Rare Earth Element Inversion for Melt Distribution: Sensitivity and Application

Inversion of rare earth element (REE) abundances in MgO-rich(MgO 6 wt.%) igneous rocks, described initially by McKenzie& O'Nions (1991), provides a useful means of constrainingthe distribution of melt fraction with depth, X(z), in the mantle.Here, we investigate the sensitivity of the REE inversion procedureto noise in measured REE abundances and to variation in parameterswhich must be specified a priori. Inverted melt distributionsare insensitive to realistic amounts of random noise but aresensitive to systematic noise. Melt distributions are also sensitiveto input parameters such as depth and range of melting, sourcecomposition, and mantle mineralogy. To determine unique meltdistributions, additional independent constraints on at leasttwo of the following input parameters are therefore required:depth and range of melting (i.e., lithospheric thickness); sourcecomposition; source mineralogy (dependent on mantle temperature).The successful replication of melt compositions where thesethree parameters are well constrained and the agreement betweeninverted melt distributions and melt distributions calculatedindependently assuming isentropic melting during adiabatic upwellingindicate that the scheme is reliable. REE inversion thereforeprovides a means for both routinely constraining X(z) and fordetermining unknown input parameters.     

Inclusions in Three S-Type Granites from Southeastern Australia

The Jillamatong Granodiorite is one of the most mafic S-typegranites in the Kosciusko regidn and is typical of widely distributed,cordierite-bearing S-type granites in the Lachlan Fold Beltof southeastern Australia. The Koetong and Granya Adamellitesbelong to the Koetong Suite of the Corryong Batholith and arerare examples in the Lachlan Fold Belt of granites that containprimary muscovite. Although subtle differences can be found,inclusions within the Jillamatong Granodiorite and the KoetongSuite are broadly similar despite the fact that the JillamatongGranodiorite belongs to a different and distinct suite (theBullenbalong Suite). Mica-rich schistose and micTogranular inclusionsdominate but other types occur, including foliated quartzofeldspathicvarieties, calcsilicates, quartzites, and pure quartz types.The total abundance of all inclusion types in each granite studiedis less than 5.1% although abundance varies from one graniteto another. All inclusions are believed to have been derived from metasedimentaryor modified metasedimentary lithologies and all inclusions,except some quartzites, were entrained at depth where the hostgranite magmas were generated by partial melting of heterogeneoussedimentary sources. The inclusions are restite but most arenot complementary to the melt component of the magma now representedby the host granite. They represent fragments from differentrefractory lithologies of a complex metasedimentary source andbecause their compositions and mineral assemblages were unsuitablefor the generation of large quantities of granite melt, theydid not melt or were melted only to small and variable extents(less than the rheological critical melt percentage of Arzi,1978). Such lithologies remained physically coherent and retainedtheir separation from the host granite magma during ascent.Lithologies that did melt extensively were physically disaggregatedand are not represented among the inclusions. Since the inclusions do not represent complementary restitecontrolling compositional variation among the host granites,their compositions cannot be used to precisely estimate thebulk compositions of the source rocks. However, the different,source-rock derived, inclusion types collectively provide informationregarding the lithologies present in the source and hence thegeneral character of the source terranes. The dominance of schistoseand microgranular inclusions in the Jillamatong Granodioriteand the Koetong Suite indicates that pelitic and quartzofeldspathiccompositions are the two dominant components in the source terranes. Inclusions of the same type from the two suites are broadlysimilar but different in detail. Inclusions reflect the mineralogicaland geochemical characteristics of their host granites and thereare textural differences between microgranular inclusions ofthe two suites examined. The differences reflect subtle butsignificant contrasts in source materials, the conditions prevailingduring partial melting and the history of emplacement and crystallizationof the host magmas.     

