Search

Experimental studies of melting relations in the MgSiO 3 –Na 2 CO 3 (±Al 2 O 3 , Fe 2 O 3) multicomponent alkaline carbonate–silicate system were carried out at 23–24 GPa and 1100–1700 °C to shed light on the conditions and mechanisms of formation of Na-bearing bridgmanite. At 1100–1300 °C, an assemblage of low-Na bridgmanite (<0.6 wt% Na 2 O), ringwoodite, periclase (ferropericlase), alkaline carbonate with composition close to Na 2 Mg(CO 3) 2 , and sodic carbonate–silicate melts is formed. With increase in temperature up to 1700 °C, carbonate disappears and the content of Na 2 O increases in all mantle phases (up to 1.6, 4.4, and 2.7 wt% in bridgmanite, ringwoodite, and ferropericlase, respectively), which indicates that Na-bearing bridgmanite may crystallize at moderate degrees of partial melting. The clear positive correlation between the contents of Na and Al, and negative correlation between Na and Fe in bridgmanite, as well as the preferable structural incorporation of Al and Na with temperature, provide evidence for the predominant crystallization of Na-rich bridgmanite via the vacancy mechanism Si4+ B + Mg2+ A + O2− O = Al3+ B + Na+ A + V O from Al-bearing alkaline carbonate–silicate melts, with composition similar to those detected as inclusions in lower-mantle diamonds. [Display omitted] • Na-rich bridgmantite and associated alkaline phases were synthesized at 23–24 GPa and 1100–1700 °C. • Na-rich bridgmanite belongs to the 'carbonatitic' lower-mantle assemblage. • High-Na bridgmanite is compatible with alkaline carbonate–silicate melts. • Na-rich bridgmanite forms via the mechanism Si4+ B + Mg2+ A + O2− O = Al3+ B + Na+ A + V O . [ABSTRACT FROM AUTHOR]

Diamonds from the Machado River alluvial deposit have been characterised on the basis of external morphology, internal textures, carbon isotopic composition, nitrogen concentration and aggregation state and mineral inclusion chemistry. Variations in morphology and features of abrasion suggest some diamonds have been derived directly from local kimberlites, whereas others have been through extensive sedimentary recycling. On the basis of mineral inclusion compositions, both lithospheric and sublithospheric diamonds are present at the deposit. The lithospheric diamonds have clear layer-by-layer octahedral and/or cuboid internal growth zonation, contain measurable nitrogen and indicate a heterogeneous lithospheric mantle beneath the region. The sublithospheric diamonds show a lack of regular sharp zonation, do not contain detectable nitrogen, are isotopically heavy (δ 13 C PDB predominantly − 0.7 to − 5.5) and contain inclusions of ferropericlase, former bridgmanite, majoritic garnet and former CaSiO 3 -perovskite. This suggests source lithologies that are Mg- and Ca-rich, probably including carbonates and serpentinites, subducted to lower mantle depths. The studied suite of sublithospheric diamonds has many similarities to the alluvial diamonds from Kankan, Guinea, but has more extreme variations in mineral inclusion chemistry. Of all superdeep diamond suites yet discovered, Machado River represents an end-member in terms of either the compositional range of materials being subducted to Transition Zone and lower mantle or the process by which materials are transferred from the subducted slab to the diamond-forming region. [ABSTRACT FROM AUTHOR]

(Mg,Fe)SiO 3 bridgmanite is the dominant phase in the lower mantle; however no naturally occurring samples had ever been found in terrestrial samples as it undergoes retrograde transformation to a pyroxene-type structure. To identify retrograde phases of former bridgmanite single-phase and composite inclusions of (Mg,Fe)SiO 3 in a series of superdeep diamonds have been examined with electron microscopy, electron microprobe, Raman spectroscopy and X-ray diffraction techniques. Our study revealed that (Mg,Fe)SiO 3 inclusions are represented by orthopyroxene. Orthopyroxenes in single-phase and composite inclusions inherit initial chemical composition of bridgmanites, including a high Al and low Ni contents. In composite inclusions they coexist with jeffbenite (ex-TAPP) and olivine. The bulk compositions of these composite inclusions are rich in Al, Ti, and Fe, which are similar but not fully resembling Al-rich bridgmanite produced in experiments on the MORB composition. The retrograde origin of composite inclusions due to decomposition of Al-rich bridgmanite may be doubtful because each of observed minerals may represent coexisting HP phases, i.e. bridgmanite or ringwoodite. • Former bridgmanite is presented in diamonds as single-phase and composite inclusions. • The retrograde phase of former bridgmanite is orthopyroxene. • Composite inclusions with bridgmanite originates from UM/LM boudary. [ABSTRACT FROM AUTHOR]

The last decade has seen the publication of a number of new and highly pertinent studies on the composition of the Earth's lower mantle, leading to a better understanding of the Deep Earth. A series of new lower-mantle minerals were found, having formed under natural conditions and received the following names: bridgmanite, jeffbenite, breyite, and ellinaite. Some other, as yet, unnamed oxides, phosphates, and fluorides were also discovered for the first time. Among the new mineral phases, of particular interest are cubic nitrogen and ice-VII. Their presence demonstrates a significant role of both nitrogen and of water in the Deep Earth. This new data allows for creation of a principal model for the composition of the Earth's lower mantle. By various evidences, it differs greatly to that of the upper mantle composition, and is heterogeneous. • The Earth's lower mantle differs in composition of the upper mantle. • The lower mantle is heterogeneous in its composition. • The lower mantle is a great reservoir of N, H 2 O, and other volatile elements. • The enstatite-chondrite model is likely for the Earth's origin. [ABSTRACT FROM AUTHOR]