Search for: All records

Massif-type anorthosites, enormous and enigmatic plagioclase-rich cumulate intrusions emplaced into Earth’s crust, formed in large numbers only between 1 and 2 billion years ago. Conflicting hypotheses for massif-type anorthosite formation, including melting of upwelling mantle, lower crustal melting, and arc magmatism above subduction zones, have stymied consensus on what parental magmas crystallized the anorthosites and why the rocks are temporally restricted. Using B, O, Nd, and Sr isotope analyses, bulk chemistry, and petrogenetic modeling, we demonstrate that the magmas parental to the Marcy and Morin anorthosites, classic examples from North America’s Grenville orogen, require large input from mafic melts derived from slab-top altered oceanic crust. The anorthosites also record B isotopic signatures corresponding to other slab lithologies such as subducted abyssal serpentinite. We propose that anorthosite massifs formed underneath convergent continental margins wherein a subducted or subducting slab melted extensively and link massif-type anorthosite formation to Earth’s thermal and tectonic evolution. 

Archaean orbicular granitoids from western Australia were investigated to better understand crystal growth processes. The orbicules are dioritic to tonalitic spheroids dispersed in a granitic host magma. Most orbicules have at least two to three concentric bands composed of elongate and radially oriented hornblendes with interstitial plagioclase. Each band consists of a hornblende-rich outer layer and a plagioclase-rich inner layer. Doublet band thicknesses increase, crystal number density decreases, and grain size increases from rim to core, suggesting crystallization was more rapid on the rims than in the core. Despite these radial differences, mineral mode and bulk composition of each band are similar, indicating limited crystal-melt segregation during crystallization. These observations lead us to suggest that the orbicules represent slowly quenched blobs of hot dioritic to tonalitic liquids injected into a cooler granitic magma. The oscillatory bands in the orbicules can be explained by rapid, disequilibrium crystallization (supercooling). In particular, a linear correlation between bandwidth and radial distance from orbicule rim can be explained by transport-limited crystallization, wherein crystallization timescales are shorter than chemical diffusion timescales. The slope of this linear relationship corresponds to the square root of the ratio between effective chemical diffusivity in the growth medium and thermal diffusivity, resulting in effective chemical diffusivities of 3 × 10−8 m2/s. These high effective diffusivities require static diffusion through a free volatile phase (fluid) and/or a strong advective/convective component in the fluid. Regardless of the mechanisms, these effective diffusivities can be used to estimate growth rates of ~10−6 m/s or 0.4 cm/hr. Our results indicate that crystals can grow rapidly, possibly facilitated by fluids and dynamic conditions. These rapid growth rates suggest that centimetre or larger crystals, such as in porphyritic and pegmatitic systems, can conceivably grow within days. 

Porphyry ore deposits, Earth’s most important resources of copper, molybdenum, and rhenium, are strongly associated with felsic magmas showing signs of high-pressure differentiation and are usually found in places with thickened crust (>45 kilometers). This pattern is well-known, but unexplained, and remains an outstanding problem in our understanding of porphyry ore deposit formation. We approach this problem by investigating the oxidation state of magmatic sulfur, which controls the behavior of ore-forming metals during magma differentiation and magmatic-hydrothermal transition. We use sulfur in apatite to reconstruct the sulfur oxidation state in the Gangdese batholith, southern Tibet. We find that magma sulfate content increased abruptly after India-Eurasia collision. Apatite sulfur content and the calculated magma S 6+ /ΣS ratio correlate with whole-rock dysprosium/ytterbium ratio, suggesting that residual garnet, favored in thickened crust, exerts a first-order control on sulfur oxidation in magmatic orogens. Our findings link sulfur oxidation to internal petrogenic processes and imply an intrinsic relationship of magma oxidation with synmagmatic crustal thickening.