Geologists have made significant strides in unraveling the mysteries of hydrous aluminosilicates, key components found deep within the Earth's mantle, by detailing how they maintain structural stability under extreme temperatures and pressures.
Hydrous aluminosilicates serve as important carriers of water from the Earth's surface down to its interior, particularly within subducting tectonic plates. Their ability to withstand the high-temperature environments of the mantle transition zone (MTZ) has long puzzled scientists. To investigate this phenomenon, researchers synthesized single crystals of various hydrous aluminosilicates under conditions mimicking the extreme pressures and temperatures of the Earth’s interior, analyzed their structural characteristics, and elucidated the mechanisms at play.
High-pressure experiments reveal these structures formed at 15.5 to 22.0 GPa and temperatures ranging from 1400 to 1700 °C. Notably, the crystals exhibited extensive structural disorders, with aluminum (Al) and silicon (Si) atoms occupying new sites within the crystal lattice, which were previously thought to be vacant. This disorder allows for increased hydrogen incorporation within the crystals, enhancing their thermal stability. The findings suggest the transition from ordered to disordered structures is fundamental to how these minerals function at high temperatures.
This research, led by Wang et al. and published in Nature Communications, indicates the structural changes under extreme conditions significantly impact the hydrous aluminosilicates' ability to store water, thereby influencing the global water cycling processes deep within the Earth. According to the research team, "the order-to-disorder transition holds the key to the high thermal stability of hydrous aluminosilicates, significantly affecting the water cycles within the deep mantle."
The existence of water within the mantle is backed by various geophysical observations and theories. The mantle transition zone is presumed to be potentially water-rich, and the hydrous aluminosilicates, particularly identified as Topaz-OH and phase Egg, are thought to play major roles as water reservoirs. The stability of these minerals suggests they could transport significant amounts of water to depths greater than previously believed, aiding our overall comprehension of mantle dynamics and geochemical processes.
One remarkable aspect of the study involved determining the Si/Al mole ratios of the synthesized samples. The ratios were found to be lower than anticipated for their ideal chemical formulas, indicative of the existing structural disorder. For example, the Si/Al ratios for Topaz-OH I and II ranged from 0.33 to 0.45, and for phase Egg, from 0.52 to 0.58.
This structural disorder not only enhances thermal stability at extreme temperatures, but it also provides numerous sites for hydrogen incorporation, with the researchers proposing various mechanisms explaining how hydrogen enters the crystal structures. The inclusion of water enhances the water content, differing from many mantle minerals where water content typically decreases with rising temperatures. The researchers observed the hydrous aluminosilicates had higher water contents than their ordered counterparts, evidenced by techniques such as electron probe microanalysis and nanoscale secondary ion mass spectrometry.
This phenomenon indicates potential significant impacts on how water circulates within deep Earth environments. By carrying substantial amounts of water to the mantle transition zone and beyond, hydrous aluminosilicates could contribute to magma generation, mantle plumes, and possibly influence earthquake activity as they release stored water.
Overall, these findings offer new insights on deep-Earth water cycles and suggest hydrous aluminosilicates may be more integral to the dynamism of Earth's mantle than previously considered. With continued exploration of these minerals, geologists hope to deepen their comprehension of their roles within geological processes, the global water cycles, and their broader significance for Earth's geology.