Seasonal growth in the bivalveMacoma balthica near the southern limit of its range

Shell-length growth inMacoma balthica from San Francisco Bay, California, as measured on living animalsin situ, is highly seasonal despite a mild Mediterranean climate: a long period of near non-growth from May to the following February is followed by a short period of rapid growth between March and May. The rapid-growth period follows the spawning period during January/February and ends as water temperature rises above about 15°C. Despite the shortness of the growth period,M. balthica grows larger at a given age in San Francisco Bay than is recorded elsewhere in the world. Application of a model, developed elsewhere from these same field measurements, shows that (1) measurable growth occurs during the summer/autumn/early winter “nongrowth” period, (2) there is an autumn recruitment, and (3) both spring and autumn recruits combine to form a single “one-year-old” size grouping. None of these features is detectable through growth-ring analysis of field samples, apparently because of indistinct climatic seasons, or through size-frequency histogram analysis because of the combined effects of slow growth and intermittent recruitment.     

Total individual ion activity coefficients of calcium and carbonate in seawater at 25°C and 35%. salinity,and implications to the agreement between apparent and thermodynamic constants of calcite and aragonite

We have calculated the total individual ion activity coefficients of carbonate and calcium, and , in seawater. Using the ratios of stoichiometric and thermodynamic constants of carbonic acid dissociation and total mean activity coefficient data measured in seawater, we have obtained values which differ significantly from those widely accepted in the literature. In seawater at 25°C and 35%. salinity the (molal) values of and are $\text{0.038 ± 0.002}$ and $\text{0.173 ± 0.010}$, respectively. These values of and are independent of liquid junction errors and internally consistent with the value . By defining and on a common scale (), the product is independent of the assigned value of and may be determined directly from thermodynamic measurements in seawater. Using the value and new thermodynamic equilibrium constants for calcite and aragonite, we show that the apparent constants of calcite and aragonite are consistent with the thermodynamic equilibrium constants at 25°C and 35%. salinity. The demonstrated consistency between thermodynamic and apparent constants of calcite and aragonite does not support a hypothesis of stable Mg-calcite coatings on calcite or aragonite surfaces in seawater, and suggests that the calcite critical carbonate ion curve of Broecker and Takahashi (1978, Deep-Sea Research25, 65–95) defines the calcite equilibrium boundary in the oceans, within the uncertainty of the data.     

Phase relations inferred from field data for mn pyroxenes and pyroxenoids

Electron microprobe analysis of manganese silicates from Balmat, N.Y., has helped elucidate phase relations for Mn-bearing pyroxenes and pyroxenoids. A compilation of these data along with published and unpublished analyses for phases plotting on the CaSiO₃-MgSiO₃-MnSiO₃ and CaSiO₃-FeSiO₃-MnSiO₃ faces of the RSiO₃ tetrahedron has constrained the subsolidus phase relations. For the system CaSiO₃-FeSiO₃-MnSiO₃, the compositional gaps between bustamite/hedenbergite, bustamite/ rhodonite and rhodonite/pyroxmangite are constrained for middle-upper amphibolite facies conditions and extensive solid solutions limit possible three phase fields. For the CaSiO₃-MgSiO₃-MnSiO₃ system much less data are available but it is clear that the solid solutions are much more limited for the pyroxenoid structures and a continuum of compositions is inferred for clinopyroxenes from diopside to kanoite (MnMgSi₂O₆) for amphibolite facies conditions (T=650° C). At lower temperatures, Balmat kanoites are unstable and exsolve into C2/c calciumrich (Ca_0.68Mn_0.44Mg_0.88Si₂O₆) and C2/c calciumpoor (Ca_0.12Mn_1.02Mg_0.86Si₂O₆) phases. At temperatures of 300–400° C the calcium-poor phase subsequently has undergone a transformation to a P2₁/c structure; this exsolution-inversion relationship is analogous to that relating augites and pigeonites in the traditional pyroxene quadrilateral. Rhodonite coexisting with Mn-clinopyroxenes is compositionally restricted to Mn_0.75–0.95Mg_0.0–0.15Ca_0.05–0.13SiO₃. For the original pyroxene+rhodonite assemblage, the Mg and Ca contents of the rhodonite are fixed for a specific P (6kbars)-T (650° C)-X(H₂O)-X(CO₂) by the coexistence of talc+quartz and calcite+quartz respectively.Contribution No. 363, from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor MI 48109, USA     

Seasonal oxygen depletion in Chesapeake Bay

The spring freshet increases density stratification in Chesapeake Bay and minimizes oxygen transfer from the surface to the deep layer so that waters below 10 m depth experiece oxygen depletion which may lead to anoxia during June to September. Respiration in the water of the deep layer is the major factor contributing to oxygen depletion. Benthic respiration seems secondary. Organic matter from the previous year which has settled into the deep layer during winter provides most of the oxygen demand but some new production in the surface layer may sink and thus supplement the organic matter accumulated in the deep layer.     

