Search

The extrusion of lava domes, their potential collapse, and subsequent pyroclastic flows represent volcanic hazards. Using high-resolution time-lapse images and numerical modeling, we study here the morphological changes of the lava dome formed at Volcán de Colima between February and March 2013 and assess the rheological properties of the dome controlling the growth. Our models of lava dome growth at the volcano incorporates the crystal growth kinetics and realistic topography of the crater floor. We consider several scenarios of the dome growth with different conduit shape and the crater geometry, initial and equilibrium crystal content, and viscosity to analyze the model parameters controlling the morphology of the growing dome. The initially steep vertical growth and subsequent lateral growth are well captured by numerical modeling. The height of a lava dome depends essentially on the lava viscosity and the effusion rate. Due to the interplay between the lava extrusion rate and the gravity, the dome reaches a height threshold, and after that a horizontal gravity spreading plays an essential role in the lava dome evolution. The viscosity of the lava dome (about 1010.8 Pa s) is higher by about an order of the magnitude than the apparent viscosity of lavas from Volcán de Colima estimated experimentally, although the lava dome's core is less crystallized and hence its viscosity is about 1010.4 Pa s. [ABSTRACT FROM AUTHOR]

The ENE-trending, ca. 306–287 Ma, Totoltepec pluton is part of a Carboniferous–Permian continental magmatic arc on the western Pangaean margin. The 15 km × 5 km pluton is bounded by two N–S Permian dextral faults, an E–W thrust to the south, and an E–W normal fault to the north. Thermobarometric data indicate that the main, ca. 289–287 Ma, part of the pluton was emplaced at ≤20 km depth and ≥700°C and was exhumed to 11 km and 400°C in 4 ± 2 million years. We have documented the following intrusive sequence: (1) the 306 Ma northern marginal mafic phase; (2) the 287 Ma main trondhjemitic phase; and (3) ca. 289–283 Ma sub-vertical dikes that vary from (a) N39E, undeformed with crystal growth perpendicular to the margins, through (b) ca. N50–73E, foliated and folded with sinistral shear indicators, to (c) N73–140E and boudinaged. The obliquity of the boundary between the folded and stretched dikes relative to the N–S dextral faults suggests sequential emplacement in a transtensional regime (with 20% E–W extension), followed by different degrees of clockwise rotation passing through a shortening field accompanied by sinistral shear into an extensional field. The ca. 289–287 Ma intrusion also contains a steep ENE-striking foliation and hornblende lineations varying from sub-horizontal to steeply plunging, probably the result of emplacement in a triclinic strain regime. We infer that magmatism ceased when some of the dextral motion was transferred from the western to the eastern bounding fault, causing thrusting to take place along the southern boundary of the pluton. This mechanism is also invoked for the rapid uplift and exhumation of the pluton between ca. 287 Ma and 283 Ma. The distinctive characteristics of the Totoltepec pluton should prove useful in identifying similar tectonic settings within continental arcs. [ABSTRACT FROM AUTHOR]

Determining the formation temperature of minerals using fluid inclusions is a crucial step in understanding rock-forming scenarios. Unfortunately, fluid inclusions in minerals formed at low temperature, such as gypsum, are commonly in a metastable monophase liquid state. To overcome this problem, ultra-short laser pulses can be used to induce vapor bubble nucleation, thus creating a stable two-phase fluid inclusion appropriate for subsequent measurements of the liquid-vapor homogenization temperature, Th. In this study we evaluate the applicability of Th data to accurately determine gypsum formation temperatures. We used fluid inclusions in synthetic gypsum crystals grown in the laboratory at different temperatures between 40 °C and 80 °C under atmospheric pressure conditions. We found an asymmetric distribution of the Th values, which are systematically lower than the actual crystal growth temperatures, Tg; this is due to (1) the effect of surface tension on liquid-vapor homogenization, and (2) plastic deformation of the inclusion walls due to internal tensile stress occurring in the metastable state of the inclusions. Based on this understanding, we have determined growth temperatures of natural giant gypsum crystals from Naica (Mexico), yielding 47 ± 1.5 °C for crystals grown in the Cave of Swords (120 m below surface) and 54.5 ± 2 °C for giant crystals grown in the Cave of Crystals (290 m below surface). These results support the earlier hypothesis that the population and the size of the Naica crystals were controlled by temperature. In addition, this experimental method opens a door to determining the growth temperature of minerals forming in low-temperature environments. [ABSTRACT FROM AUTHOR]

