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The problem of depth in geology: When pressure does not translate into depth |
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| Authors: | E Moulas Y Y Podladchikov L Ya Aranovich D Kostopoulos |
| Institution: | 1. Department of Earth Sciences, ETH-Zürich, Sonneggstrasse 5, 8092, Zürich, Switzerland 2. Institute of Earth Sciences, University of Lausanne, CH-1015, Lausanne, Switzerland 3. Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, IGEM, Russian Academy of Sciences, Staromonetnyi per., 35, Moscow, 109017, Russia 4. Department of Geology, University of Johannesburg, Auckland Park, Johannesburg, 2006, South Africa 5. Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens, Athens, 15784, Greece |
| Abstract: | We review published evidence that rocks can develop, sustain and record significant pressure deviations from lithostatic values. Spectroscopic studies at room pressure and temperature (P-T) reveal that in situ pressure variations in minerals can reach GPa levels. Rise of confined pressure leads to higher amplitude of these variations documented by the preservation of α-quartz incipiently amorphized under pressure (IAUP quartz), which requires over 12 GPa pressure variations at the grain scale. Formation of coesite in rock-deformation experiments at lower than expected confined pressures confirmed the presence of GPa-level pressure variations at elevated temperatures and pressures within deforming and reacting multi-mineral and polycrystalline rock samples. Whiteschists containing garnet porphyroblasts formed during prograde metamorphism that host quartz inclusions in their cores and coesite inclusions in their rims imply preservation of large differences in pressure at elevated pressure and temperature. Formation and preservation of coherent cryptoperthite exsolution lamellae in natural alkali feldspar provides direct evidence for grain-scale, GPa-level stress variations at 680°C at geologic time scales from peak to ambient P-T conditions. Similarly, but in a more indirect way, the universally accepted’ pressure-vessel’ model to explain preservation of coesite, diamond and other ultra-high-pressure indicators requires GPa-level pressure differences between the inclusion and the host during decompression at temperatures sufficiently high for these minerals to transform into their lower pressure polymorphs even at laboratory time scales. A variety of mechanisms can explain the formation and preservation of pressure variations at various length scales. These mechanisms may double the pressure value compared to the lithostatic in compressional settings, and pressures up to two times the lithostatic value were estimated under special mechanical conditions. We conclude, based on these considerations, that geodynamic scenarios involving very deep subduction processes with subsequent very rapid exhumation from a great depth must be viewed with due caution when one seeks to explain the presence of microscopic ultrahigh-pressure mineralogical indicators in rocks. Non-lithostatic interpretation of high-pressure indicators may potentially resolve long-lasting geological conundrums. |
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