Ultramafic inclusions from San Carlos, Arizona: Petrologic and geochemical data bearing on their petrogenesis

Hydrothermal vents jetting out water at 380° ± 30°C have been discovered on the axis of the East Pacific Rise. The hottest waters issue from mineralized chimneys and are blackened by sulfide precipitates. These hydrothermal springs are the sites of actively forming massive sulfide mineral deposits. Cooler springs are clear to milky and support exotic benthic communities of giant tube worms, clams, and crabs similar to those found at the Galapagos spreading center. Four prototype geophysical experiments were successfully conducted in and near the vent area: seismic refraction measurements with both source (thumper) and receivers on the sea floor, on-bottom gravity measurements, in situ magnetic gradiometer measurements from the submersible Alvin over a sea-floor magnetic reversal boundary, and an active electrical sounding experiment. These high-resolution determinations of crustal properties along the spreading center were made to gain knowledge of the source of new oceanic crust and marine magnetic anomalies, the nature of the axial magma chamber, and the depth of hydrothermal circulation.

https://science.sciencemag.org/content/sci/207/4438/1421.full.pdf

East Pacific Rise: Hot Springs and Geophysical Experiments

In many coastal plain estuaries light attenuation by suspended sediments confines the photic zone to a small fraction of the water column, such that light limitation is a major control on phytoplankon production and turnover rate. For a variety of estuarine systems (e.g. San Francisco Bay, Puget Sound, Delaware Bay, Hudson River plume), photic-zone productivity can be estimated as a function of phytoplankton biomass times mean irradiance of the photic zone. Net water column productivity also varies with light availability, and in San Francisco Bay net productivity is zero (estimated respiratory loss of phytoplankton balances photosynthesis) when the ratio of photic depth ( Z p ) to mixed depth ( Z m ) is less than about 0.2. Thus whenever Z p : Z m Much of the spatial and temporal variability of phytoplankton biomass or productivity in estuaries is explained by variations in the ratio of photic depth to mixed depth. For example, phytoplankton blooms often coincide with stratification events that reduce the depth of the surface mixed layer (increase Z p : Z m ). Shallow estuarine embayments (high Z p : Z m ) are often characterized by high phytoplankton biomass relative to adjacent channels (low Z p : Z m ). Many estuaries have longitudinal gradients in productivity that mirror the distribution of suspended sediments: productivity is low near the riverine source of sediments (low Z p : Z m ) and increases toward the estuary mouth where turbidity decreases. Some of these generalizations are qualitative in nature, and detailed understanding of the interaction between turbidity and estuarine phytoplankton dynamics requires improved understanding of vertical mixing rates and phytoplankton respiration.

Turbidity as a control on phytoplankton biomass and productivity in estuaries

The mineralogy, geochemistry and origin of Iherzolite inclusions in Victorian basanites

The bulk chemical compositions, mineralogy, and mineral proportions of sands and muds of the Mallacoota Basin in southeastern Australia reflect the composition of weathering profiles mantling source rocks, rather than bedrock. Muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles. Sands are strongly enriched in quartz relative to source rocks, even in the headwaters of the fluvial system, demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system. K-feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or K-feldspar in weathering profiles. Subsequent erosion and sorting produce sands enriched in quartz, with high K-feldspar:plagioclase ratios relative to source rocks. The composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat...

Effects of Chemical Weathering and Sorting on the Petrogenesis of Siliciclastic Sediments, with Implications for Provenance Studies

Problems of petrologic mixing have been solved using a two-stage computer-based calculation. First, linear programming is used to obtain an approximate solution and to identify non-negative solution values. Then a conventional least squares calculation is performed using the analyses represented by non-negative solution values as input to yield an optimum set of solution values. The error attached to each solution value is estimated by an empirical procedure. Petrologic application of the program has been demonstrated with three types of calculations: chemical mode, magma mixing, and liquid line of descent.

Patrick C. Doherty

Papers

A Linear Programming and Least Squares Computer Method for Solving Petrologic Mixing Problems

Location of the non-tidal current null zone in northern San Francisco Bay