Mineralogical processes taking place close to equilibrium, or with very slow kinetics, are difficult to quantify precisely. The determination of ultraslow dissolution/precipitation rates would reveal characteristic timing associated with these processes that are important at geological scale. We have designed an advanced high-resolution white-beam phase-shift interferometry microscope to measure growth rates of crystals at very low supersaturation values. To test this technique, we have selected the giant gypsum crystals of Naica ore mines in Chihuahua, Mexico. a challenging subject in mineral formation. They are thought to form by a self-feeding mechanism driven by solution-mediated anhydritegypsum phase transition, and therefore they must be the result of an extremely slow crystallization process close to equilibrium. To calculate the formation time of these crystals we have measured the growth rates of the {010} face of gypsum growing from current Naica waters at different temperatures. The slowest measurable growth rate was found at 55°C, 1.4±0.2×10-5nm/s, the slowest directly measured normal growth rate for any crystal growth process. At higher temperatures, growth rates increase exponentially because of decreasing gypsum solubility and higher kinetic coefficient. At 50°C neither growth nor dissolution was observed indicating that growth of giant crystals of gypsum occurred at Naica between 58°C (gypsum/an hydrite transition temperature) and the current temperature of Naica waters, confirming formation temperatures determined from fluid inclusion studies. Our results demonstrate the usefulness of applying advanced optical techniques in laboratory experiments to gain a better understanding of crystal growth processes occurring at a geological timescale. [ABSTRACT FROM AUTHOR]

Abstract: Three hypogenic caves within the Naica mine of Mexico (Cueva de los Cristales — CLC, Ojo de la Reina — OR, and Cueva de las Velas — CLV) host spectacular gypsum crystals up to 11m in length. These caves are close to another shallow cave of the area (Cueva de las Espadas — CLE), with which they cover a 160m-deep vertical section of the local drainage basin. Similar to other hypogenic caves, all these caves lack a direct connection with the land surface and should be unrelated with climate. A record of multi-technique fluid inclusion data and pollen spectra from cave and mine gypsum indicates surprisingly that climatic changes occurring at Naica could have controlled fluid composition in these caves, and hence crystal growth. Microthermometry and LA-ICP-Mass Spectrometry of fluid inclusions indicate that the shallow, chemically peculiar, saline fluid (up to 7.7eq. wt.%NaCl) of CLE could have formed from evaporation, during a dry and hot climatic period. The fluid of the deep caves was instead of low salinity (∼3.5eq. wt.% NaCl) and chemically homogeneous, and was poorly affected by evaporation. We propose that mixing of these two fluids, generated at different depths of the Naica drainage basin, determined the stable supersaturation conditions for the gigantic gypsum crystals to grow. Fluid mixing was controlled by the hydraulic communication between CLE and the other deep caves, and must have taken place during cycles of warm-dry and fresh-wet climatic periods, which are known to have occurred in the region. Pollen grains from a 35ka-old gypsum crystal of CLC corresponds to a fairly homogenous catchment basin made of a mixed broadleaf wet forest, which suggests precipitation during a fresh-wet climatic period and confirms our interpretation of the fluid inclusion data. The unusual combination of geological and geochemical factors of Naica suggests that other hypogenic caves found elsewhere may not host similar crystals. However, this work shows that fluid inclusions and pollen spectra represent a useful tool for cave studies in general, and if used in future studies might be essential to unravel the mechanisms of hypogenic deposition. [Copyright &y& Elsevier]

The article cites a study focusing on the formation of giant gypsum crystals in deep caves of the Naica mine in Mexico. According to the researchers, the growth of such sizable crystals requires precise maintenance of specific environmental conditions. Analyses of fluid inclusions, trapped sequentially in the crystals show that the temperature in the large fluid-filled caves was maintained near 54 degree Celsius for thousands of years. This temperature is just below the maximal solubility point for gypsum in low-salinity water and also slightly below the thermodynamic stability range of anhydrite. Thus, this temperature, which is close to the equilibrium allowed the formation of only a few crystal nuclei in the deep large cavities.