Cumulates rocks

Fractional crystallization (fractionation) is that process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains. The process of fractional crystallization is responsible for the bulk of differentiation that is occurs in igneous rocks. Igneous rocks formed by sedimentation are termed cumulates (Latin "cumulus", a heap), and characteristically display cumulate textures.

Mechanisms of crystal fractionation

• Crystal Settling/Floating: In general, crystals forming from a magma will have different densities than the liquid. 1) If the crystals (olivine, pyroxene) have a higher density than the liquid, they will tend to sink or settle to the floor of the magma body. The first layer that settles will still be in contact with the magma, but will later become buried by later settling crystals so that they are effectively removed from the liquid.

2) If the crystals (feldspar) have a lower density in the magma, they will tend to float or rise upward through the magma. Again the first layer that accumulates at the top of the magma body will initially be in contact with the liquid, but as more crystals float to the top and accumulate, the earlier formed layers will be effectively removed from contact with the liquid.

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Schematic representation of the crystal settling/floating process in a magma chamber. Modified from Prof. Stephen A. Nelson

• Inward Crystallization
Because a magma body is hot and the country rock which surrounds it is expected to be much cooler, heat will move outward away from the magma. Thus, the walls of the magma body will be coolest, and crystallization would be expected to take place first in this cooler portion of the magma near the walls. The magma would then be expected to crystallize from the walls inward. Just like in the example above, the first layer of crystals precipitated will still be in contact with the liquid, but will eventually become buried by later crystals and effectively be removed from contact with the liquid.

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Schematic representation of the inward crystallization process in a magma chamber. Modified fromProf. Stephen A. Nelson

Schematic diagrams showing the principles behind fractional crystallisation in a magma. While cooling, the magma evolves in composition because different minerals crystallize from the melt. 1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclase crystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock forms. From Wikipedia

Terminology

Cumulates can be subdivided on the basis of the proportion of cumulate crystals relative to crystals formed from the trapped intercumulus liquid (Hall, 1996).

Adcumulates comprise >95% cumulate crystals.
Mesocumulates have 95-85% cumulate crystals.
Orthocumulates consist of <85% cumulus crystals.

Magmatic Layering

Many mafic and ultramafic exhibit igneous layering. In its most obvious form, layering means the upward repetition of planar or shallow trough-shaped layers (centimetres to metres thick) that differ in relative mineral proportions (modal composition). This can vary from a simple succession of thin bands richer in ferromagnesian minerals to layers that are beautifully internally graded, from melanocratic at the base of each layer to mesocratic or leucocratic at the top, either stacked directly upon each other or interspersed with homogeneous layers.

To form such layers in a plutonic rock requires some kind of crystal-sorting process to have operated during crystallization and deposition. Usually the ferromagnesian minerals have accumulated preferentially at the base of each layer in whereas felsic minerals are concentrated at the top. Analysis of a cumulate rock, unlike a volcanic rock, does not accurately record the composition of the melt from which it crystallized; only at a chilled margin, where cool wall - rocks have quenched the melt too rapidly for any accumulation to occur, can an estimate be made (not always reliably) of the initial melt or magma composition. Whereas in a volcanic rock mineralogy is dictated by melt composition, the reverse is true in a cumulate rock: its chemical composition depends primarily on the minerals it contains and the proportions in which they happen to be have accumulated.

When describing layered intrusions, petrologists distinguish seven types of igneous layering:

• Uniform layering: Each layer is mineralogically and texturally homogeneous.
• Modal layering: layering deï¬ned by smooth and gradual variations in the modal abundances of the cumulus minerals which are commonly plagioclase and pyroxene (norites and gabbronorites) or plagioclase and olivine (troctolites).
• Phase layering: layering deï¬ned by the appearance or disappearance of cumulus minerals in the crystallization sequence.
• Cryptic layering: Systematic variation in the chemical composition of cumulus minerals (pyroxenes, olivines, plagioclase feldspars) with stratigraphic height in a layered sequence.
• Rhythmic layering: Layering pattern is repeated. Macrorhythmic layering: individual layers are several meters thick. Microrhythmic layering, layers are a few cm thick.
• Size graded layering: Smooth and gradual variations in the grain sizes of cumulus minerals. This type of layering is not common, except in the Duke Island Intrusion.
• Intermittent layering: A common type consists of rhythmically graded layers interlayered with uniform layers.

