Depth-dependent peridotite-melt interaction and the origin of variable silica in the cratonic mantle

Peridotites from the thick roots of Archaean cratons are known for their compositional diversity, whose origin remains debated. We report thermodynamic modelling results for reactions between peridotite and ascending mantle melts. Reaction between highly magnesian melt (komatiite) and peridotite leads to orthopyroxene crystallisation, yielding silica-rich harzburgite. By contrast, shallow basalt-peridotite reaction leads to olivine enrichment, producing magnesium-rich dunites that cannot be generated by simple melting. Komatiite is spatially and temporally associated with basalt within Archaean terranes indicating that modest-degree melting co-existed with advanced melting. We envisage a relatively cool mantle that experienced episodic hot upwellings, the two settings could have coexisted if roots of nascent cratons became locally strongly extended. Alternatively, deep refractory silica-rich residues could have been detached from shallower dunitic lithosphere prior to cratonic amalgamation. Regardless, the distinct Archaean melting-reaction environments collectively produced skewed and multi-modal olivine distributions in the cratonic lithosphere and bimodal mafic-ultramafic volcanism at surface.

DTA-TG Analysis of Gd0.95La0.05Ba1.95Sr0.05Cu3Oy Compounds

The sintering temperature is played a vital role in the evolution of phase structure, microstructure, and the properties of the superconductor. In this study, the Gd0.9La0.1Ba1.95Sr0.05Cu3O7-d phase compound has been synthesized by the wet method using HNO3 as a solvent. The samples were divided into two groups. The first sample was calcined at 400 °C for 2 hours + 500 °C for 2 hours + 600 °C for 6 hours. The second sample treated by the same process and then continued by heating at 900 °C for 15 minutes. The effect of the calcination temperature for the synthesis of Gd0.9La0.1Ba1.95Sr0.05Cu3O7-d bulks was investigated using the DTA-TG method. The results showed that the optimum reaction temperature for the formation of Gd0.9La0.1Ba1.95Sr0.05Cu3O7-d phase was 938 °C. The additional heating temperature e.g. 900 °C for 15 minutes on the calcination process can reduce the optimum formation temperature of Gd0.9La0.1Ba1.95Sr0.05Cu3O7-d compounds by 20 °C. The peritectic melting reaction temperatures of the sample without the addition of heating and with the addition of heating at temperature 900 °C for 15 minutes are 1032°C and 1035°C, respectively. The melting temperatures of both samples are 1164 °C and 1200 °C.

Constraining the timing and character of crustal melting in the Adirondack Mountains using multi-scale compositional mapping and in-situ monazite geochronology

Migmatites are common in the hinterland of orogenic belts. The timing and mechanism (in situ vs. external, P-T conditions, reactions, etc.) of melting are important for understanding crustal rheology, tectonic history, and orogenic processes. The Adirondack Highlands has been used as an analog for mid/deep crustal continental collisional tectonism. Migmatites are abundant, and previous workers have interpreted melting during several different events, but questions remain about the timing, tectonic setting, and even the number of melting events. We use multiscale compositional mapping combined with in situ geochronology and geochemistry of monazite to constrain the nature, timing, and character of melting reaction(s) in one locality from the eastern Adirondack Highlands. Three gray migmatitic gneisses, studied here, come from close proximity and are very similar in microscopic and macroscopic (outcrop) appearance. Each of the rocks is interpreted to have undergone biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). Full-section compositional maps show the location of reactants and products of the melting reaction, especially prograde and retrograde biotite, peritectic K-feldspar, and leucosome, in addition to all monazite and zircon in context. In addition, the maps provide constraints on kinematics during melting and a context for interpretation of accessory phase composition and geochronology. More so than zircon, monazite serves as a monitor of melting and melt loss. The growth of garnet during melting leaves monazite depleted in Y and HREEs while melt loss from the system leaves monazite depleted in U. Results show that in all three localities, partial melting occurred during at ca. 1160–1150 Ma (Shawinigan orogeny), but the samples show high variability in the location and degree of removal of the melt phase, from near complete to segregated into layers to dispersed. All three localities experienced a second high-T event at ca. 1050 Ma, but only the third (non-segregated) sample experienced further melting. Thus, in addition to bulk composition, the fertility for melting is an important function of the previous history and the degree of mobility of earlier melt and fluids. Monazite is also a sensitive monitor of retrogression; garnet breakdown leads to increased Y and HREE in monazite. Results here suggest that all three samples remained at depth between the two melting events but were rapidly exhumed after the second event.

The dual origin of I-type granites: the contribution from experiments

New laboratory experiments using granulite xenoliths support a dual origin for I-type granites as primary and secondary. Primary I-type granites represent fractionated liquids from intermediate magma systems of broadly andesitic composition. Fluid-fluxed melting of igneous rocks that resided in the continental crust generates secondary I-type granites. The former are directly related to subduction, with Cordilleran batholiths as the most characteristic examples. Experiments with lower crust granulite sources, in the presence of water, show that amphibole is formed by a water-fluxed peritectic rehydration melting reaction. Entrainment of only 10% of restites composed of amphibole, pyroxene, plagioclase and magnetite, is sufficient to account for discrepancies in aluminium saturation index and maficity in secondary I-type granites. Lower crust granulite xenoliths, attached to a sanukitoid containing 6 wt% water, have been used in two-layer capsules to test fluid-fluxed melting reactions as the origin of secondary I-type granites. It is proposed that sanukitoid magmas act as water donors that trigger extensive melting of the lower crust, giving rise to granodioritic liquids. Because primary granites are related to coeval subduction, and secondary ones are crustal melts from older subduction-related rocks, the distinction between both I-types is essential in tectonic reconstructions of ancient orogenic belts.