Mechanisms of Replacement Reactions of Single Cerussite Pbco3 Crystals

1532 Goldschmidt2013 Conference Abstracts

Mechanisms of replacement reactions Vertical lithofacies changes of the of single cerussite PbCO3 crystals by Jeonchong tuff cone in the Miocene pyromorphite, mimetite and Eoil Basin, SE Korea: Implication of vanadinite a series of eruptive and depositional MONIKA KWA(NIAK-KOMINEK1 AND MACIEJ MANECKI1 processes 1AGH – University of Science and Technology, Krakow, C.W. KWON Poland ([email protected]) Geological Research Division, Korea Institute of Geosciences

and Mineral Resource, SE Korea ([email protected]) Formation of thermodynamically stable phases like pyromorphite Pb (PO ) Cl, mimetite Pb (AsO ) Cl and 5 4 3 5 4 3 The Miocene Eoil Basin, SE Korea, contains abundant vanadinite Pb (VO ) Cl are common in the environment. It is 5 4 3 volcanic and volcaniclastic deposits because of active hypothesised here that lead carbonate PbCO , cerussite, can be 3 volcanism during basin formation. The Jeonchon tuff cone readily replaced by polycrystalline lead apatites. This process refers to an about 25 m-thick sequence of basaltic lapilli tuff in is analogous to the calcium carbonate – calcium the central part of the basin. The tuff cone is underlain by hydroxylapatite system, where such transformation is pahoehoe lavas (the Lower Eoil Basalt), lacustrine mudstone, observed. However, the mechanisms of formation of and mouth-bar sandstone in ascending order, indicating pyromorphite, mimetite and vanadinite in the presence of hydrovolcanic eruption in a shallow lake. It is composed of cerussite are unknown. seven sedimentary facies: massive tuff breccia (TBm), crudely The aim of this study was to experimentally determine the stratified lapilli tuff (LTb), inverse-to-normally graded lapilli mechanism of the transformation of cerussite to pyromorphite, tuff (LTin), normally graded lapilli tuff (LTn), massive coarse mimetite and vanadinite and the conditions under with these tuff ( Tm) and massive medium to fine tuff ( Tm & Tm). It phases can form. The parameters of the experiments include C M F can be divided into five depositional units (unit I to V in pH (alkalic or acidic), the presence or absence of chloride ion, ascending order) based on facies characteristics and and temperature (140 or 5 °C). Experiments were carried out componentry. Unit I, about 4 m thick, consists of quartz- in order to produce partially reacted crystals to provide bearing massive tuffs ( Tm & Tm), suggesting contact- information about structural and textural relationships between M F surface steam explosivity within mouth-bar sand. Unit II, parent and product phases. Cerussite crystals were put into 2M about 1.5 m thick, consists of graded lapilli tuffs (LTin & (NH )H PO , 2M (NH )H VO or 2M (NH )H AsO to form 4 2 4 4 2 4 4 2 4 LTn) with abundant accidental basalt clasts, suggesting bulk- pyromorphite, vanadinite and mimetite, respectively. interaction steam explosivity within the Lower Eoil Basalt and X-Ray powder diffraction was used to identify and deposition from pyroclastic surges and ballistic fallouts of characterize the reaction products. Microstructural relationship basalt clasts. Unit III consists of tuff breccia (TBm; about 3.5 between parent and product phases were determined by m thick) and overlying stratified lapilli tuff (LTb; about 0.5 m scanning electron microscopy (SEM-EDS) and electron thick), which are interpreted to have resulted from debris microprobe analysis. flows and Surtseyan fallouts associated with vent-widening In every case single cerussite crystal were replaced by explosions. Unit IV, 0.8 m-thick and composed of massive pollycrystaline pyromorphite, mimetite and vanadinite through coarse tuff ( Tm), suggests remobilization of pyroclasts by dissolution of PbCO followed by precipitation of lead apatite. C 3 debris flows during volcanic quiescence. Crudely stratified In acidic conditions formation of shultenite PbHAsO or 4 lapilli tuff (LTb) of Unit V, about 15 m thick, is interpreted to phospho-shultenite PbHPO were also observed. Products of 4 have formed by sustained Surtseyan finger jets without further the reaction in every case are porous, allowing fluid transport excavation of the substrate. The vertical lithofacies changes to the reaction interface. At all temperatures the product phase suggest that the Jeonchon tuff cone experienced a series of and its morphology are the same. Interface coupled eruptive and depositional processes during growth because of dissolution-precipitation mechanisms are confirmed by the the changes in depositional environments, types of the textural relationships. substrate (sand vs. lava), and vent geometry. This project is supported by Polish NCN grant No 2011/01/M/ST10/06999.