Chromitite layers indicate the existence of large, long-lived, and entirely molten magma chambers

Abstract

The classical paradigm of the ‘big magma tank’ chambers in which the melt differentiates, is replenished, and occasionally feeds the overlying volcanos has recently been challenged on various grounds. An alternative school of thought is that such large, long-lived and largely molten magma chambers are transient to non-existent in Earth’s history. Our study of stratiform chromitites in the Bushveld Complex—the largest magmatic body in the Earth’s continental crust—tells, however, a different story. Several chromitites in this complex occur as layers up to 2 m in thickness and more than 400 kms in lateral extent, implying that chromitite-forming events were chamber-wide phenomena. Field relations and microtextural data, specifically the relationship of 3D coordination number, porosity and grain size, indicate that the chromitites grew as a 3D framework of touching chromite grains directly at the chamber floor from a basaltic melt saturated in chromite only. Mass-balance estimates imply that a few km thick column of this melt is required to form each of these chromitite layers. Therefore, an enormous volume of melt appears to have been involved in the generation of all the Bushveld chromitite layers, with half of this melt being expelled from the magma chamber. We suggest that the existence of thick and laterally extensive chromitite layers in the Bushveld and other layered intrusions supports the classical paradigm of big, albeit rare, ‘magma tank’ chambers.

In situ crystallization of non-cotectic and foliated igneous rocks on a magma chamber floor

“…This allows plagioclase grains to nucleate heterogeneously at the bottom of the boundary layer with different orientations. Random orientations for heterogeneously nucleated crystals appear to be possible based on recent observations on chromitite layers 15 . Blue lines indicate the orientation of the a-axis of plagioclase grains along which growth is generally faster 65 .…”

“…1a). Grains may have random orientations (e.g., randomly oriented chromite grains nucleating heterogeneously on overturned contacts 15 ), unless large compositional gradients are present 14 . However, it is not uncommon for layered intrusions to have rocks that do not contain cotectic proportions of primocrysts 16 .…”

A comparison of trace element concentrations in chromite from komatiites, picrites, and layered intrusions: implications for the formation of massive chromite layers

“…However, Roeder and Reynolds (1991) found that chromite is, in fact, slightly more soluble at high pressure. Latypov et al (2017Latypov et al ( , 2022 proposed that a rapid transfer of a superheated magma from depth would lower pressure in the magma and could bring chromite only onto the liquidus which they modelled using the MELTS program. r An increase in fO 2 would ensure that most of the Cr is converted to Cr 3+ (the form of Cr in chromite) leading to chromite saturation (Ulmer 1969;Murck and Campbell 1986;Roeder and Reynolds 1991).…”

Abstracts from the early-career researchers virtual International Platinum Symposium, May 2022

“…The mass balance problem of chromitites (and stratiform PGE deposits) offers support to this concept of a large magma tank. [1] This is based on the weak assumption that individual layers of chromitite (in the RLS) are laterally continuous for 10–100 s of kilometres.…”

“…There is, however, a growing body of evidence that suggests continental basaltic layered intrusions are formed by non-sequential sill emplacement into a resident mush (± transient melt lenses) [2] (Figure 1a). The RLS is also compartmentalised, [3,4] with compartments extending ≤ 50 km, and each being characterised by their own stratigraphic sequence of chromitites.…”