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<p>We discovered a brucite, quartz and moissanite bearing natural rock of mantle affinity containing relics of two now decomposed minerals, so far known from meteorites only, constituting ~ 4 vol% and scattered in the groundmass of the ~ 100 gr sample. At the TEM/SAED scale, <i>phase-I</i> now consist of porous-less intergrowths of 100–500&#xa0;nm-sized forsterite-enstatite-Fe-oxide, where Fo<sub>99.9</sub> and En<sub>99.7</sub> show both individual and combined [100] and [001] preferred orientations. At the EMPA/WDS scale, <i>phase-I</i> shows (Mg<sub>2.71</sub>Fe<sup>2+</sup><sub>0.24</sub>Al<sub>0.02</sub>)<sub>∑=2.97</sub>(Si<sub>1.99</sub>Al<sub>0.01</sub>)<sub>∑=2</sub>O<sub>7</sub> composition. We interpret EMPA chemical and TEM structural combined data according to the decomposition reaction (Mg, Fe)<sub>3</sub>Si<sub>2</sub>O<sub>7</sub> → Mg<sub>2</sub>SiO<sub>4</sub> + MgSiO<sub>3</sub> + Fe<sub>x</sub>O<sub>y</sub>. At the EMPA/WDS scale <i>phase-II</i> corresponds to (Mg<sub>3.84</sub>Fe<sup>2+</sup><sub>0.18</sub>)<sub>∑=4.02</sub>(Si<sub>2.97</sub>Al<sub>0.01</sub>)<sub>∑=2.98</sub>O<sub>10</sub>, while at the TEM/SAED scale it consists of 80–90 vol% amorphous material comprising clusters of ~ 100&#xa0;nm-sized forsterite. All collected data would suggest former High-Temperature and Ultra-High-Pressure (HT-UHP) phases, now preserved as low-P transformed relics. The mineral precursors would potentially be main constituents of rocky planets’ mantle like ours. As their compositions would be very close to the pyrolite (pristine) model mantle, they would be likely common in the deep mantle. Our discovery would open the possibility for fine-tuning models of Earth seismic discontinuities to novel minerals and structures.</p>

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Relics of possible (Mg,Fe)3Si2O7 and (Mg,Fe)4Si3O10: potential messengers from the Earth’s mantle

  • 期刊论文

  • 发表日期：

    2026年1月20日

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  • Simonpietro Di Pierro,
  • Edwin Gnos,
  • Damien Jacob,
  • Bertrand Devouard,
  • Hugues Leroux,
  • Pascal Roussel

摘要

We discovered a brucite, quartz and moissanite bearing natural rock of mantle affinity containing relics of two now decomposed minerals, so far known from meteorites only, constituting ~ 4 vol% and scattered in the groundmass of the ~ 100 gr sample. At the TEM/SAED scale, phase-I now consist of porous-less intergrowths of 100–500 nm-sized forsterite-enstatite-Fe-oxide, where Fo99.9 and En99.7 show both individual and combined [100] and [001] preferred orientations. At the EMPA/WDS scale, phase-I shows (Mg2.71Fe2+0.24Al0.02)∑=2.97(Si1.99Al0.01)∑=2O7 composition. We interpret EMPA chemical and TEM structural combined data according to the decomposition reaction (Mg, Fe)3Si2O7 → Mg2SiO4 + MgSiO3 + FexOy. At the EMPA/WDS scale phase-II corresponds to (Mg3.84Fe2+0.18)∑=4.02(Si2.97Al0.01)∑=2.98O10, while at the TEM/SAED scale it consists of 80–90 vol% amorphous material comprising clusters of ~ 100 nm-sized forsterite. All collected data would suggest former High-Temperature and Ultra-High-Pressure (HT-UHP) phases, now preserved as low-P transformed relics. The mineral precursors would potentially be main constituents of rocky planets’ mantle like ours. As their compositions would be very close to the pyrolite (pristine) model mantle, they would be likely common in the deep mantle. Our discovery would open the possibility for fine-tuning models of Earth seismic discontinuities to novel minerals and structures.