Ringwoodite lamellae in olivine: Clues to olivine–ringwoodite phase transition mechanisms in shocked meteorites and subducting slabs

Abstract

The first natural occurrence of ringwoodite lamellae was found in the olivine grains inside and in areas adjacent to the shock veins of a chondritic meteorite, and these lamellae show distinct growth mechanism. Inside the veins where pressure and temperature were higher than elsewhere, ringwoodite lamellae formed parallel to the {101} planes of olivine, whereas outside they lie parallel to the (100) plane of olivine. The lamellae replaced the host olivine from a few percent to complete. Formation of these lamellae relates to a diffusion-controlled growth of ringwoodite along shear-induced planar defects in olivine. The planar defects and ringwoodite lamellae parallel to the {101} planes of olivine should have been produced in higher shear stress and temperature region than that parallel to the (100) plane of olivine. This study suggests that the time duration of high pressure and temperature for the growth of ringwoodite lamellae might have lasted at least for several seconds, and that an intracrystalline transformation mechanism of ringwoodite in olivine could favorably operate in the subducting lithospheric slabs in the deep Earth. N atural ringwoodite had been found in many shocked chondritic meteorites, in which the ringwoodite occurs as finegrained polycrystalline aggregates formed through a phase transition of olivine during exogenous dynamic events on the parent asteroids (1-4). It was also suggested that the olivineringwoodite transformation could take place in cold subducting slabs at the deep transition zone, where pressure oversteps the ringwoodite stability field (5-9). An experiment-produced texture characteristic of this phase transition in the slabs is the formation of ringwoodite lamellae within individual olivine crystals (6,7,(10)(11)(12). A number of laboratory experiments on compositions like Mg 2 GeO 4 , Fe 2 SiO 4 , and (Mg,Fe) 2 SiO 4 showed that the spinel-structured lamellae lie parallel to the (100) planes of olivine (6,(10)(11)(12)(13)(14)(15)(16)(17)(18). These ringwoodite lamellae form platelets from several unit cells to 100 nm in thickness (6,(10)(11)(12)(14)(15)(16)(17)(18). The formation of ringwoodite lamellae in olivine was attributed either to a martensitic transformation mechanism (6,13,(15)(16)(17)(18)(19) or an intracrystalline nucleation and growth mechanism by which coherent nucleation of ringwoodite occurs on the stacking faults of olivine (10-12). However, no natural occurrence of ringwoodite lamellae had been found so far either in the shocked meteorites or exhumed subducted slabs.Sixiangkou meteorite is a heavily shock-metamorphosed L6-chondrite with a number of shock-produced veins up to 10 mm in thickness. The shock veins contain abundant high-pressure minerals including ringwoodite, majorite, garnet, and magnesiowüstite, for which the shock-produced pressure and temperature of Ϸ20 GPa and 2,000°C were inferred (4). Polycrystalline aggregates of ringwoodite and majorite up to 100 m in size and porphyroclastic grains of olivine and pyroxene up to 1.5 mm in size distr...

Shock-Induced Olivine-Ringwoodite Transformation in the Shock Vein of Chondrite GRV053584

“…This indicates the transformation begin from the rims of the clasts. This hypothesis can be supported by the investigations of ringwoodite lamellae in olivine in the shock veins of Sixiangkou chondrite [31] and Yamato 791384 chondrite [39]. Chen et al [31] reported that ringwoodite lamellae in the rim of olivine grains are thicker and have higher densities than in their interior.…”

“…Natural ringwoodite was first discovered in the Tenham L6 chondrite [21], and so far it was identified only in shocked meteorites, such as ordinary chondrites, carbonaceous chondrites, lunar meteorites, and Martian meteorites [6,8,[22][23][24][25][26][27][28], except one terrestrial sample, as an inclusion in a tiny diamond crystal [29]. Ringwoodites in meteorites are generally found within or directly adjacent to shock-melt veins and pockets, where it mainly occurs as fine grained polycrystalline aggregates [27,30] and lamellar [22,31]. Phase transformation is a complex process.…”

The Tissint Martian meteorite as evidence for the largest impact excavation

“…The maximum shock pressure and temperature, as constrained by the former perovskite, are 25-40 GPa and c2,000 1C, respectively [14][15][16][17] . Other occurrences of high-P minerals indicate shock pressures of 13-25 GPa and temperatures of 1,700-2,600 1C in melt pockets [17][18][19][20][21][22][23][24][25] .…”

“…Other occurrences of high-P minerals indicate shock pressures of 13-25 GPa and temperatures of 1,700-2,600 1C in melt pockets [17][18][19][20][21][22][23][24][25] . Furthermore, the absence of lingunite decomposition products (calcium ferrite-type NaAlSiO 4 þ stishovite) suggests a P-T upper limit of 24 GPa at B1,800-2,000 1C (refs 26,27).…”

Ringwoodite microstructures in L-chondrite NWA 5011: implications for transformation mechanism and source region in L parent body

“…A partially transformed texture was also found in some olivine grains which were located close to the vein edge. In some cases ringwoodites appeared as lamellae, as it is shown in optical and reflected light images (Figs 4 and 5); this was first published by Chen et al (2004). The lamellae system shows at least three different orientations in a given section, which might have corresponded to rational crystallographic planes of olivine.…”