The Near-Surface Stress Profile and Conditional Averaging of Turbulence

Using the conditional average formulation, we suggest a new explanation for why the stress in the atmospheric surface layer is often observed to vary with height. In essence, because turbulence series are always correlated for small lags, the steady-state equations of motion with negligible viscous terms that traditionally require vertical fluxes to be constant with height accordingly now require the vertical fluxes to vary with height. This result has implications for interpreting and validating Monin–Obukhov similarity theory.     

Evidence for plunging river plume deposits in the Pahrump Hills member of the Murray formation,Gale crater,Mars

Recent robotic missions to Mars have offered new insights into the extent, diversity and habitability of the Martian sedimentary rock record. Since the Curiosity rover landed in Gale crater in August 2012, the Mars Science Laboratory Science Team has explored the origins and habitability of ancient fluvial, deltaic, lacustrine and aeolian deposits preserved within the crater. This study describes the sedimentology of a ca 13 m thick succession named the Pahrump Hills member of the Murray formation, the first thick fine‐grained deposit discovered in situ on Mars. This work evaluates the depositional processes responsible for its formation and reconstructs its palaeoenvironmental setting. The Pahrump Hills succession can be sub‐divided into four distinct sedimentary facies: (i) thinly laminated mudstone; (ii) low‐angle cross‐stratified mudstone; (iii) cross‐stratified sandstone; and (iv) thickly laminated mudstone–sandstone. The very fine grain size of the mudstone facies and abundant millimetre‐scale and sub‐millimetre‐scale laminations exhibiting quasi‐uniform thickness throughout the Pahrump Hills succession are most consistent with lacustrine deposition. Low‐angle geometric discordances in the mudstone facies are interpreted as ‘scour and drape’ structures and suggest the action of currents, such as those associated with hyperpycnal river‐generated plumes plunging into a lake. Observation of an overall upward coarsening in grain size and thickening of laminae throughout the Pahrump Hills succession is consistent with deposition from basinward progradation of a fluvial‐deltaic system derived from the northern crater rim into the Gale crater lake. Palaeohydraulic modelling constrains the salinity of the ancient lake in Gale crater: assuming river sediment concentrations typical of floods on Earth, plunging river plumes and sedimentary structures like those observed at Pahrump Hills would have required lake densities near freshwater to form. The depositional model for the Pahrump Hills member presented here implies the presence of an ancient sustained, habitable freshwater lake in Gale crater for at least ca 10³ to 10⁷ Earth years.     

Application of new thermodynamic data to grossular phase relations

Recent low temperature, adiabatic calorimetric heat capacity measurements for grossular have been combined with DSC measurements to give entropies up to 1000 K. In conjunction with enthalpy of solution values for grossular, these data have yielded H _f ^o (298.15K) and G _f ^o (298.15K) values of –1583.2 ± 3.5 and –1496.74 ± 3.7 kcal mol^–1 respectively. For 15 reactions in the CaO-Al₂O₃-SiO₂-H₂O system, thermodynamically calculated P-T curves have been compared with experimental reversals and have shown good agreement in most cases. Calculations indicate that gehlenite is probably totally disordered. Estimates of zoisite and lawsonite entropies are consistent with the phase equilibrium and grossular data, but estimates of the entropies of pyrope and andradite show large discrepancies when compared with experimental reversals.Contribution no. 600 from the Mineralogical Laboratory, The Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan 48109, USA     

Experimental investigation of the formation of cordierite-orthopyroxene parageneses in pelitic rocks

Cordierite and orthopyroxene (or orthoamphibole) are widespread in migmatitic terranes, and partial melting of pelitic rocks may be important in their production. In particular, the reaction quartz +albite+biotite+garnet+water vapor = cordierite +orthopyroxene or orthoamphibole+melt was among reactions discussed by Grant (1973) but poorly constrained in pressure-temperature space.This reaction involves too many phases to be readily studied experimentally. Therefore simpler melting and dehydration reactions involving quartzalbite-biotite-cordierite-orthopyroxene were investigated.In conjunction with the work of Hoffer (1976, 1978) these experiments place useful constraints on the above reaction and on the reaction quartz+albite+aluminosilicate+biotite+vapor = cordierite+garnet+melt. In pelitic rocks near the second illimanite isograd, cordierite and garnet may coexist with melt as low as 660° C and cordierite and orthopyroxene may coexist with melt at temperatures less than 675° C. In the absence of significant Mn or Ca, in pelitic rocks within the realm of melting, biotite+garnet assemblages are probably limited to pressures greater than 2kb and aluminosilicate+biotite assemblages to pressures greater than 3kb.     

