What Does Bowen's Reaction Series Describe

The Bowen's Reaction Series elegantly describes the order in which minerals crystallize from cooling magma, a cornerstone concept in understanding the formation of igneous rocks. This principle, developed by Norman L. Bowen in the early 20th century, revolutionized petrology by providing a framework for predicting the composition of rocks based on the temperature at which they formed.

Introduction to Bowen's Reaction Series

Norman L. Bowen, a Canadian petrologist, conducted a series of experiments in the early 1900s to understand how magma, or molten rock, cools and solidifies. Through meticulous laboratory work, he observed that minerals don't all crystallize at the same temperature. Instead, they form in a specific sequence as the magma gradually cools. This sequence is now known as Bowen's Reaction Series, and it's a fundamental concept in geology, particularly in the study of igneous rocks.

The series is divided into two main branches:

• Discontinuous Series (also known as the Ferromagnesian Series): This branch describes the formation of ferromagnesian minerals, which are rich in iron and magnesium. These minerals react with the remaining magma as the temperature decreases, forming new minerals in a stepwise fashion.
• Continuous Series (also known as the Plagioclase Feldspar Series): This branch describes the formation of plagioclase feldspar, a group of minerals that form a solid solution. The composition of the plagioclase changes gradually as the temperature drops, with the early-formed crystals being more calcium-rich and the later-formed crystals being more sodium-rich.

Bowen's Reaction Series provides a predictable order of crystallization. At high temperatures, minerals like olivine and calcium-rich plagioclase crystallize first. As the temperature drops, these minerals react with the remaining magma to form minerals like pyroxene and sodium-rich plagioclase. At even lower temperatures, minerals like amphibole, biotite, orthoclase feldspar, muscovite mica, and quartz crystallize.

Understanding Bowen's Reaction Series is crucial for several reasons:

• Predicting Mineral Compositions: It allows geologists to predict which minerals are likely to be found together in igneous rocks based on their crystallization temperatures.
• Explaining Rock Textures: It helps explain the textures of igneous rocks, such as porphyritic textures, where large crystals are embedded in a fine-grained matrix.
• Understanding Magmatic Differentiation: It sheds light on the process of magmatic differentiation, where the composition of a magma body changes as minerals crystallize and are removed from the melt.
• Interpreting Geological History: By analyzing the mineral composition of igneous rocks, geologists can infer the conditions under which the rocks formed, providing insights into the Earth's geological history.

The Discontinuous (Ferromagnesian) Series: A Step-by-Step Transformation

The discontinuous series, also known as the reaction series or ferromagnesian series, is characterized by a series of reactions where early-formed minerals react with the remaining melt to produce different minerals. This series proceeds in a stepwise fashion, with each mineral stable only within a specific temperature range. As the temperature decreases, the early-formed mineral becomes unstable and reacts with the remaining liquid to form a new, more stable mineral.

Here's a breakdown of the minerals in the discontinuous series, in order of crystallization:

1. Olivine (Highest Temperature): Olivine is the first mineral to crystallize in the discontinuous series. It's a ferromagnesian mineral with a high magnesium content. Olivine is stable at high temperatures and is commonly found in ultramafic rocks like peridotite, which are found in the Earth's mantle. The general formula of olivine is (Mg,Fe)2SiO4.

2. Pyroxene: As the temperature decreases, olivine reacts with the remaining magma to form pyroxene. Pyroxene is another ferromagnesian mineral, but it contains less magnesium and more silica than olivine. Pyroxene is a chain silicate mineral with a general formula of (Mg,Fe)SiO3. It's commonly found in mafic rocks like basalt and gabbro.

3. Amphibole: As the temperature continues to drop, pyroxene reacts with the remaining magma to form amphibole. Amphibole is a more complex ferromagnesian mineral that contains water in its crystal structure. Amphibole is a double-chain silicate mineral with a complex formula that generally includes (Mg,Fe)7Si8O22(OH)2. It's commonly found in intermediate rocks like andesite and diorite.

