Jill A. VanTongeren

How sharp is the sharp Archean Moho? Example from eastern Superior Province

The Superior Province of North America has not experienced major internal deformation for nearly 2.8 Gyr, preserving the Archean crust in its likely original state. We present seismological evidence for a sharp (less than 1 km) crust-mantle boundary beneath three distinct Archean terranes and for a more vertically extensive boundary at sites likely affected by the 1.2–0.9 Ga Grenville orogeny. At all sites crustal thickness is smaller than expected for the primary crust produced by melting under higher mantle potential temperature conditions of Archean time. Reduced thickness and an abrupt contrast in seismic properties at the base of the undisturbed Archean crust are consistent with density sorting and loss of the residues through gravitational instability facilitated by higher temperatures in the upper mantle at the time of formation. Similar sharpness of crust-mantle boundary in disparate Archean terranes suggests that it is a universal feature of the Archean crustal evolution.

Iron isotopic evolution during fractional crystallization of the uppermost Bushveld Complex layered mafic intrusion

We present δ56Fe (56Fe/54Fe relative to standard IRMM-014) data from whole rock and magnetite of the Upper and Upper Main Zones (UUMZ) of the Bushveld Complex. With it, we assess the role of fractional crystallization in controlling the Fe isotopic evolution of a mafic magma. The UUMZ evolved by fractional crystallization of a dry tholeiitic magma to produce gabbros and diorites with cumulus magnetite and fayalitic olivine. Despite previous experimental work indicating a potential for magnetite crystallization to drastically change magma δ56Fe, we observe no change in whole rock δ56Fe above and below magnetite saturation. We also observe no systematic change in whole rock δ56Fe with increasing stratigraphic height, and only a small variation in δ56Fe in magnetite separates above magnetite saturation. Whole rock δ56Fe (errors twice standard deviation, ±2σ) throughout the UUMZ ranges from -0.01 ±0.03‰ to 0.21 ±0.09‰ (δ56FeaverageWR = 0.10 ±0.09‰; n=21, isotopically light outlier: δ56FeWR = -0.15‰), and magnetites range from 0.28 ±0.04‰ to 0.86 ±0.07‰ (δ56FeaverageMgt = 0.50 ±0.15‰; n=20), similar to values previously reported for other layered intrusions. We compare our measured δ56FeWR to a model that incorporates the changing normative mineralogy, calculated temperatures, and published fractionation factors of Fe-bearing phases throughout the UUMZ and produces δ56FeWR values that evolve only in response to fractional crystallization. Our results show that the Fe isotopic composition of a multiply-saturated (multiple phases on the liquidus) magma is unlikely to change significantly during fractional crystallization of magnetite due to the competing fractionation of other Fe-bearing cumulus phases. This article is protected by copyright. All rights reserved.

Lateral Variability in the Upper Main Zone, Bushveld Complex, owing to Directional Magma Recharge and Emplacement from North to South

Recharge and magma mixing into shallow crustal reservoirs is a critical parameter in understanding magma diversity and predicting volcanic activity and hazards. Direct observation of magma mixing within the crust, however, is impossible. The solidified remnants of large magma chambers in layered mafic intrusions are therefore some of the most important natural laboratories for measuring and understanding past magma chamber dynamics. Here we provide in situ major and trace element compositions of all major mineral phases throughout a single stratigraphic section of a well-defined magma recharge interval in the 2 06 Ga Bushveld Complex layered intrusion of South Africa. This section, the Roossenekal Traverse, is located 75 km south of a previously documented section of the same stratigraphy, the Leolo Mountain Traverse. Despite their distance, the two profiles show remarkably similar thicknesses and compositional variations; however, both resident and incoming magmas recorded in the Roossenekal Traverse are slightly more compositionally evolved. We show that the lateral compositional variability is a direct result of the locus of magma recharge originating in the north, near the Thabazimbi–Murchison Lineament. New primitive magma was emplaced in the north, mixed with more evolved magma towards the south, and fractionated as it filled the magma chamber progressively to the south.