Basalt with Quartz: US Guide for Geologists

Basalt with quartz, a seemingly paradoxical combination, presents unique challenges and opportunities for geologists across the United States, particularly when understood through the lens of petrogenesis and interpreted using tools such as the polarizing microscope. The formation of basalt with quartz, often linked to specific tectonic settings within regions like the Columbia River Basalt Group, challenges conventional models of magma evolution. Fieldwork led by researchers at institutions like the United States Geological Survey (USGS) provides crucial data for understanding the conditions under which these atypical rock compositions arise. Detailed analysis of these formations, referencing the work of pioneers like Norman L. Bowen, helps to refine our understanding of fractional crystallization and silica activity in basaltic systems, thereby shedding light on the complex geological history of various US regions.

The Unexpected Guest: Quartz in Basalt

Basaltic rocks, the dark and fine-grained extrusive igneous formations, constitute a significant portion of Earth’s oceanic crust and are widespread in continental flood basalt provinces. Their genesis, typically in the upper mantle, gives rise to a relatively consistent mineralogical composition.

Typical Basalt Composition

A characteristic basalt comprises predominantly plagioclase feldspar and pyroxene minerals. These minerals crystallize from a relatively low-silica, high-temperature melt, resulting in a mafic (magnesium and iron-rich) composition. Olivine is also commonly present, and small amounts of iron-titanium oxides may occur.

The overall silica (SiO2) content in basalt typically ranges from 45 to 52 weight percent. This silica level is crucial because it dictates which minerals can stably crystallize from the melt. Notably absent in this mineral assemblage is quartz (SiO2), a silica-rich mineral.

The Quartz Anomaly: An Enigmatic Presence

The presence of quartz in basaltic rocks, therefore, presents a paradox. Quartz, with its high silica content, is typically associated with felsic (feldspar and silica-rich) igneous rocks like granite or rhyolite, which form from silica-rich, lower-temperature melts. Its occurrence in basalt raises fundamental questions about the petrogenesis of the host rock.

Why is quartz sometimes found within a rock type that, by its very nature, should not contain it? This anomaly challenges conventional models of magma evolution and requires a deeper investigation into the potential geological processes that could lead to this unexpected association.

Implications for Magma Genesis and Evolution

The existence of quartz within basalt suggests that the basaltic magma either underwent unusual differentiation processes or interacted with external silica-rich sources. These processes may include:

• Crustal contamination: Assimilation of silica-rich crustal rocks into the basaltic melt.
• Magma mixing: Interaction between basaltic and more evolved, silica-rich magmas.
• Xenocryst incorporation: Entrainment of quartz crystals from pre-existing rocks.

Understanding the origin of quartz in basalt can provide valuable insights into the complex interplay of magmatic processes. It can help unravel the thermal history of the magma, the extent of crustal involvement in magma genesis, and the dynamics of magma chambers beneath active volcanoes.

Scope of Discussion

Our exploration will focus on specific geological formations and processes that offer plausible explanations for the presence of quartz in basalt. We will examine locations such as the Columbia River Basalt Group, the Snake River Plain, and Yellowstone National Park. These serve as natural laboratories for investigating the geological mechanisms responsible for this fascinating geological anomaly.

Geological Playgrounds: Quartz-Bearing Basalt Formations

Having established the unexpected nature of quartz in basaltic environments, it’s time to explore real-world examples. Certain geological formations offer compelling evidence of this phenomenon, providing valuable insights into the processes that facilitate quartz incorporation. Let’s journey to a few key locations where this geological puzzle unfolds.

Columbia River Basalt Group (CRBG): A Flood of Intrigue

The Columbia River Basalt Group (CRBG) stands as one of the most extensive flood basalt provinces on Earth. This massive outpouring of basaltic lava, covering parts of Washington, Oregon, and Idaho, presents an intriguing case for the occurrence of quartz.

A Voluminous Eruption

The CRBG’s scale is truly impressive, with individual flows extending for hundreds of kilometers. These flows erupted over a relatively short period, between approximately 17 and 6 million years ago.

