The Evolution of the Igneous Rocks
"The Evolution of the Igneous Rocks" is a classic book written by Norman L. Bowen, a Canadian petrologist and one of the most influential geologists of the 20th century. The book was first published in 1928 and has been a cornerstone of petrology and the study of igneous rocks.
In this book, Bowen presents his research on the crystallization of magma and the formation of igneous rocks. He introduced several important concepts and principles, including the "Bowen's Reaction Series," which describes the sequence of mineral crystallization in cooling magmas. The reaction series is divided into two branches: the continuous and discontinuous series. The continuous series describes the crystallization of plagioclase feldspars, while the discontinuous series explains the formation of mafic minerals like olivine, pyroxene, amphibole, and biotite.
-
Bowen attempts to classify the wide range of igneous rocks into three general series based primarily on their chemical composition and their cooling rate.
-
Gabbro, Diorite, Quartz Diorite, Granodiorite, Granite
This series represents a range of intrusive (or plutonic) igneous rocks, which form when magma cools and solidifies beneath the Earth's surface. These rocks are arranged in increasing order of silica content and decreasing order of iron and magnesium content:
Gabbro: A mafic rock composed mainly of plagioclase feldspar, pyroxene, and, sometimes, olivine. It is dark in color and coarse-grained, with low silica content and high iron and magnesium content.
Diorite: An intermediate rock composed mainly of plagioclase feldspar and amphibole, with minor amounts of pyroxene, biotite, and quartz. It has a salt-and-pepper appearance, with a mix of light and dark minerals.
Quartz Diorite: Similar to diorite, but with a higher quartz content. It represents a transitional phase between diorite and granodiorite.
Granodiorite: A felsic rock composed mainly of plagioclase feldspar, quartz, and lesser amounts of alkali feldspar, biotite, and amphibole. It has a lighter color and higher silica content than diorite.
Granite: A felsic rock composed mainly of quartz, alkali feldspar, and plagioclase feldspar, with minor amounts of biotite and amphibole. It is light in color and coarse-grained, with the highest silica content in this series.
-
Gabbro, Diorite, Monzonite, Syenite:
This series represents another range of intrusive igneous rocks, with varying mineral compositions:
Gabbro and Diorite: Described in the first series.
Monzonite: An intermediate rock composed mainly of plagioclase feldspar and alkali feldspar, with lesser amounts of pyroxene, amphibole, and biotite. It is transitional between diorite and syenite.
Syenite: A felsic rock composed mainly of alkali feldspar, with lesser amounts of plagioclase feldspar and mafic minerals like amphibole, pyroxene, or biotite. It is similar to granite but has less quartz content and a higher proportion of alkali feldspar.
-
This series represents a continuum of extrusive (volcanic) rocks with increasing alkali content (sodium and potassium) and decreasing silica content.
Basalt: A dark-colored, fine-grained mafic extrusive rock composed mainly of plagioclase feldspar, pyroxene, and olivine, with or without minor amounts of amphibole or biotite.
Nephelinite: A dark-colored, fine-grained mafic extrusive rock similar to basalt but containing the feldspathoid mineral nepheline instead of plagioclase feldspar. This occurs when the magma has a lower silica content and higher alkali content than typical basalt.
Melilitite: A rare, dark-colored, fine-grained mafic to ultramafic extrusive rock composed predominantly of melilite, a group of minerals that form in silica-poor, alkaline magmas. Melilitites often contain other feldspathoid minerals, such as nepheline or leucite, and may also have olivine, pyroxene, or carbonates.
Phonolite: A light-colored, fine-grained felsic extrusive rock characterized by the presence of alkali feldspar and feldspathoid minerals, such as nepheline or leucite. It has a lower silica content and higher alkali content than more common felsic rocks like rhyolite. Phonolites may also contain minor amounts of mafic minerals, such as pyroxene or amphibole.