Although there are many mafic intrusive bodies found on Earth, there are comparatively few exceptional layered intrusions exposed today. Members of that exceptional group include the Skaergaard Intrusion, the Bushveld Complex, the Great Dyke, the Kiglapait Intrusion, the Stillwater Complex (Montana, USA) and the Rum Layered Suite. These intrusions have represented natural laboratories for studying magmatic processes for more than a century and lie at the heart of fundamental and classic petrological theories of magmatic differentiation. The formation of layering and cumulate textures of mafic and ultramafic intrusions are still incompletely understood after a half century of intensive study.

The Processes of Layering

Cumulus theory: The fundamental principle underpinning the ideas proposed by Wager and Brown (1968) is cumulus theory. Gravity-driven crystal settling, leading to accumulation of crystals on a substrate, was invoked to explain many of the textures and geochemical trends observed in layered intrusions. Rhythmic modal layering can be explained by crystal settling interrupted by periodic large-scale convective overturn of the entire cooling unit. Reinjection of more primitive magma may explain major compositional shifts and cases of irregular cryptic variations.

In-Situ Processes: Nucleation and growth of minerals in a thin stagnant boundary layer along the margins of the chamber (e.g. Campbell 1978; McBirney and Noyes 1979).

Double-diffusive convection (Turner and Campbell 1986): An important property of multicomponent fluids is that individual component (including heat) can have different diffusivities. As consequence, such fluids may become vertically stratified with respect to density, composition and temperature. If opposing gradients of two components with different diffusivities are stet up, the system may separate into a series of independently convicting layers, bounded by sharp diffusive interfaces, across which heat and chemical components are transported by molecular diffusion.

A double-diffusive interface may form when a layer of hot dense magma is overlain by a layer of cooler, less dense magma. such a situation may occur when a new pulse of primitive magma is injected into the base of the magma chamber through vents in the floor. Heat in transported between the layers faster than chemical components, driving convection in both layers and maintaining a sharp interface between them. Eventually, crystallization of the lower layer will occur, causing a reduction in its density, and it may then mix with the overlaying layer. layering2019(10).jpg

The distribution of density variation due to compositional and temperature effects in a multi-component fluid layer before it breaks up into double-diffusive layers. After Turner and Campbell 1986.

Double-diffusive layers: ρ composition is the variation of density in the system due to composition gradients; ρ temperature is the variation of density in the system due to temperature gradients. After Turner and Campbell 1986.

Uniform Layering: gabbro from Giles Complex, South Australia. From Jim Talbot

Uniform Layering: uniform chromite layers alternating with plagioclase-rich layers. Critical Zone, UG1 of the Bushveld Complex, S. Africa. From Kevin Walsh

Size graded layering: smooth size variations in cumulus minerals. Duke Island Intrusio, Alaska. From Richard Arculus

Size graded layering: smooth size variations in cumulus minerals. Duke Island Intrusio, Alaska. From Richard Arculus

Modal and graded layering (rhythmic): Close-up view of thin graded beds with characteristic dark bottoms and light tops. They are widely spaced with homogeneous rock in between that is like the ridge rock between troughs.Skaergaard intrusion, East Greenland. From Union College, Schenectady, NY

Graded layering (rhythmic): Modally graded layers in the. Commonly referred to as rhythmic layering. Graded layers are separated by uniform layers (hammer for scale). Skaergaard intrusion, East Greenland. From Union College, Schenectady, NY

Rhythmic layering: inch-scale layering with anorthosite-pyroxene couplets. Stillwater, Montana, Usa. From Union College, Schenectady, NY

Bibliography

• C.H. and Troll, V.R. A geological excursion guide to Rum: the Palaeocene igneous rocks of the Isle of Rum, Inner Hebrides. Edinburgh : Edinburgh Geological Society in association with NMS Enterprises, 2008.
• Wilson, B. Marjorie. Igneous petrogenesis a global tectonic approach. Springer Science & Business Media, 2007