The sheer volume of basalt raises questions about the source of the magma and its composition, particularly the presence of minerals not typically associated with basaltic melts.

Potential Sources of Quartz

The presence of quartz within the CRBG basalts suggests either crustal contamination or interaction with silicic melts.

Crustal contamination occurs when basaltic magma, as it rises through the Earth’s crust, incorporates surrounding rock material. The continental crust is rich in silica. The assimilation of such material could introduce quartz into the basaltic system.

Another possibility involves the mixing of basaltic magma with more silicic magmas. Silicic magmas, characterized by their high silica content, can contain quartz as a primary mineral. The interaction of these two magma types could lead to the presence of quartz crystals within the resulting basaltic rock.

Further research, including detailed geochemical analyses, is needed to fully understand the sources of quartz within the CRBG.

Snake River Plain, Idaho: A Dance of Magmas

Moving eastward, the Snake River Plain in Idaho presents another fascinating geological setting where quartz-bearing basalts can be found. This region is known for its bimodal volcanism, characterized by both basaltic and silicic eruptions.

A Hotspot’s Trail

The Snake River Plain is thought to be the surface expression of the Yellowstone hotspot. This hotspot has migrated eastward over millions of years, leaving a trail of volcanic activity in its wake. The interplay between basaltic volcanism and silicic volcanism is a defining feature of the region.

Basalt-Silica Interaction

The proximity and interaction of basaltic and silicic magmas are crucial for understanding the presence of quartz. As basaltic magma ascends through the crust, it may encounter pockets of silicic magma.

Mixing between these magma types can result in the incorporation of quartz crystals from the silicic magma into the basaltic melt. This process can lead to the formation of basaltic rocks with unusual mineral compositions.

Yellowstone National Park: A Volcanic Puzzle

Yellowstone National Park, situated at the eastern end of the Snake River Plain, is a volcanic landscape of immense complexity. The park’s geology provides a unique opportunity to study the processes that lead to quartz occurrence in basaltic rocks.

Magmatic Complexity

Yellowstone is underlain by a large magma chamber. This chamber fuels the region’s hydrothermal activity and occasional volcanic eruptions. The magmatic system beneath Yellowstone is far from simple, involving multiple magma sources and complex interactions between magmas of different compositions.

Quartz Incorporation Mechanisms

Given the magmatic complexity, several mechanisms could contribute to the presence of quartz in Yellowstone’s basalts.

Magma mixing is a likely candidate, as basaltic and silicic magmas coexist within the Yellowstone system. The assimilation of crustal material is another potential mechanism, particularly given the thick continental crust beneath the park.

The study of quartz-bearing basalts in Yellowstone is essential for understanding the dynamics of large silicic systems. These systems often involve small volumes of mafic rocks that contain quartz, as a result of magma mixing or crustal assimilation.

Understanding the interplay between these processes is a key focus of ongoing research.

The Culprits: Geological Processes Behind the Quartz Anomaly

Having explored specific geological locations where quartz manifests within basaltic rocks, we now turn our attention to the underlying mechanisms that allow this seemingly paradoxical combination to occur. Several geological processes can introduce or generate quartz within a basaltic environment, each leaving its own unique signature. Understanding these processes is crucial to deciphering the history of magma genesis and evolution.

Xenocrysts: Foreign Crystals in a Basaltic Sea

The term xenocryst refers to a crystal that is foreign to the magma in which it is found. In the context of quartz-bearing basalt, xenocrysts represent quartz crystals that were incorporated into the basaltic magma from an external source.

Sources and Characteristics

These foreign crystals can originate from several sources:

• Country Rock Entrainment: As magma ascends through the Earth’s crust, it can incorporate fragments of the surrounding rock. If this country rock is rich in quartz, such as granite or gneiss, quartz crystals can be physically plucked and carried along by the magma.

• Residue from Partially Melted Rocks: During partial melting events, not all minerals melt completely. Quartz, being relatively refractory, can remain as a solid residue that is then entrained in subsequent magma batches.

Xenocrysts often exhibit distinct characteristics that distinguish them from crystals that formed directly from the basaltic melt. These may include:

• Reaction Rims: The quartz crystal may react with the surrounding basaltic melt, forming a rim of different minerals around the xenocryst.