Pliocene subaqueous fans and Gilbert-type deltas in maar crater lakes, Hopi Buttes, Navajo Nation (Arizona), USA

Maar craters of the Mio-Pliocene Hopi Buttes volcanic field of Arizona formed within a broad playa system, and accumulated a variety of lacustrine sedimentary deposits. Many craters initially held isolated, groundwater-fed lakes. Ephemeral streams crossing the playa entered some of the lake-filled craters, and built coarse grained Gilbert-type deltas and subaqueous fans along the margins of these craters. The small, coarse grained fans and deltas have many features in common with much larger coarse grained deltaic and fan deltaic deposits. However, the local production of coarse grained volcanic sediment, low gradients in the local stream catchment, steep subaqueous relief and the small size of the receiving ‘basins’resulted in a unique combination of features. Cone-shaped subaqueous fans initially formed at the mouths of incised feeder streams. The fans are small accumulations of steeply dipping gravelly tephra that consist almost entirely of overlapping lobes constructed by density-modified grain flows. Gravelly Gilbert-type tephra deltas formed in brimfull craters fed by a freely migrating feeder stream. They are concave lakeward, mimicking the underlying crater wall topography. Complex deltaic geometries are defined by topset strata that steeply onlap tall foreset beds. They suggest that feeding stream floods caused rapid and comparatively large variations in lake level within the small crater lakes. Bed-specific carbonate alteration is common, and probably resulted from both influx of detrital carbonate across the playa and alteration of tephra beds by carbonate-saturated lakewaters during between flood periods of high net evaporation.     

Devolatilization of metabasic rocks during greenschist–amphibolite facies metamorphism

The production of large volumes of fluid from metabasic rocks, particularly in greenstone terranes heated across the greenschist–amphibolite facies transition, is widely accepted yet poorly characterized. The presence of carbonate minerals in such rocks, commonly as a consequence of sea‐floor alteration, has a strong influence, via fluid‐rock buffering, on the mineral equilibria evolution and fluid composition. Mineral equilibria modelling of metabasic rocks in the system Na₂O‐CaO‐FeO‐MgO‐Al₂O₃‐SiO₂‐CO₂‐H₂O (NCaFMASCH) is used to constrain the stability of common metabasic assemblages. Calculated buffering paths on T–X_CO2 pseudosections, illustrate the evolution of greenstone terranes during heating across the greenschist‐amphibolite transition. The calculated paths constrain the volume and the composition of fluid produced by devolatilization and buffering. The calculated amount and composition of fluid produced are shown to vary depending on P–T conditions, the proportion of carbonate minerals and the X_CO2 of the rocks prior to prograde metamorphism. In rocks with an initially low proportion of carbonate minerals, the greenschist to amphibolite facies transition is the primary period of fluid production, producing fluid with a low X_CO2. Rocks with greater initial proportions of carbonate minerals experience a second fluid production event at temperatures above the greenschist to amphibolite facies transition, producing a more CO₂‐rich fluid (X_CO2 = 0.2–0.3). Rocks may achieve these higher proportions of carbonate minerals either via more extensive seafloor alteration or via infiltration of fluids. Fluid produced via devolatilization of rocks at deeper crustal levels may infiltrate and react with overlying lower temperature rocks, resulting in external buffering of those rocks to higher X_CO2 and proportions of carbonate minerals. Subsequent heating and devolatilization of these overlying rocks results in buffering paths that produce large proportions of fluid at X_CO2 = 0.2–0.3. The production of fluid of this composition is of importance to models of gold transport in Archean greenstone gold deposits occurring within extensive fluid alteration haloes, as these haloes represent the influx of fluid of X_CO2 = 0.2–0.3 into the upper crust.     

Calculated phase equilibria in K2O-FeO-MgO-Al2O3-SiO2-H2O for silica-undersaturated sapphirine-bearing mineral assemblages

Silica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al₂O₃‐SiO₂ (FMAS) and KFMASH (+K₂O + H₂O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.     

Sediment magnetic properties of glacial till deposited since the Little Ice Age maximum for selected glaciers at Svartisen and Okstindan, northern Norway

Samples of glacial till deposited since the Little Ice Age (LIA) maximum by two glaciers, North Bogbre at Svartisen and Corneliussen-breen at Okstindan, northern Norway, were obtained from transects running from the current glacier snout to the LIA (c. AD 1750) limit. The samples were analysed to determine their sediment magnetic properties, which display considerable variability. Significant trends in some magnetic parameters are evident with distance from the glacier margin and hence length of subaerial exposure. Magnetic susceptibility (χ) decreases away from the contemporary snout, perhaps due to the weathering of ferrimagnetic minerals into antiferromagnetic forms, although this trend is generally not statistically significant. Trends in the ratios of soft IRM/hard IRM which are statistically significant support this hypothesis, suggesting that antiferromagnetic minerals are increasing relative to ferrimagnetic minerals towards the LIA maximum. Backfield ratios (IRM -100 mT/SIRM) also display a significant and strong trend towards magnetically harder behaviour with proximity to the LIA maximum. Thus, by employing a chronosequence approach, it may be possible to use sediment magnetics data as a tool for reconstructing glacier retreat in areas where more traditional techniques, such as lichenometry, are not applicable.     

