Search

Crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae. Spherulites typically range from microscopic to a few centimeters in diameter. Larger spherulites, termed "megaspherulites," are rare but have been reported in the USA and Mexico. The uncommon nature of such structures supports the need for a study to explore the factors and special conditions that allow them to reach such a large size. In the Southern Puna of Argentina, El Viejo Coulee includes megaspherulites up to 4 m in diameter. We present observations from field work, petrography, scanning electron microscopy, bulk geochemistry, and Fourier transform infrared spectroscopy. The megaspherulites occur in obsidian lenses that differ in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee. The obsidian lacks vesicles and microlites and is unaltered. The megaspherulites comprise growth cones consisting of micropoikilitic texture where quartz encloses potassium feldspar. The growth cones are separated by interconal areas composed of lithophysae. We propose that the megaspherulites were formed above the glass transition temperature (Tg) and are the product of primary crystallization of rhyolitic melt. The exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperatures above Tg during crystallization. The large size also suggests scarcity of nucleation sites, which is consistent with the megaspherulites being hosted by unaltered microlite- and vesicle-free glass. The position of the obsidian lens at the base of the coulee may have played a critical role in maintaining the temperature above the Tg long enough to allow the crystallization of the megaspherulites. These conditions also favored crystallization in the most advanced stage where micropoikilitic texture replaced the fans of crystal fibers typical of spherulites. Crystallization of anhydrous quartz and feldspar in the growth cones led to the concentration of volatiles in the melt in the interconal areas, resulting in volatile exsolution and formation of vesicles that became nucleation sites for lithophysae. The study advances our understanding of some of the special processes that are involved in the cooling and solidification of rhyolitic magmas. Fundamentally, we find that the position of the obsidian at the base of the coulee was critically important because this position favored maintenance of the temperature above the Tg which, in turn, favored high diffusion rates. Also, the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulites to nucleate; those that nucleated therefore grew very large. The meter-scale megaspherulites may have taken ~ 55 years to create. [ABSTRACT FROM AUTHOR]

The biosphere reserve El Triunfo (REBITRI for its Spanish acronym) is an important regulator of streams that provide water resources for different watersheds in Mexico. Nevertheless, the assessment of ecosystem services provided by the Biosphere Reserve is still rare. In the Cuxtepeques river basin a water balance infiltration capacity was studied with the aim to point out the importance of the REBITRI as a recharge zone. The results show that the REBITRI, located in the upper part of the watershed, plays a decisive role in water catchment, being 2,5 times more than outside of the biosphere reserve. Forest conservation in the REBITRI is highly important as this land use is fundamental to maintain ecosystem services and its functioning. [ABSTRACT FROM AUTHOR]

The Water Seed Foundation looks for the effective investment of conservation areas in six tributary subbasins in order to reduce the reservoir bottom profile (sediment deposition) of Chicoasen and La Angostura dams at Chiapas, México. As inputs to the conservation criteria workshop it was calculated the sediment and erosion contribution of each subbasin by map algebra for the Universal Soil Lost Equation (USLE) and Soil and Water Assessment Tool (SWAT). As a result only 14% (120 759,18 ha) of the total area of subbasins (850 770 ha) are prospected for conservation. [ABSTRACT FROM AUTHOR]

Nine municipalities have territorial connectivity with the biosphere reserve El Triunfo (BIRETRI) and eleven with water connectivity through 14 hydrographic basins. The water balance in four basins shows BIRETRI as a strategic zone for the recharge and water regulation. The importance of the water resource is evidenced by: 1) the 1,804 titles of deed of use and the tap water supply for 325,000 residents and 2) the flood damage risk. The resulting generated information will be useful in the development of mechanisms that may improve competitiveness of the territory based on a basin perspective. [ABSTRACT FROM AUTHOR]

With the objective of determining the influence as a recharge area over the Biosphere Reserve The Triumph (REBITRI), it was estimated the water balance of Pijijiapan river watershed since its high basin is included into the reserve polygon. It was estimated an annual rainfall of 653,8 millions of m³, 12% of which is runoff, 64% is evapotranspirated and 24% becomes infiltration. The potential recharge volume of the watershed is 2,6 higher at the area inside the REBITRI than outside. The results remark the role of the REBITRI as a water recharge zone, as well as its erosion and flood regulation effect in the watershed of Pijijiapan. [ABSTRACT FROM AUTHOR]