• Corroded or Rounded Shapes: The crystal may show signs of dissolution or abrasion, indicating that it was not in equilibrium with the melt.

Magma Mixing: When Basalt Meets Silica

Magma mixing is a fundamental process in magmatic systems, involving the interaction of two or more chemically distinct magmas. When basaltic magma encounters a more silicic magma (e.g., rhyolitic or dacitic), the resulting mixture can exhibit unexpected mineral assemblages, including quartz within a dominantly basaltic rock.

The key here is disequilibrium. Basaltic magmas are typically undersaturated with silica. Silicic magmas, on the other hand, are silica-rich. When these magmas mix, quartz can be inherited from the silicic component, even if it wouldn’t normally crystallize from the basalt alone.

The efficiency of magma mixing can vary significantly, ranging from complete homogenization to incomplete mixing. Incomplete mixing often results in textures where quartz crystals are clearly associated with the silicic component, providing evidence of the mixing process.

Assimilation: Consuming the Crust

Assimilation refers to the process by which magma incorporates surrounding crustal material through melting or dissolution. This is different than the mechanical incorporation of xenocrysts. Crustal rocks, particularly those in continental settings, are often rich in silica. The assimilation of such material into basaltic magma can effectively increase the silica content of the melt, potentially leading to quartz saturation and crystallization.

Sources and Mechanisms of Crustal Assimilation

• Melting of Wall Rocks: As magma resides in or ascends through the crust, it can transfer heat to the surrounding rocks, causing them to partially or completely melt. This molten crustal material can then be incorporated into the magma.

• Dissolution of Crustal Fragments: In addition to melting, magma can also dissolve solid fragments of crustal rock. The rate of dissolution depends on the temperature and composition of both the magma and the crustal material.

• AFC Processes: A combined process of assimilation and fractional crystallization (AFC) can significantly alter the composition of magma. As magma crystallizes, it can simultaneously assimilate crustal material, leading to complex geochemical trends.

Partial Melting: Extracting the Silica

Partial melting is the process by which only a portion of a rock melts, resulting in a melt that is chemically different from the original rock. While basaltic magmas are typically generated by partial melting of the mantle, the partial melting of crustal rocks can produce silica-rich melts.

If a basaltic magma interacts with or incorporates these silica-rich partial melts, it can become saturated with quartz. The degree of partial melting and the composition of the source rock are critical factors in determining the silica content of the resulting melt.

Fractional Crystallization: Pushing Towards Silica

Fractional crystallization is a process in which minerals crystallize from a magma and are physically separated from the remaining melt. As certain minerals crystallize and are removed (typically early-forming, mafic minerals like olivine and pyroxene), the composition of the residual melt changes.

This process tends to enrich the remaining melt in silica and other incompatible elements. If fractional crystallization proceeds far enough, the residual melt can eventually become saturated with quartz, even if the original magma was basaltic in composition. While not directly introducing quartz, it sets the stage for its stability and eventual crystallization.

Quartz Unveiled: Forms and Textures in Basalt

Having explored specific geological locations where quartz manifests within basaltic rocks, we now turn our attention to the underlying mechanisms that allow this seemingly paradoxical combination to occur. Several geological processes can introduce or generate quartz within a basaltic environment, resulting in distinct forms and textures observable in the field and under the microscope.

Quartz Xenocrysts: Captured Crystals

One of the most direct ways quartz appears in basalt is as xenocrysts. These are crystals that originated from a source external to the basaltic magma itself. Essentially, they are foreign inclusions "captured" by the ascending or emplaced basaltic melt.

Recognizing Xenocrysts: A Microscopic Hunt

Identifying quartz xenocrysts requires careful petrographic examination. Key features include:

• Reaction rims: These are zones of alteration or reaction surrounding the quartz crystal where it interacts with the surrounding basaltic melt. The basaltic melt, being out of equilibrium with the quartz, will react with it, forming a new mineral assemblage around the quartz grain.

