Garnet–orthopyroxene geothermometry and geobarometry: error propagation and equilibration effects

Garnet–orthopyroxene geothermometry and geobarometry are widely used in high-grade metamorphic terranes. These techniques may provide an insight into pressure–temperature ( P–T   ) paths followed by such terranes, provided various sources of uncertainty are taken into consideration. Analytical uncertainties, particularly with regard to their effect on ferric iron estimation in orthopyroxene, can contribute to the overall uncertainty on the calculated P–T  . Additionally, retrograde cation diffusion can affect the Fe–Mg distribution between coexisting garnet and orthopyroxene, consequently affecting P–T  estimates. Recognizing the importance of these effects, and care with both the choice of the grains analysed and analytical techniques, may lead to more reliable P–T  estimation.     

Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith,southern California

Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and typically represent fragments of mafic magmas, cumulates, restites, or country rocks. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with the host tonalite, the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have geochemical characteristics that overlap those of literature-compiled igneous dark enclaves. When compared with modelled igneous differentiation paths, metasedimentary enclaves can have anomalous CaO and K₂O contents for a given SiO₂, but other major-element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that literature-compiled mafic enclaves trend towards high K₂O + CaO suggests that not all mafic enclaves are of igneous origin. In this work, we provide criteria for identifying enclaves of possible metasedimentary origin.     

Geochemistry of microgranular enclaves and host granite from Telões (Vila Pouca de Aguiar), Northern Portugal

At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe²⁺-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and (⁸⁷Sr/⁸⁶Sr)₀=0.7063±0.0011. Microgranular enclaves have δ¹⁸O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas.     

The truth and beauty of chemical potentials

This essay in honour of Mike Brown addresses aspects of chemical equilibrium and equilibration in rocks, with a focus on the role that chemical potentials play. Chemical equilibrium is achieved by diffusive flattening of chemical potential gradients. The idea of equilibration volume is developed, and the way equilibration volumes may evolve along a pressure–temperature path is discussed. The effect of the environment of an equilibration volume is key to understanding the evolution of the equilibration volume with changing conditions. The likely behaviour of equilibration volumes is used to suggest why preservation of equilibrium mineral assemblages and mineral compositions from metamorphism tends to occur. This line of logic then provides the conceptual support to conventional equilibrium thermodynamic approaches to studying rocks, using, for example, thermobarometry and pseudosections.     

Isotopic fractionation induced by a surface effect influences the estimation of the hydrological process of topsoil

Isotopic heterogeneity in soil water has hindered the application of isotope compositions (δ¹⁸O and δ²H) in soil water dynamics. This heterogeneity has been suggested to be caused by soil properties such as organic matter (OM) and clay content. However, this is yet to be verified in field soil. We sampled the organic layer (O-horizon soil) with highly decomposed organic material and the A-horizon soil in western Sichuan, China, and equilibrated these samples with vapour created by unconfined labelling water. The relationship between soil properties and isotopic fractionation (ε_T/U) between unconfined water and the total soil water was used to determine the line-conditioned excess (lc-excess) and source rain of A-horizon field soil by removing the influence of confined water. Equilibration experiments demonstrated a significant isotopic difference between the ε_T/U levels in the A-horizon and O-horizon soils, indicating that OM plays an important role in isotopic fractionation. In field samples, the lc-excess of the unconfined A-horizon water was, on an average, 2.5‰ higher than that of bulk soil water. The average offsets between the annual rain and the estimated source rain of soil water decreased by 5.0 and 0.5‰ for hydrogen and oxygen after removing the influence of confined water. Isotopic heterogeneity should not be ignored while examining the evaporation of soil water, soil source rain, and hence the recent ‘two water worlds’ hypothesis, which is especially true for cases in which the soils contain high levels of OM.     

