Strong Compositional Zonation in a Silicic Magmatic System: Los Humeros, Mexican Neovolcanic Belt

The Los Humeros volcanic center, located 180 km east of MexicoCity, is one of several silicic centers in the ‘back-arc’portion of the Mexican Neovolcanic Belt. Eruptive products spanthe compositional range from high-silica rhyolite to basalt.During the last 0?46 Ma, three major explosive eruptions andan extended period of lava flow emplacement periodically sampledan integrated magma reservoir that was initially zoned fromrhyolitic uppermost levels to andesitic and perhaps basalticlower levels, with compositional gaps in the ranges 63–67and 72–75 per cent SiO₂. The compositional zonation canlargely be explained by fractional crystallization, but mustbe accompanied by assimilation to explain the range of Sr andNd isotopic ratios. Higher than predicted concentrations ofNi, Cr, and strongly incompatible elements such as Rb and Bain andesites suggest continuous replenishment of a fractionatingchamber by mantle-derived basalts. The volumetric predominanceof rhyolite in the early history of the center points to a longperiod of accumulation of differentiates without eruptive withdrawal. Once volcanic activity started tapping the chamber, eruptiverates seem to have exceeded the rate of regeneration of differentiatedmagma. Although there is overlap in the compositional zonationof the products of successive eruptive events, the dominantvolume of each is always more mafic than that of the precedingeruption, indicating only limited regeneration of differentiatedmagmas during repose periods. This seems to have been a consequenceof the chamber remaining in approximate thermal balance duringthe last 0?46 Ma, as shown by similar Fe-Ti oxide temperaturesfor given magma compositions, regardless of age. Calculationssuggest that the chamber received thermal input through theinjection of basalt at an average rate of 0?1 km³ per thousandyears. Apparently this thermal input was too small to generatedifferentiated magma by partial melting of the wall rocks ofthe chamber, but large enough to offset conductive or hydrothermalcooling that would promote differentiation by extensive crystallization.     

Trapping of sustained turbidity currents by intraslope minibasins

Depositional turbidity currents have filled many intraslope minibasins with sediment creating targets for petroleum exploration. The dynamics of sustained turbidity currents and their depositional characteristics are investigated in a scaled physical model of a minibasin. Each turbidity current deposited a downstream thinning wedge of sediment near the inlet. Farther downstream the turbidity current was ponded by a barrier. The ponded part of the turbidity current was separated from the sediment‐free water above by a relatively sharp, horizontal settling interface indicating highly Froude‐subcritical flow. The very slow moving flow within the ponded zone created conditions for the passive rainout of suspended sediment onto the bed. In the lower part of the ponded zone, the concentration and mean grain‐size of the sediment in suspension tended to be relatively uniform in both the vertical and streamwise directions. As a result, the deposit emplaced in the ponded zone showed only a weak tendency toward downstream fining and was passively draped over the bed in such a way that irregularities in the inerodible bed were accurately reflected. The discharge of suspended sediment overflowing the downstream end of the minibasin was significantly less than the inflow discharge, resulting in basin sediment trapping efficiencies >95%. A simple model is developed to predict the trapping of sediment within the basin based on the relative magnitudes of the input discharge of turbid water and the detrainment discharge of water across the settling interface. This model shows a limiting case in which an intraslope basin captures 100% of the sediment from a ponded turbidity current, even through a succession of sustained flow events, until sediment deposition raises the settling interface above the downstream lip of the minibasin. This same process defines one of the mechanisms for minibasin filling in nature, and, when this mechanism is operative, the trap efficiency of sediment can be expected to be high until the minibasin is substantially filled with sediment.