• Rounded or corroded shapes: Unlike euhedral (well-formed) crystals that grew directly from the basaltic melt, xenocrysts often exhibit rounded edges or embayments due to partial dissolution within the magma.

• Compositional zoning that is unrelated to the host basalt: Xenocrysts may show compositional zoning, reflecting their original growth environment, which is distinct from the chemical signature of the basaltic magma.

Sources of Quartz Xenocrysts

The source of quartz xenocrysts can vary depending on the geological context. Potential sources include:

• Continental crust: Basaltic magmas traversing continental crust can incorporate quartz grains from pre-existing silicic rocks like granite or gneiss.

• Sedimentary rocks: If the basaltic magma passes through sedimentary formations containing quartz sandstones, these grains can be incorporated as xenocrysts.

• Hydrothermal veins: Quartz veins formed by hydrothermal activity in the surrounding country rock can be ripped up and entrained in the magma.

Quartz Veins and Amygdules: Filling the Cracks and Voids

Quartz can also be found filling fractures (veins) or vesicles (amygdules) within basalt. These occurrences indicate post-crystallization alteration and fluid flow within the basaltic rock.

Veins: Pathways of Silica-Rich Fluids

Quartz veins form when silica-rich fluids circulate through fractures in the basalt. As the fluid cools or experiences changes in pressure or chemical environment, quartz precipitates out of solution, gradually filling the crack.

The thickness and extent of quartz veins can vary significantly, depending on the size and connectivity of the fractures, and the duration of fluid flow.

Amygdules: Bubbles Transformed

Amygdules are vesicles (gas bubbles) in the basalt that have been subsequently filled with secondary minerals. Quartz can be one of these secondary minerals, along with other phases like calcite, zeolites, or chlorite.

The formation of quartz amygdules indicates that silica-rich fluids permeated the basalt after it had solidified, precipitating quartz within the open pore spaces.

Quartz in Associated Silicic Rocks: A Proximal Relationship

The presence of silicic volcanism in proximity to basalt formations significantly impacts the potential for quartz occurrence. This proximity facilitates interactions that can introduce quartz into basaltic systems.

Magma Mixing: A Volcanic Blend

Magma mixing occurs when two or more distinct magma compositions combine within a magma chamber or during ascent.

If a basaltic magma encounters a more silicic magma (e.g., rhyolite or dacite), the two can mix to varying degrees. This mixing can result in the incorporation of quartz crystals or even partially melted silicic material into the basalt.

The textures resulting from magma mixing can be complex, often showing disequilibrium features like reaction rims and mingled domains of different compositions.

Crustal Assimilation: Incorporating the Surroundings

In addition to magma mixing, basaltic magmas can also assimilate crustal rocks as they ascend or reside in magma chambers. If the surrounding crust is rich in silica (e.g., granite or quartzite), assimilation can introduce quartz into the basaltic system.

This process typically involves the partial melting and incorporation of the surrounding rock into the magma. This can lead to an increase in the silica content of the basalt and the presence of quartz crystals derived from the assimilated material.

Decoding the Clues: Analytical Techniques for Studying Quartz in Basalt

Having explored specific geological locations where quartz manifests within basaltic rocks, we now turn our attention to the underlying mechanisms that allow this seemingly paradoxical combination to occur. Several geological processes can introduce or generate quartz within a basaltic environment, resulting in a fascinating interplay of mineral compositions and textures. To fully understand these intricate formations, scientists employ a variety of powerful analytical techniques.

These methods act as crucial detective tools, helping to unravel the origin, history, and conditions under which quartz found its way into the basaltic matrix. Let’s delve into the specific techniques that geologists utilize in their quest to decode the clues hidden within these rocks.

Geochemistry: Unveiling Chemical Fingerprints

Geochemistry forms the cornerstone of understanding the genesis and evolution of both the basaltic host rock and the embedded quartz. By analyzing the chemical composition of these materials, researchers can identify unique chemical fingerprints that provide invaluable insights.

