very high (up to 11.5 wt%) MnO contents. The rare "olivine" inclusions coexisting with ferropericlase have low Mg# (87–89), and higher Ca, Cr and Zn contents than typical diamond-inclusion olivines. They are interpreted as inverted from spinel-structured (Mg, Fe)2Si2O4. This suite of inclusions is consistent with derivation of most of the diamonds from depths near 670 km, and adds ilmenite and relatively low-Cr, high-Ti spinel to the known phases of the superdeep paragenesis. Diamonds from the Juina area are characterized by a narrow range of carbon isotopic composition (δ13C=–7.8 to –2.5‰), except for the one majorite-bearing diamond (δ13C=–11.4‰). There are high proportions of nitrogen-free and low-nitrogen diamonds, and the aggregated B center is predominant in nitrogen-containing diamonds. These observations have practical consequences for diamond exploration: Low-Mg olivine, low-Mg and high-Mn ilmenite, and low-Cr spinel should be included in the list of diamond indicator minerals, and the role of high-Cr, low-Ti spinel as the only spinel associated with diamond, and hence as a criterion of diamond grade in kimberlites, should be reconsidered.

The crystal structure of “Tetragonal Almandine-Pyrope Phase” (TAPP): A reexamination

X-ray intensities for the “tetragonal almandine-pyrope phase” (TAPP), *(FeMg) VIII (AlMgFe) 4 [ IV (SiAl)O 4 ] 3 where * refers to a capped tetrahedral site, reported by Harris et al. (1997) were recollected using the identical crystal, and the crystal structure was refined from these new data. The basic structure of the previous report is confirmed; however, some of the details regarding cation occupancies are revised. The chemical composition of this phase corresponds to a high-pressure origin; however, significant questions remain regarding the crystallization pressure of this phase

Ferric Iron Content of Mineral Inclusions in Diamonds from São Luiz: A View into the Lower Mantle

Mössbauer spectroscopy on inclusions in diamonds from São Luiz, Brazil, show that (Mg,Fe)O inclusions contain little Fe 3+ (Fe 3+ /ΣFe ≤ 7%), whereas inclusions with pyroxene composition (believed to have been originally in the perovskite structure) and tetragonal almandine-pyrope phase (TAPP) inclusions contain high relative amounts of Fe 3+ (Fe 3+ /ΣFe = 20 to 75%). These observations are consistent with high-pressure experiments on synthetic samples and support the idea that the inclusions originated in the lower mantle. Fe-Mg partitioning data for co-existing (Mg,Fe,Al)(Si,Al)O 3 – (Mg,Fe)O pairs also indicate that the phases were in equilibrium at the time of diamond formation in the lower mantle.

A new tetragonal silicate mineral occurring as inclusions in lower-mantle diamonds

Aluminium is one of a small group of elements—Si, Mg, Fe, Al, Ca and O–that form the bulk of the Earth's mantle. In the upper mantle, Al is largely contained in minerals with a garnet structure. But the mineral transformations associated with the breakdown of garnet structures in the uppermost lower mantle have been a matter of uncertainty1–9. Here we report the discovery of a new aluminous mineral, present as inclusions in diamonds of lower-mantle origin found at Sao Luiz, Brazil10. This phase has a stoichiometric garnet composition similar to pyrope–almandine but has a distinct tetragonal structure (space group I42d), and is here referred to by the acronym TAPP (tetragonal almandine–pyrope phase). Although TAPP has not been recognized in experimental studies, we suggest that it is nevertheless a primary rather than retrogressive phase and has a limited stability field in relatively aluminous bulk compositions in the uppermost lower mantle. TAPP, like garnet, would lead to relatively low densities for basic rocks compared with peridotites (assumed to be the dominant mantle rocks), and we suggest therefore that it may play an important role in determining density differences and the dynamics of segregation between ultrabasic and basic compositions in the mantle7,11,12.