4. Biotite Mica: As the temperature decreases further, amphibole reacts with the remaining magma to form biotite mica. Biotite is a sheet silicate mineral that contains potassium, iron, magnesium, and aluminum. Biotite has a general formula of K(Mg,Fe)3AlSi3O10(OH)2. It's commonly found in felsic rocks like granite and rhyolite.

Why is it called Discontinuous? The term "discontinuous" refers to the fact that each mineral in the series is distinct and stable only within a specific temperature range. As the temperature changes, the earlier-formed mineral becomes unstable and reacts to form a completely new mineral, rather than gradually changing its composition. The reaction is abrupt, hence the term discontinuous.

The Continuous (Plagioclase Feldspar) Series: A Gradual Transition

The continuous series, also known as the plagioclase feldspar series, involves a solid solution of two end-member minerals: albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Unlike the discontinuous series, the continuous series doesn't involve abrupt reactions. Instead, the composition of the plagioclase feldspar changes gradually as the temperature decreases.

Here's a breakdown of the plagioclase feldspar series:

1. Anorthite (Calcium-Rich): At high temperatures, the plagioclase feldspar that crystallizes is rich in calcium (anorthite). This is because calcium ions are more readily incorporated into the crystal structure at higher temperatures.

2. Gradual Change in Composition: As the temperature drops, the plagioclase feldspar becomes progressively richer in sodium (albite) and poorer in calcium. This is because sodium ions become more stable in the crystal structure at lower temperatures. The plagioclase crystals that form at intermediate temperatures have a composition that falls between anorthite and albite, with varying proportions of calcium and sodium. This continuous change in composition is what gives the series its name.

3. Albite (Sodium-Rich): At the lowest temperatures, the plagioclase feldspar that crystallizes is rich in sodium (albite).

Why is it called Continuous? The term "continuous" refers to the fact that the composition of the plagioclase feldspar changes gradually and continuously as the temperature decreases. There are no abrupt reactions or formation of completely new minerals, but rather a smooth transition in composition. The plagioclase crystals formed at different temperatures are all part of the same solid solution series.

The Significance of Bowen's Reaction Series: Applications and Implications

Bowen's Reaction Series is more than just a list of minerals and their crystallization temperatures; it's a powerful tool for understanding the formation and evolution of igneous rocks. Here are some key applications and implications of the series:

• Understanding Igneous Rock Compositions: The series helps explain why certain minerals are commonly found together in igneous rocks. For example, rocks that form at high temperatures, like peridotite and basalt, are typically rich in olivine, pyroxene, and calcium-rich plagioclase. Rocks that form at lower temperatures, like granite and rhyolite, are typically rich in quartz, orthoclase feldspar, and sodium-rich plagioclase.

• Explaining Magmatic Differentiation: Bowen's Reaction Series is crucial for understanding the process of magmatic differentiation. As magma cools, minerals crystallize and are removed from the melt. The removal of these minerals changes the composition of the remaining magma, leading to the formation of different types of igneous rocks. For example, if olivine and pyroxene crystallize early in the cooling process, the remaining magma will become enriched in silica and aluminum, leading to the formation of more felsic rocks like granite.

• Interpreting Rock Textures: The series can help explain the textures of igneous rocks. For example, porphyritic textures, where large crystals are embedded in a fine-grained matrix, can be explained by the fact that some minerals crystallize early at depth where cooling is slow and large crystals can grow. Then, the remaining magma is erupted onto the surface, where it cools rapidly, forming a fine-grained matrix around the pre-existing crystals.

• Predicting Mineral Deposits: Bowen's Reaction Series can be used to predict the occurrence of certain mineral deposits. For example, chromite, an ore of chromium, is often associated with ultramafic rocks like peridotite, which are rich in olivine. This is because chromite tends to crystallize at high temperatures, along with olivine.

• Understanding Weathering Processes: The series also provides insights into the weathering of igneous rocks. Minerals that crystallize at high temperatures, like olivine and pyroxene, are less stable at the Earth's surface and are more susceptible to weathering. Minerals that crystallize at lower temperatures, like quartz and feldspar, are more stable and resistant to weathering.

