Arkadiusz Derkowski

A hydrothermal origin for isotopically anomalous cap dolostone cements from south China

The release of methane into the atmosphere through destabilization of clathrates is a positive feedback mechanism capable of amplifying global warming trends that may have operated several times in the geological past. Such methane release is a hypothesized cause or amplifier for one of the most drastic global warming events in Earth history, the end of the Marinoan ‘snowball Earth’ ice age, ∼635u2009Myr ago. A key piece of evidence supporting this hypothesis is the occurrence of exceptionally depleted carbon isotope signatures (δ13CPDB down to −48‰; ref. 8) in post-glacial cap dolostones (that is, dolostone overlying glacial deposits) from south China; these signatures have been interpreted as products of methane oxidation at the time of deposition. Here we show, on the basis of carbonate clumped isotope thermometry, 87Sr/86Sr isotope ratios, trace element content and clay mineral evidence, that carbonates bearing the 13C-depleted signatures crystallized more than 1.6u2009Myr after deposition of the cap dolostone. Our results indicate that highly 13C-depleted carbonate cements grew from hydrothermal fluids and suggest that their carbon isotope signatures are a consequence of thermogenic methane oxidation at depth. This finding not only negates carbon isotope evidence for methane release during Marinoan deglaciation in south China, but also eliminates the only known occurrence of a Precambrian sedimentary carbonate with highly 13C-depleted signatures related to methane oxidation in a seep environment. We propose that the capacity to form highly 13C-depleted seep carbonates, through biogenic anaeorobic oxidation of methane using sulphate, was limited in the Precambrian period by low sulphate concentrations in sea water. As a consequence, although clathrate destabilization may or may not have had a role in the exit from the ‘snowball’ state, it would not have left extreme carbon isotope signals in cap dolostones.

THE CHARGE OF COMPONENT LAYERS OF ILLITE-SMECTITE IN BENTONITES AND THE NATURE OF END-MEMBER ILLITE

AbstractThe nature of component layers of mixed-layer illite-smectite and their possible evolution in the course of illitization have been debated since the 1960s. The present study is a new attempt to solve these problems, using samples collected from diverse geological formations around the world. Twenty three purified illite-smectites from bentonites and hydrothermal rocks, covering the complete range of expandability, were analyzed chemically, including

ON THE PROBLEMS OF TOTAL SPECIFIC SURFACE AREA AND CATION EXCHANGE CAPACITY MEASUREMENTS IN ORGANIC-RICH SEDIMENTARY ROCKS

{rm{NH}}_4^ +

Rehydration of dehydrated-dehydroxylated smectite in a low water vapor environment

NH4+ determination, and their structural formulae were calculated. The exchangeable cations (EXCH) were plotted vs. the fixed cations (FIX) yielding the following experimental regression:n

New insight into the structural transformation of partially dehydroxylated pyrophyllite

{rm{EXCH}} = - 0.43 times {rm{FIX}} + 0.41;;left( {{{rm{R}}^2} = 0.98} right)

Nature of rehydroxylation in dioctahedral 2:1 layer clay minerals

EXCH=−0.43×FIX+0.41(R2=0.98)FIX and EXCH depend on the charge of the illite (Qi) and smectite (Qs) interlayers, and the fractions of these interlayers in the bulk clay leading to:n

Nature of the structural and crystal-chemical heterogeneity of the Mg-rich glauconite (Riphean, Anabar Uplift)

{rm{EXCH}} = - {Q_{rm{s}}}{rm{/}}{Q_{rm{i}}} times {rm{FIX}} + {Q_{rm{s}}}