Trace and Rare Earth Element Distribution and Mobility During Diagenetic Alteration of Volcanic Ash to Bentonite in Eastern Iranian Bentonite Deposits

Abstract

The chemical composition of the source magma along with the physiochemical conditions of the depositional environment are the main controlling factors in determining the behavior and mobility of trace elements and rare earth elements (REEs) during the transformation of volcanic ash to bentonite. The purpose of the present study was to determine the distribution pattern and mobility of trace elements and REEs in several bentonite deposits formed by diagenetic alteration of volcanic ash in shallow alkaline water in eastern Iran. Using geochemical and statistical data, the degree of weathering in the parent rocks and the distribution and mobility of trace elements and REEs during the alteration process at seven deposits (Chah-Taleb, Chah-Keshmir, Chah-Golestan, Chah-Pirouz, Gholeh-Gelia, Kharman-Sar, and Khal-Kooh) were studied. None of the parent rocks showed an advanced degree of depositional reworking and, therefore, their chemical composition is representative of the volcanic ash from which the bentonites were formed. In the chondrite-normalized REE patterns of both parent rocks and bentonites, the light rare earth elements (LREEs) were found to be enriched relative to the heavy rare earth elements (HREEs). The variation in δEu and δCe values suggested a high-temperature (<200°C), suboxic, aquatic environment in which the conversion of volcanic ash to bentonite occurred. In the bivariate correlation analysis, Si showed a strong inverse relationship with Al and LREEs, while large ion lithophile elements (LILEs), high field strength elements (HFSEs), HREEs, and LREEs displayed a positive correlation between the elements of their respective groups. The R2 values in the binary diagram of potential immobile elements against Al suggest a qualitative classification in which Ti, Gd, Ga, Pr, Tb, Nd, Sm, Ce, and Nb are considered immobile, and U, Dy, In, Sc, Hf, Zr, La, and Eu are considered to have had poor mobility during formation of the Eastern Iranian bentonite deposits.

Assessing trace-element mobility during alteration of rhyolite tephra from the Dinaride Lake System using glass-phase and clay-separate laser ablation inductively coupled plasma mass spectrometry

“…Altered volcanic ash may provide insights into the dynamics of trace-element depletion and enrichment during the low-temperature, post-emplacement alteration of primary igneous lithologies due to the increased solubility of trace elements under such conditions (McHenry, 2009; Hong et al , 2019; Namayandeh et al , 2020). Distal volcanic tephra, and ash in particular, consist largely of metastable volcanic glass, which is prone to deuteric alteration in aqueous environments (Christidis & Huff, 2009; Huff, 2016).…”

Middle Miocene volcanic flare up preceding and synchronous with the Langhian/Serravallian sea‐level decline in the North Pannonian Basin: Insights from 40Ar/39Ar dating, geo‐seismic analysis and 3D visualization of the subterranean Kráľová stratovolcano

“…To minimize misinterpretations only non‐mobile elements were used for interpretations (Figure 6a–c; Hastie et al., 2007; Pearce, 1983, 1996). However, also trace element diagrams can be impacted by the mobility of some ‘immobile’ elements during the formation of bentonite or zeolite minerals (Christidis, 1998; Namayandeh et al., 2020). For example, the mobility of Y can cause a shift of vitric tuff samples towards a more alkaline composition in the diagram modified by Pearce (1996).…”

Fluorine speciation and origin of Early-Middle Triassic bentonite deposits in Sichuan Basin, South China

“…Bentonites are volcanic ashes that have undergone significant devitrification to dioctahedral smectite, and volcanic ashes are a precursor material for bentonites (Altaner et al, 1984;Huff, 2016;Namayandeh et al, 2020). Generally, smectite formed in marine subaqueous environments during diagenetic alteration of volcanic glass shards, releasing alkalis and alkaline Earth elements after initial hydration and cation exchange between the fluids and volcanic glass shards (De La Fuente et al, 2000;Huff, 2016;Hong et al, 2019).…”