Major and Trace Element Analysis

Major element analysis reveals the fundamental building blocks of the minerals, while trace element analysis acts like a microscopic GPS, pinpointing the source and pathways of the involved materials. The ratios of certain trace elements can be particularly diagnostic. For example, rare earth element (REE) patterns can distinguish between mantle-derived magmas and those that have interacted with crustal rocks. This is crucial for assessing potential contamination or magma mixing events.

By carefully comparing the elemental signatures of quartz and the surrounding basalt, geochemists can infer whether the quartz originated from a different source and was later incorporated, or whether it crystallized directly from the basaltic magma itself under unusual conditions.

Petrography: A Microscopic Window into the Past

Petrography, the study of rocks under a microscope, provides a crucial first step in characterizing quartz-bearing basalts.

Thin Section Analysis

By preparing thin sections – incredibly thin slices of rock – and examining them under a polarized light microscope, petrologists can identify the minerals present, their textural relationships, and any signs of alteration or deformation. This is essential for identifying quartz and assessing its relationship to other minerals in the basalt.

The textural relationships, such as the size, shape, and arrangement of minerals, are particularly insightful. For example, the presence of corroded or embayed quartz crystals suggests that they were not in equilibrium with the surrounding melt and may have been xenocrysts – foreign crystals brought in from another source.

Geochronology: Dating the Basaltic Events

Understanding the timing of magmatic events is crucial for piecing together the history of quartz-bearing basalts. Geochronology, the science of dating rocks and minerals, provides the necessary temporal framework.

Radioisotope Dating Techniques

Radioisotope dating techniques, such as argon-argon (⁴⁰Ar/³⁹Ar) dating and uranium-lead (U-Pb) dating, can be used to determine the age of the basalt and, in some cases, the quartz itself. By comparing the ages of the basalt and quartz, researchers can determine whether the quartz formed at the same time as the basalt or was incorporated later.

Geochronological data can also be used to correlate basaltic volcanism with other geological events, such as regional tectonics or climate change.

Advanced Microscopic and Spectroscopic Techniques

Beyond traditional petrography, several advanced techniques provide more detailed insights into the composition and structure of quartz in basalt.

Optical Microscopy (Petrographic Microscope)

This technique involves examining thin sections of rocks under a polarized light microscope. It allows for the identification of minerals based on their optical properties, such as birefringence and pleochroism.

Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS)

SEM provides high-resolution images of the sample surface, revealing micro-textures and the spatial distribution of elements. EDS is used to determine the elemental composition of specific points or areas on the sample. Together, SEM and EDS can identify and map the distribution of quartz and other minerals in basalt.

Electron Microprobe Analysis (EMPA)

EMPA allows for precise, quantitative chemical analysis of small areas within a sample. This is particularly useful for determining the composition of individual quartz crystals and surrounding minerals.

X-Ray Diffraction (XRD)

XRD is used to identify the crystalline phases present in a sample. By analyzing the diffraction pattern of X-rays passed through a powdered sample, researchers can determine the mineral composition of the basalt and identify the presence of quartz.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS)

ICP-MS is a highly sensitive technique used to measure the concentrations of trace elements in rocks and minerals. This can provide valuable information about the origin and evolution of the basalt and quartz.

Stable Isotope Analysis: Tracing Origins Through Isotopic Signatures

Stable isotope analysis, particularly oxygen isotopes, can provide valuable insights into the source and history of quartz in basalt.

Oxygen Isotopes as Tracers

The ratio of ¹⁸O to ¹⁶O in quartz and other minerals varies depending on the temperature and source of the fluids from which they formed. By analyzing the oxygen isotope composition of quartz, researchers can infer its origin and whether it interacted with different fluids during its formation.

For example, quartz that formed from high-temperature magmatic fluids will typically have a different oxygen isotope signature than quartz that formed from low-temperature hydrothermal fluids. This information can help determine whether the quartz in basalt is of magmatic origin or was introduced later by hydrothermal alteration.

By combining these analytical techniques, geologists can piece together a comprehensive picture of the origin and evolution of quartz-bearing basalts. Each technique provides unique information that, when integrated, helps to unravel the mysteries of these fascinating rocks.