Nonlinear baroclinic equilibration at finite supercriticality

The nonlinear equilibration of finite amplitude baroclinic waves in Phillips two-layer model is investigated at finite supercriticality. The aims are to quantify the robustness and relevance of the nonlinear theory of Warn, Gauthier and Pedlosky (WGP) for the evolution of the developing baroclinic wave, and to assess the tightness of pseudomomentum and improved pseudoenergy bounds for disturbance amplitude and energy. A high-resolution numerical model is used to perform a parameter sweep in (β,?W)-space, where β is the inverse criticality of the initial flow, and W is the ratio of the channel width to the (internal) Rossby radius. At low supercriticalities, the main predictions of WGP are found to be accurate at short times, but at long times the fully nonlinear results are found to diverge from WGP's solution. The mechanism for equilibration involves the elimination of the lower layer potential vorticity (PV) gradient, but as the supercriticality increases this is achieved by the roll-up of a train of opposite-signed vortices, rather than by coarse-grain PV homogenization as in WGP. Peak wave amplitudes are typically ≈90% of the maximum attainable under the pseudomomentum bound. New formulae are given for the pseudoenergy bound on disturbance energy which, unlike the WGP solution and the pseudomomentum bound, have non-trivial dependence on W. A detailed assessment is made of the extent to which these bounds are attained.     

NaCl不饱和的海水体系卤水—矿物平衡研究

本文用实验的方法研究了Ｋ，Ｎａ，Ｍｇ，Ｃｌ，ＳＯ４－Ｈ２Ｏ体系在ＮａＣｌ不饱和条件下的卤水－矿物平衡关系，并与该体系对ＮａＣｌ饱和的相图作了对比。研究成果有利于更深入地探讨盐类矿床的形成和演化，对钾盐生产工艺的制订亦有重要指导作用。本文对四川盆地富钾卤水的成因作了合理的解释。     

Chemical Indicator of Spinel During Partial Melting and Subsolidus Equilibration of Mantle Peridotite:Experimental Study and Application to Natural Rocks

Systematic experimental studies on partial melting and subsolidus equilibration of three reconstituted spinel-peridotites have been carried out at temperatures ranging from 1000 to 1380℃ and pressures of 1.0 and 1.5 GPa. The results shows (1) during partial melting, Mg# [=Mg/(Mg+Fe)]and Cr# [=Cr/(Cr-Al)]of spinel increase with an increase in degrees of melting) (2) during subsolidus equilibration, with increasing temperatures, Mg# of spinel increases but Cr# of spinel remains almost unchanged in dunite and increases slightly in Iherzolite and harzburgite. The negative and quasi-linear Mg# -Cr# correlation of spinel represents an isotherm of total non-equilibrium. The same results have been obtained by means of mathematical modelling. It is also proved by Mg# -Cr# correlation of spinel in natural peridotites. As a result of the spinel subsolidus equilibration, only variation of Cr# of spinel is limited, and then Cr# of spinel can be used to estimate the relative degree of melting undergone by host rocks     

Grain‐scale dependency of metamorphic reaction on crystal plastic strain

The Breaksea Orthogneiss in Fiordland, New Zealand preserves water‐poor intermediate and mafic igneous rocks that were partially recrystallized to omphacite granulite and eclogite, respectively, at P ≈ 1.8 GPa and T ≈ 850°C. Metamorphic reaction consumed plagioclase and produced grossular‐rich garnet, jadeite‐rich omphacite, clinozoisite and kyanite. The extent of metamorphic reaction, identified by major and trace element composition and microstructural features, is patchy on the grain and outcrop scale. Domains of re‐equilibration coincide with areas that exhibit higher strain suggesting a causal link between crystal plastic strain and metamorphic reaction. Quantitative orientation analysis (EBSD) identifies gradual and stepped changes in crystal lattice orientations of igneous phenocrysts that are surrounded by homophase areas of neoblasts, characterized by high grain boundary to volume ratios and little to no internal lattice distortion. The narrow, peripheral compositional modification of less deformed garnet and omphacite phenocrysts reflects limited lattice diffusion in areas that lacked three‐dimensional networks of interconnected low‐angle boundaries. Low‐angle boundaries acted as elemental pathways (pipe diffusion) that enhanced in‐grain element diffusion. The scale of pipe diffusion is pronounced in garnet relatively to clinopyroxene. Strain‐induced mineral transformation largely controlled the extent of high‐T metamorphic reaction under relatively fluid‐poor conditions.