Preliminary U/Th dating and the evolution of gypsum crystals in Naica caves (Mexico)

Abstract

Lauritzen: Predhodni rezultati datiranja z metodo U-Th in razvoj sadrinih kristalov v jami Naica (Mehika) Izvor, razvoj in me�anizmi rasti veliki� selenitni� kristalov v jama� Naica, so del raziskav v okviru mednarodnega projekta »Naica«. V okviru te� smo določali natančen čas začetka nukleacije sadre in stalnost rasti kristalov v času. Prvi podatki datiranj z uran-torijevo metodo kažejo na pomembne razlike v starosti kristalov sadre ( med 191 ± 13 ky za kristale v jami Ojo de la Reina in 57±1.7 ky v korenu kristala Espada) in veliko časovno obdobje rasti. Hitrost rasti kristalov je med 0,56 in 1,22 mm/ky, kar se odlično ujema z eksperimentalnimi rezultati v trenutni� pogoji� v najglobljem delu rudnika. Rezultati podpirajo tezo o več stopenjskem izločanju kristalov (prvotno v jama� gornjega nivoja, kjer je bila sadra naknadno raztopljena in kasneje v stabilni� razmera� globoko v vodonosniku) in omogočajo nove interpretacije speleogeneze na obravnavanem območju. Ključne besede: Datiranje z metodo U-Th, sadra, jame v rudniku, �itrost rasti kristalov, Naica, Me�ika. The origin and t�e evolution of giant selenite crystals in Naica caves, toget�er wit� t�e understanding of t�eir growt� mec�a-nisms, is one of t�e aims of t�e international multidisciplinary researc�, called t�e "Naica Project". In t�is context, t�e exact timing of w�en t�e gypsum nucleation started and w�et�er its growt� �as been constant over time, �ave been investigated. The preliminary data obtained wit� t�e U-Th disequilibrium met�od s�ow significant differences in ages for gypsum (between 191 ± 13 kyr for one of t�e Ojo de la Reina cave crystals and 57 ± 1.7 kyr for t�e base of Espadas cave's spar) and �ave produced a coarse c�ronological interval of growt�. The crystal depositional rates vary from 0.56 to 1.22 mm/kyr, in excellent agreement wit� t�e laboratory tests for gypsum deposition under present conditions performed in t�e deepest part of t�e mine. These results are also consistent wit� a multistage precipitation started at different times in t�e Naica caves (first in caves at t�e upper level, w�ere gypsum was subsequently dissolved, and only later in t�e deeper part of t�e aquifer under stable conditions) and allow us to improve t�e knowledge on t�e speleogenetic evolution of t�ese caves.

Wind erosion on the north‐eastern Tibetan Plateau: constraints from OSL and U‐Th dating of playa salt crust in the Qaidam Basin

“…This can be identified by small 230 Th/ 232 Th activity ratios. To correct for admixed detritus with a uniform 230 Th/ 232 Th activity ratio at the time of formation, a simplistic correction model (Peng et al, 2001;Yang and Scuderi, 2010;Sanna et al, 2011;Ma et al, 2012) is applied. The method is based on the assumption of a two-component mixing of radiogenic and detrital 230 Th to test the sensitivity of 230 Th ages.…”

Ancient Gypsum Mortars from Ornaments of Seyyed Shams al-Din Mausoleum(Yazd, Iran) Characterization; Identification of Technology of Making

“…Studies have covered the formation and growth of gypsum crystals during setting, proposing some insights: gypsum 1 crystallizes in the monoclinic system and the prismatic class [5]; gypsum crystals grow in three directions 2 [6], also (1¯11) and (1¯03) forms have convex surfaces and edges [7]; and refractive indices in gypsum crystals are 1.530, 1.523, 1.521 depending on the directions of observation [8].…”

Dating of multi-period earthquake-triggered rockfalls: a method for revealing paleo-seismic events that occurred along the Yushu fault in the eastern Tibetan Plateau

“…identified by measuring the 230 Th/ 232 Th ratio of samples, and to obtain credible 230 Th ages, they need to be corrected by adopting the general international dating pattern (Yang and Scuderi 2010;Sanna et al 2011;Ma et al 2012;Han et al 2013), which assumes that the 230 Th constituent of calcareous coat samples are composed of a radiogenic part and a detrital part. In this respect, it is key to determine how to ascertain the rational correction value of a calcareous coat.…”