Seekers of Basaltic Secrets: Institutions and Experts

Having decoded the chemical clues and textural signatures of quartz within basalt, it’s only natural to wonder where one can delve deeper into this fascinating field. Fortunately, a wealth of resources exists for those eager to unravel the mysteries of basaltic rocks and their unusual quartz inhabitants. From governmental surveys to academic institutions and individual experts, the path to knowledge is paved with readily accessible information.

United States Geological Survey (USGS): A National Repository of Geological Data

The United States Geological Survey (USGS) stands as a cornerstone of geological knowledge in the nation. Its role in mapping and researching basalt and quartz occurrences is paramount. The USGS provides an unparalleled wealth of data, from detailed geological maps to comprehensive research publications.

The USGS’s publications cover a vast range of topics, including the geochemistry, petrology, and geochronology of basaltic provinces. These resources are invaluable for researchers and enthusiasts alike, offering a solid foundation for understanding the geological context of quartz-bearing basalts.

Navigating the USGS Resources

The USGS website is a treasure trove of information, but navigating it effectively is key. Start by exploring the "Geology and Geophysics" section, which houses a wealth of reports, maps, and data portals. Keyword searches such as "basalt," "quartz," and specific locations (e.g., "Columbia River Basalt") can yield targeted results.

Furthermore, the USGS library system provides access to a vast collection of books, journals, and unpublished reports, offering a comprehensive overview of basaltic research.

State Geological Surveys: Regional Gateways to Understanding

While the USGS provides a national perspective, state geological surveys offer invaluable regional insights. Each state survey focuses on the specific geological features within its borders, providing detailed information on local basalt formations and associated quartz occurrences.

State surveys often conduct detailed mapping projects, publish regional geological guides, and maintain databases of geological information. These resources are particularly useful for understanding the local context of quartz-bearing basalts and identifying specific research opportunities.

Accessing State Survey Data

State geological survey websites are the primary gateway to accessing their data. These sites typically offer interactive maps, downloadable reports, and contact information for survey geologists.

Reaching out to state survey personnel can provide invaluable insights and guidance, especially for researchers interested in specific locations or geological formations.

Universities with Geology Departments: Academic Frontiers of Research

Universities with strong geology departments serve as vibrant centers of research and education. Their contributions to understanding basalt and volcanic processes are immense, particularly in areas related to quartz genesis and its interaction with basaltic magmas.

University researchers conduct cutting-edge studies, publish their findings in peer-reviewed journals, and train the next generation of geoscientists. Exploring university websites and databases can reveal a wealth of information on ongoing research projects, published papers, and expert faculty members.

Engaging with Academic Research

To delve into academic research, explore university geology department websites. Look for faculty profiles that highlight research interests and publications. Utilize academic search engines like Google Scholar or Web of Science to find relevant research papers.

Consider attending geology conferences and seminars to network with researchers and learn about the latest findings in basalt petrology and geochemistry.

Geologists Specializing in Basalt Petrology and Geochemistry: Consulting the Experts

Ultimately, connecting with individual geologists specializing in basalt petrology and geochemistry is crucial for gaining in-depth knowledge and personalized insights. These experts possess a wealth of experience and can provide valuable guidance for research projects or general inquiries.

Identifying and engaging with these experts can unlock access to specialized knowledge, unpublished data, and invaluable perspectives on the complexities of quartz-bearing basalts. They serve as critical resources for understanding the nuances of basaltic systems.

Identifying and Contacting Experts

Identifying experts in basalt petrology and geochemistry can be achieved through various channels. Start by exploring university faculty directories, searching for researchers who focus on volcanic rocks, magma genesis, or petrology.

Consulting professional organizations such as the Geological Society of America (GSA) or the American Geophysical Union (AGU) can provide access to member directories and conference proceedings, showcasing the work of leading experts in the field.

Once potential experts are identified, contacting them via email or phone can open the door to valuable conversations and collaborative opportunities.

So, next time you’re out in the field and spot something dark and intriguing, remember the odd couple – basalt with quartz! It might just tell you a fascinating story about the earth’s ever-changing processes and maybe, just maybe, lead to a new discovery. Happy hunting, geologists!