Earth’s clay mineral inventory and its climate interaction: A quantitative assessment

Abstract

Much work has been undertaken quantifying the abundance of phyllosilicate clay minerals in soils, sediments and rocks of Earth's crust. However, no detailed global compilation of these minerals has yet been presented. Such information has implications for understanding the interactions between weathering, soil formation, plate tectonics, climate change, diagenesis, low-temperature metamorphism and hydrothermal alteration. It is also central to understanding the distribution of bound (crystalline) and adsorbed water in upper crustal environments. This database presents the first clay mineral inventory based on mineral quantifications of 21 lithological units representing the brittle continental and oceanic crust characterised by varying degrees of water-rock interaction at temperatures between ∼ −25 and +350 °C. Published mineral abundances for >24,000 samples, determined largely by the Rietveld X-ray diffraction-based technique, indicates 21.5 weight% (wt%) of the brittle crust is composed of clay minerals: 12.9 wt% located in the upper (< 12 km) continental crust and 8.6 wt% in the thinner oceanic crust down to average depths of 6.57 km. In terms of clay mineral types, the 2:1 clay minerals with little or no expandability (commonly illite) are most abundant in Earth's upper crustal environments and total 7.7 wt%. The remaining 13.8 wt% is distributed more equally between the 1:1 kaolin-serpentine minerals (5.7 wt%), the highly expandable 2:1 smectites (3.0 wt%) and the 2:1:1 chlorites (5.1 wt%). Whereas continental soils and the underlying regolith represent only a small part of the clay mineral inventory (just 0.02 wt%), they do constitute important generating zones. These minerals and other products of surface weathering are redistributed to the main depositional sink located in sedimentary basins where 8.2 wt% of hydrous phyllosilicates reside and are modified during diagenesis or hydrothermal activity. A further 1.0 wt% are redistributed as oceanic sediment. The remaining 4.7 wt% of clay minerals in the continental crust are found in the altered igneous and crystalline rocks. Although less well quantified, the altered basalts, gabbros and ultramafic rocks of the oceanic crust, together with the overlying oceanic sediments, represent a significant part of the inventory characterised by notably high amounts of smectite, chlorite and serpentine minerals. The total amount of stored water (adsorbed and crystalline water) held in the inventory is equivalent in volume to 22% of today's surface water with approximately half located in altered oceanic crust. The trapping and release of surface water in and from the clay mineral sink likely influenced both interior crustal processes and climate change throughout Earth's history. On a shorter time scale, clay minerals aid climate stability by influencing atmospheric CO2 concentrations and the carbon cycle.