• Relating to Goldich Dissolution Series: The Goldich Dissolution Series is the weathering counterpart of Bowen's Reaction Series. While Bowen's series describes the order in which minerals crystallize from magma, Goldich's series describes the order in which minerals weather at the Earth's surface. Minerals that crystallize at high temperatures in Bowen's series are the least stable and most susceptible to weathering in Goldich's series, and vice versa.

Limitations of Bowen's Reaction Series: A Simplified Model

While Bowen's Reaction Series is a valuable tool, it's important to remember that it is a simplified model of a complex process. There are several limitations to the series:

• Simplified Composition: The series assumes a relatively simple magma composition. In reality, magmas can be very complex, containing a wide range of elements and compounds. The presence of other elements can affect the crystallization temperatures and the order in which minerals form.

• Igneous Rock Type Variations: The series doesn't account for all types of igneous rocks. Some igneous rocks, like kimberlites and lamproites, have very unusual compositions and don't fit neatly into the framework of Bowen's Reaction Series.

• Role of Volatiles: The series doesn't explicitly consider the role of volatiles, such as water and carbon dioxide. Volatiles can significantly affect the crystallization temperatures and the types of minerals that form.

• Pressure Factor: The series is primarily based on experiments conducted at atmospheric pressure. In reality, magmas can crystallize at a wide range of pressures, which can also affect the crystallization process.

• Non-Equilibrium Conditions: The series assumes that the magma cools slowly and that equilibrium is maintained throughout the crystallization process. In reality, cooling can be rapid, and equilibrium may not be achieved, leading to the formation of metastable minerals.

Despite these limitations, Bowen's Reaction Series remains a valuable tool for understanding the formation and evolution of igneous rocks. It provides a framework for predicting mineral compositions, explaining rock textures, and understanding magmatic differentiation.

Bowen's Reaction Series: A Quick Reference Chart

To summarize the key concepts, here's a quick reference chart of Bowen's Reaction Series:

Frequently Asked Questions (FAQ) About Bowen's Reaction Series

• What is the main purpose of Bowen's Reaction Series?

  • The main purpose is to describe the order in which minerals crystallize from cooling magma, allowing geologists to predict the mineral composition of igneous rocks.

• What are the two main branches of Bowen's Reaction Series?

  • The discontinuous (ferromagnesian) series and the continuous (plagioclase feldspar) series.

• What is the difference between the discontinuous and continuous series?

  • The discontinuous series involves abrupt reactions where early-formed minerals react with the remaining melt to form completely new minerals. The continuous series involves a gradual change in the composition of plagioclase feldspar as the temperature decreases.

• What are some limitations of Bowen's Reaction Series?

  • The series simplifies magma composition, doesn't account for all igneous rock types, doesn't explicitly consider the role of volatiles, and is primarily based on experiments at atmospheric pressure.

• How can Bowen's Reaction Series be used in mineral exploration?

  • It can be used to predict the occurrence of certain mineral deposits by understanding the association of specific minerals with particular rock types. For example, the presence of chromite is often associated with rocks rich in olivine, as both crystallize at high temperatures.

• What is the Goldich Dissolution Series, and how does it relate to Bowen's Reaction Series?

  • The Goldich Dissolution Series describes the order in which minerals weather at the Earth's surface, and it is essentially the weathering counterpart of Bowen's Reaction Series. Minerals that are most stable at high temperatures are the least stable at the Earth's surface.

Conclusion: Bowen's Enduring Legacy

Bowen's Reaction Series remains a cornerstone of igneous petrology, providing a fundamental framework for understanding the formation and evolution of igneous rocks. While it is a simplified model with certain limitations, its predictive power and explanatory capabilities have stood the test of time. By understanding the principles of Bowen's Reaction Series, geologists can gain valuable insights into the Earth's geological history, the formation of mineral deposits, and the processes that shape our planet. The series continues to be a valuable tool for both students and researchers in the field of geology, solidifying Norman L. Bowen's place as a pioneer in the study of igneous rocks.