Impact of grain size and rock composition on simulated rock weathering

Abstract

Abstract. Both chemical and mechanical processes act together to control the weathering rate of rocks. In rocks with micrometer size grains, enhanced dissolution at grain boundaries has been observed to cause the mechanical detachment of particles. However, it remains unclear how important this effect is in rocks with larger grains, and how the overall weathering rate is influenced by the proportion of high-and low-reactivity mineral phases. Here, we use a numerical model to assess the effect of grain size on chemical weathering and chemo-mechanical grain detachment. Our model shows that as grain size increases, the weathering rate initially decreases; however, beyond a critical size no significant decrease in the rate is observed. This transition occurs when the density of reactive boundaries is less than ∼ 20 % of the entire domain. In addition, we examined the weathering rates of rocks containing different proportions of high-and low-reactivity minerals. We found that as the proportion of low-reactivity minerals increases, the weathering rate decreases nonlinearly. These simulations indicate that for all compositions, grain detachment contributes more than 36 % to the overall weathering rate, with a maximum of ∼ 50 % when high-and low-reactivity minerals are equally abundant in the rock. This occurs because selective dissolution of the high-reactivity minerals creates large clusters of low-reactivity minerals, which then become detached. Our results demonstrate that the balance between chemical and mechanical processes can create complex and nonlinear relationships between the weathering rate and lithology.

Differentiated Neogene bauxitization of volcanic rocks (western Cameroon): Morpho-geological constraints on chemical erosion

“…Although olivines are known to be more weathering-reactive than plagioclase and pyroxenes (Eggleton et al, 1987;Schwarz, 1997), these minerals present grossly similar weathering rates (Nesbitt and Wilson, 1992), no matter the grain size (Israeli and Emmanuel, 2018). This suggests no direct (or little) influence of differential mineral sensitivity on weathering rates and geochemical transfers on different volcanic rocks.…”

Impact of textural patterns on modeled rock weathering rates and size distribution of weathered grains

“…These surrounded elements are considered to be detached physically and their elements are also reassigned to the fluid phase (Figure 1; see also Supplementary Video S1). Israeli & Emmanuel (2018) used the model with different ratios of reactive and nonreactive minerals and found a nonlinear impact of mineral composition on rock weathering rates. However different patterns of discontinuities in rocks were not examined.…”

“…In studies focused on rock weathering at the submicron scale, enhanced dissolution at grain boundaries was shown to cause the mechanical detachment of particles into the fluid phase (Emmanuel & Levenson, 2014;Fischer & Luttge, 2017;Krklec et al, 2016;Silveira & Aarão Reis, 2013). Such chemo-mechanical rock weathering was observed in micritic limestone (Levenson & Emmanuel, 2016), however particle detachment can occur in various types of rocks with larger grain sizes and different mineral compositions (Israeli & Emmanuel, 2018;Krklec et al, 2013;Levenson & Emmanuel, 2016).…”

Comment on gchron-2021-5

“…3b). Studies have shown that coarser grain sizes are more susceptible to mechanical weathering via grain detachment induced by chemical weathering (Israelli and Emmanuel, 2018); thus, although care was taken, it is possible that the luminescence depth profile (likely <10 mm based on BALL02 and BALL03) was lost during sampling and/or sample preparation due to the presence of a fragile weathering crust, potentially with a sub-surface zone of weakness (e.g. Robinson and Williams, 1987).…”