The thermodynamic properties and phase relations of some minerals in the system CaO-AI2O3-SiO2-H2O

The heat capacities of lawsonite, margante, prehnite and zoisite have been measured from 5 to 350 K with an adiabatic-shield calorimeter and from 320 to 999.9 K with a differential-scanning calorimeter. At 298.15 K, their heat capacities, corrected to end-member compositions, are 66.35, 77.30, 79.13 and 83.84 cal K^?1 mol^?1; their entropies are 54.98, 63.01, 69.97 and 70.71 cal K^?1 mol^?1, respectively. Their high-temperature heat capacities are described by the following equations (in calories, K, mol): Lawsonite (298–600 K): Cp° = 66.28 + 55.95 × 10^?3T ? 15.27 × 10⁵T^?2 Margarite (298–1000 K): Cp° = 101.83 + 24.17 × 10^?3T ? 30.24 × 10⁵T^?2 Prehnite (298–800 K): Cp° = 97.04 + 29.99 × 10^?3T ? 25.02 × 10⁵T^?2 Zoisite (298–730 K): Cp° = 98.92 + 36.36 × 10^?3T ? 24.08 × 10⁵T^?2 Calculated Clapeyron slopes for univariant equilibria in the CaO-Al₂O₃-SiO₂-H₂O system compare well with experimental results in most cases. However, the reaction zoisite + quartz = anorthite + grossular + H₂O and some reactions involving prehnite or margarite show disagreements between the experimentally determined and the calculated slopes which may possibly be due to disorder in experimental run products. A phase diagram, calculated from the measured thermodynamic values in conjunction with selected experimental results places strict limits on the stabilities of prehnite and assemblages such as prehnite + aragonite, grossular + lawsonite, grossular + quartz, zoisite + quartz, and zoisite + kyanite + quartz. The presence of this last assemblage in eclogites indicates that they were formed at moderate to high water pressure.     

An analytical electron microscopic study of a pyroxene-amphibole intergrowth

Samples of a garnet granulite from the mafic border units of the Lake Chatuge, Georgia alpine peridotite body were found to contain lamellar intergrowths of a pargastic amphibole in augite having the typical appearance of an exsolution feature. Single crystal X-ray diffraction, optical, electron microprobe and conventional and analytical electron microscopic studies have provided data limiting the compositions and structures of the coexisting phases. Individual lamellae of both materials are from 0.5 to 2.0 m in width with the lamellar interface parallel to {0 1 0}. The formulae of the minerals, as determined by a combination of electron microprobe and analytical electron microscopy, are (Na_0.1Ca_1.0Mg_0.6Fe³⁺ _0.3)(Si_1.8Al_0.2)O₆ for the pyroxene and Na_0.7Ca_1.9(Mg_2.1Fe²⁺ _1.4Fe³⁺ _0.5Ti_0.1Cr_0.1Al_0.8)(Si_5.9Al_2.1) O₂₂(OH)₂ for the amphibole. Several other studies have described intergrowths similar to those observed in this work, in general favoring exsolution as the formation mechanism for the intergrowths. In the Lake Chatuge samples however, replacement of pyroxene by amphibole is in part indicated by continuous gradation of amphibole lamellae into amphiboles rimming the clinopyroxenes.Contribution No. 368 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan     

Petrogenetic model for the origin of carbonatites

The petrological significance and distribution of igneous carbonatites is discussed. Particular attention is paid to the relationship that exists between carbonatites and volcanism in East Africa. Using a simple model of a carbonatite-nephelinite-ijolite volcanic complex it was discovered that all the peralkaline and carbonatitic rocks found in such a complex could be derived from a single primary nephelinitic magma. The origin of such a primary magma, and the subsequent evolution carbonatitic sub-magmas, is examined.     

Mössbauer spectra of synthetic Ca-Fe pyroxenoids and lunar pyroxferroite

Resolution of the iron sites in the Mössbauer spectra of Ca-Fe pyroxenoids is only partial, two doublets being apparent in each case, whereas there are up to nine cation sites in each structure. Thus the inner and outer doublets represent, respectively: two sites and two sites in Ca_0.5Fe_0.5SiO₃ (ferrobustamite); five sites and two sites in Ca_0.15Fe_0.85SiO₃ (pyroxferroite); and five sites and four sites in FeSiO₃ (ferrosolite III). Disorder of iron and calcium which exists in the two calcium-bearing pyroxenoids is not readily apparent in the spectra.Publication authorized by the Director, U. S. Geological Survey.     

Evolution of volcanic islands

It is shown that there is no significant correlation between the chemistry of volcanic islands and their distance from the axes of oceanic rises. The composition and the distribution of those oceanic volcanic islands that are not associated with subduction zones is considered to be related to the distribution of a series of thermal plumes that originated either within the upper mantle or at the coremantle interface.