Bowen vs Immiscibility
Chapter 2 covers the formation of immiscible liquids in silicate melts has been proposed as a way for natural magmas to produce various rock types, but this theory has not been widely accepted by petrologists due to insufficient observational and experimental evidence. Bowen suggested that if immiscibility existed in rock melts, it should be observable in glassy lavas like obsidian, where discrete spheres of one glass would be embedded in another glass of different composition, but this has not been observed. Greig's laboratory investigations on immiscibility in binary and ternary silicate systems found areas of liquid immiscibility that did not align with rock compositions, even abnormal ones. However, Roedder later discovered a region of low-temperature immiscibility in the FeO-K2O-Al2O3-SiO2 system, which was closer to the compositions of some abnormal rock types. Despite these findings, direct evidence of silicate immiscibility in nature remains lacking.
What if separation happened much earlier?
Fractional Crystallization
Crystallization in Silicate Systems
The Reaction Principal
The Reaction Principle describes the sequence of mineral crystallization from a cooling magma. The two pronged series is based on the observation that minerals crystallize at different temperatures as magma cools, and it helps explain the formation of various igneous rock compositions. Bowen's Reaction Series is divided into two branches: the continuous and discontinuous series.
-
This series represents the crystallization of plagioclase feldspar, where the mineral's composition changes continuously from calcium-rich (anorthite) to sodium-rich (albite) as the magma cools. The plagioclase composition in the resulting rock depends on the temperature at which the magma solidified.
-
This series outlines the sequential crystallization of mafic minerals, which occurs in distinct steps as the magma cools. The discontinuous series proceeds as follows: olivine, pyroxene, amphibole, and biotite. As each mineral crystallizes, it reacts with the remaining magma, causing a change in the magma's composition and promoting the crystallization of the next mineral in the series.
The Fractional Crystallization of Basaltic Magmas
-
Fractional crystallization is a process by which different minerals crystallize from a magma at different temperatures as it cools, causing the remaining magma to change in composition. When applied to basaltic magmas, fractional crystallization plays a significant role in the formation of various igneous rock types and in the evolution of the Earth's crust.
Basaltic magmas are typically mafic, meaning they are rich in iron, magnesium, and calcium, and have a relatively low silica content. As basaltic magma cools, minerals with higher melting points crystallize first, while minerals with lower melting points remain in the liquid phase. This process can be described by Bowen's Reaction Series, which outlines the sequential crystallization of minerals from a cooling magma.
In the context of basaltic magmas, the following minerals typically crystallize in order:
Olivine: The first mineral to crystallize from basaltic magma, olivine is an iron-magnesium silicate with a high melting point.
Pyroxene: As olivine reacts with the remaining melt, pyroxene begins to crystallize. Pyroxenes are a group of chain silicate minerals composed of iron, magnesium, calcium, and aluminum.
Plagioclase feldspar: Plagioclase feldspar begins to crystallize along with pyroxene, usually forming calcium-rich compositions initially and gradually transitioning to sodium-rich compositions as the magma continues to cool.
As these minerals crystallize and are removed from the magma (either by settling to the bottom of the magma chamber or being carried away by movement), the remaining liquid becomes progressively more silica-rich and evolves to a more felsic composition. This can lead to the formation of different rock types, such as andesites, dacites, or even rhyolites, depending on the extent of fractional crystallization and other geological processes at play.
Fractional crystallization of basaltic magmas can also result in the formation of layered intrusions, where different minerals accumulate in distinct layers within a magma chamber. This process is responsible for some economically important ore deposits, such as the Bushveld Complex in South Africa, which contains significant reserves of platinum group elements, chromium, and vanadium.
“There are no known glasses (extrusive texture igneous rocks) corresponding with any of the ultrabasic types, peridotites, pyroxenites or anorthosites. It is probably that peridotite and pyroxenite magmas cloud not be chilled rapidly enough to give a glass.”
Crystal Sorting- Olivine Basalts
