Mineralogy and structural specification of ODP Leg 125 peridotite samples

Abundant serpentinite seamounts are found along the outer high of the Mariana forearc at the top of the inner slope of the trench. One of them, Conical Seamount, was drilled at Sites 778, 779, and 780 during Leg 125. The rocks recovered at Holes 779A and 780C, respectively, on the flanks and at the summit of the seamount, include moderately serpentinized depleted harzburgites and some dunites. These rocks exhibit evidence of resorption of the orthopyroxene, when present, and the local presence of very calcic-rich diopside in veins oblique to the main high-temperature foliation of the rock. The peridotites, initially well-foliated with locally poikiloblastic textures, show overprints of a two-stage deformation history: (1) a high-temperature (>1000°C), low-stress (0.02 GPa), homogeneous deformation that has led to the present Porphyroclastic textures displayed by the rocks and (2) heterogeneous ductile shearing at a much higher stress (0.05 GPa). This heterogeneous shearing probably describes a single tectonic event because it began at high temperatures, producing dynamic recrystallization of olivine in the shear zone, and ended at low temperatures in the stability field of chlorite and serpentine. In a few samples, olivine shows evidence of quasi-hydrostatic recrystallization at a very high temperature. Here, we propose that this recrystallization was related to fluid/magma percolation, a process that can also account for the resorption of the orthopyroxene and for the late crystallization of diopside veins in the rock. The impregnation by fluid or magma, development of the main high-temperature, low-stress deformation, and subsequent migration recrystallization of olivine probably occurred in a mantle fragment involved in the arc formation. In addition, this mantle has preserved structures that may have formed earlier in the oceanic lithosphere upon which the arc formed. Heterogeneous ductile shear zones in the peridotites may have developed during uplift. The "cold" deformation may have taken place during diapiric rise of hot mantle that underwent subsequent serpentinization or gliding along normal faults associated with the extension of the eastern margin of the forearc.

Magnetic properties and chemical composition at DSDP Hole 77-538A

Magnetic properties of doleritic and some metamorphic basement rocks underlying Catoche Knoll are studied. Doleritic rocks show a high saturation magnetic moment (2-5 emu/g) compared to metamorphic rocks (0.1-1 emu/g). Magnetic minerals of rocks from this hole show a high stability when heated in vacuo up to 600°C at a fixed rate of heating. Curie temperatures are distributed close to 550°C. These properties differ markedly from those of common submarine basalts observed before. X-ray microprobe analysis techniques were used to determine internal structures of ferromagnetic minerals; in most of ferromagnetic minerals there exist two different types of magnetic phases (i.e., products of high-temperature and low-temperature oxidations). Interpretations on the coexisting, seemingly contradictory, phases can be made based upon present analyses.

Energía geotérmica de baja entalpía

El actual aumento de concentraciones de gases de efecto invernadero, el agotamiento y encarecimiento de combustibles fósiles, y el cuidado del ambiente, son los motivos principales por los cuales el uso de las energías a partir de fuentes renovables está incrementando. La energía geotérmica es un tipo de energía potencialmente renovable. La misma presenta varias tipologías, siendo la de baja entalpía la que posee mayor disponibilidad en todo el mundo. Este tipo de energía se puede utilizar para climatizar ambientes, durante todo el año, debido a la estabilidad térmica que la misma brinda. Se ha realizado un análisis del recurso presente en los primeros metros de profundidad de suelo, bajo determinadas condiciones externas y de extracción, y dentro de estas circunstancias se obtuvieron datos de temperatura de suelo, y de aire (tanto arriba como debajo de la tierra), los cuales, luego de ser estudiados, fueron comprobados matemáticamente y pudieron brindar un ábaco que sirve como herramienta para ser utilizado en distintos aprovechamientos utilizando el prototipo elaborado, pudiendo reflejar en la realidad práctica las investigaciones realizadas.

Curie temperatures and hysteresis properties of ODP Leg 183 sites, Kerguelen Plateau and Broken Ridge

In this manuscript, we present rock magnetic results of samples recovered during Leg 183. The Leg 183 cores were recovered from six drill sites and display variable rock magnetic properties. The differences in the rock magnetic properties are a function of mineralogy and alteration. Cretaceous subaerial basalt samples with titanomagnetite exhibit a strong Verwey transition in the vicinity of 110 K and have frequency-dependent susceptibility curves that resemble those of synthetic (titano) magnetites. These results are in good agreement with the thermomagnetic characteristics where titanomagnetites with Curie temperatures of ~580°C were identified. The hysteresis ratios suggest that the bulk magnetic grain size is in the psuedo-single-domain boundary. These subaerial basalts experienced high-temperature oxidation and maintained reliable paleomagnetic records. In contrast, the 34-Ma submarine pillow basalts do not show the Verwey transition during the low-temperature experiments. Thermomagnetic analysis shows that the remanent magnetization in this group is mainly carried by a thermally unstable mineral titanomaghemite. The frequency-dependent relationships are opposite of those from the first group and show little sign of titanomagnetite characteristics. Rocks from the third group are oxidized titanomagnetites and have multiple magnetic phases. They have irreversible thermaomagnetic curves and hysteresis ratios clustering toward the multidomain region (with higher Hcr/Hc ratios). The combined investigation suggests that variations in magnetic properties correlate with changes in lithology, which results in differences in the abundance and size of magnetic minerals. The rock magnetic data on Leg 183 samples clearly indicate that titanomagnetite is the dominant mineral and the primary remanence carrier in subaerial basalt. The generally good magnetic stability and other properties exhibited by titanomagnetite-bearing rocks support the inference that the ChRM isolated from the Cretaceous sites were acquired during the Cretaceous Normal Superchron. The stable inclinations identified from these samples are therefore useful for future tectonic studies.