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.     

Substitution mechanisms and solubility of titanium in phlogopites from rocks of probable mantle origin

Previously proposed substitution mechanisms for Ti in phlogopites, based on experimental studies and crystal chemistry, have been examined using data for 81 phlogopites from mantle-derived rocks (primarily as nodules in kimberlites and also from alkali basalts, lamprophyres and carbonatites), 49 phlogopites from high-K rocks with basaltic affinities, and from 32 phlogopites crystallized in high pressure experiments mainly on high-K rock compositions. For the majority of phlogopites from the kimberlite group and for all those crystallized in the experimental studies, the substitution of Ti can be represented by a combination of the mechanisms represented by 2Mg^[VI]⇌ Ti^[VI]□^[VI] and Mg^[VI]2Si^[IV]⇌Ti^[VI]2Al^[IV]. Some phlogopites in ultrapotassic rocks have only the former substitution mechanism. The Ti contents of phlogopites generally increase with decreasing octahedral site occupancy and decreasing Si+Al^[VI]. For the phlogopites crystallized in the experiments on high-K rocks, the solubility of Ti increases with increasing fO₂ and temperature, and possibly with decreasing pressure at constant fO₂. The effect of the composition of the liquids used in the experimental studies from which these phlogopites have crystallized has only minor effect on either the substitution mechanism or the solubility of Ti in phlogopites. This suggests that phlogopite in high-K rocks may be a potential geothermometer and possibly a geobarometer.     

Upper mantle source for some hawaiites,mugearites and benmoreites

Chemical analyses of over seventy lavas or dykes containing spinel lherzolite inclusions of high pressure mineralogy, show that most host magmas are of alkali olivine basalt or basanite composition with relatively rare olivine nephelinites, and olivine melilitites. The 100 Mg/Mg+Fe⁺⁺ ratios of host magmas display a strong maximum at about Mg₇₀ consistent with partial melting of source peridotite with olivine of Fo_88–90. In contrast to these primary magmas, there occur some host magmas with 100 Mg/Mg+Fe⁺⁺<60 and with chemical compositions resembling those of classical hawaiite, mugearite, and nepheline benmoreite magmas. It is inferred that these magmas have been produced by crystal fractionation, within the upper mantle, of parental basanites or alkali olivine basalts. The presence of kaersutitic hornblende xenocrysts accompanying the lherzolite inclusions, and the nature of the chemical variation between associated basanites and nepheline benmoreites suggests that crystal fractionation has been dominated by kaersutitic hornblende, together with olivine and, in some cases, probably clinopyroxene. The mantle-derived nepheline benmoreite magmas also show similarities to some plutonic nepheline syenites.     

Parameterizing turbulent exchange over sea ice: the ice station weddell results

A 4-month deployment on Ice Station Weddell (ISW) in the western Weddell Sea yielded over 2000 h of nearly continuous surface-level meteorological data, including eddy-covariance measurements of the turbulent surface fluxes of momentum, and sensible and latent heat. Those data lead to a new parameterization for the roughness length for wind speed, z₀, for snow-covered sea ice that combines three regimes: an aerodynamically smooth regime, a high-wind saltation regime, and an intermediate regime between these two extremes where the macroscale or `permanent' roughness of the snow and ice determines z₀. Roughness lengths for temperature, z_T, computed from this data set corroborate the theoretical model that Andreas published in 1987. Roughness lengths for humidity,z_Q, do not support this model as conclusively but are all, on average, within an order of magnitude of its predictions. Only rarely arez_Tand z_Q equal to z₀. These parameterizations have implications for models that treat the atmosphere-ice-ocean system.