Progress relating to calculation of partial melting equilibria for metapelites

Improved activity–composition relationships for biotite, garnet and silicate liquid are used to construct updated P–T grids and pseudosections for high‐grade metapelites. The biotite model involves Ti charge‐balanced by hydrogen deprotonation on the hydroxyl site, following the substitution , where HD represents the hydroxyl site. Relative to equivalent biotite‐breakdown melting reactions in P–T grids in K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH), those in K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (KFMASHTO) occur at temperatures close to 50 °C higher. A further consequence of the updated activity models is that spinel‐bearing equilibria occur to higher temperature and higher pressure. In contrast, the addition of Na₂O and CaO to KFMASH to make the Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) system lowers key biotite‐breakdown melting reactions in P–T space relative to KFMASH. Combination of the KFMASHTO and NCKFMASH systems to make Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O₂ (NCKFMASHTO) results in key biotite‐breakdown melting reactions occurring at temperatures intermediate between those in KFMASHTO and those in NCKFMASH. Given such differences, the choice of model system will be critical to inferred P–T conditions in the application of mineral equilibria modelling to rocks. Further, pseudosections constructed in KFMASH, NCKFMASH and NCKFMASHTO for several representative rock compositions show substantial differences not only in the P–T conditions of key metamorphic assemblages but also overall topology, with the calculations in NCKFMASHTO more reliably reflecting equilibria in rocks. Application of mineral equilibria modelling to rocks should be undertaken in the most comprehensive system possible, if reliable quantitative P–T information is to be derived.     

Computed X-ray Tomography Investigation of Porosity and Permeability of the Liujiagou Formation Sandstone Exposed to CO2-Saturated Brine

In order to improve CO2 capture, utilization and storage (CCUS) to solve carbon emission, sandstone from the Triassic Liujiagou Formation (LF) from the Ordos Basin in China was investigated using permeability tests and computed X-ray tomography (CT) scanning. The presence of reactive minerals within the geological CO2 sequestration target storage formation can allow reaction with injected CO2, which changes the porosity and permeability of the LF beds, affecting storage effectiveness. To investigate the effect of chemical reactions on the pore structure and permeability of sandstone cores representing the LF CO2 storage, tests were conducted to analyze the changes in porosity and permeability of sandstone cores induced by CO2-saturated brine at different reaction times (28-day maximum reaction period). Porosity and permeability of the sandstone increased after reaction with CO2-saturated brine due to mineral dissolution. The sandstone exhibited an increase in porosity and permeability after 15 days of reaction with CO2-saturated brine. Moreover, there was an increase in the volume of large pores in the sandstone after the 28-day period. The pore network of the sandstone was established through CT results, and the porosity calculated based on the obtained pore network was close to that measured in the test, demonstrating the feasibility to use CT to study the evolution of the microstructure of sandstone after long-time exposure to CO2-saturated brine.     

The Significance of Primary Scapolite in Granulitic Inclusions from Deep-seated Pipes

Sulphur-rich scapolite is described as a primary constituentof several granulite inclusions occurring, along with eclogiticand rare ultrabasic rocks, in the breccia- and basalt-filledpipes of eastern Australia. The wide distribution of such inclusionsin space and time indicates that granulites, some scapolite-bearing,form an important part of the base of the crust of this region. Two inclusions with the assemblage orthopyroxene-clinopyroxene-plagioclase-scapolite,two with the assemblage clinopyroxene-garnet-plagioclase-scapolite,and one with the assemblage orthopyroxene-clinopyroxene-plagioclase,have been analysed and chemical and optical data on the constituentminerals are given. Since it is impossible to adequately separatescapolite from primary and secondary plagioclase, partial analysesof these minerals have been made with an A.R.L. electron probeX-ray microanalyser. From our data and that obtained from the literature, we suggestthat scapolites with more than 10 mol per cent sulphur in theanion group are confined to rocks of the granulite meta-morphicfacies. Within the granulite facies there appears to be a progressiveincrease in the sulphur content of scapolite in equilibriumwith such sulphur-rich minerals as pyrrhotite, with increasingmetamorphic grade.