AFM and sediments

Summary

The video discusses igneous AFM diagrams, focusing on chemical trends in basalt formation. It highlights the differences between mid-ocean ridge and arc basalts, emphasizing the iron enrichment trend in the former and the alkaline trend in the latter. The lecture explores how differing crystallization processes impact magma composition, with a notable emphasis on the roles of water and oxidation in magmas from subduction zones. Additionally, it introduces beryllium-10 as a tracer for sediment involvement in volcanic processes and discusses the implications of boron content in oceanic crust, connecting various arc systems and their unique compositional trends.

Takeaways

• 🌋 The AFM diagram is crucial for understanding igneous rock compositions, specifically the weight percent of sodium, potassium, magnesium, and iron oxides.
• 🔍 Most basalts, whether mid-ocean ridge or arc basalts, start from a similar composition on the AFM diagram.
• 📈 Mid-ocean ridge basalts exhibit an iron enrichment trend, contrasting with the more straightforward compositional trend of continental arc basalts, which often lead to rhyolite formation.
• ⚗️ The alkaline trend is associated with higher aluminum contents, while calc-alkaline trends involve the co-precipitation of high magnesium and iron-rich minerals.
• 💧 Water plays a significant role in the composition of magmas, with arc magmas typically having a higher water content than mid-ocean ridge basalts.
• 🌊 Different types of subduction zones, including ocean-ocean and ocean-continent, significantly influence volcanic activity and magma characteristics.
• 🚫 Flat slab subduction, characterized by a shallow dip, can result in a lack of volcanic activity due to insufficient magma generation.
• 🧪 Beryllium-10 serves as an important tracer for geological processes, indicating rapid movement and mixing within magma systems.
• 📊 Boron concentrations in sediments and altered oceanic crust highlight the mixing of materials in volcanic processes.
• 🔄 The presence of multiple magma pulses in andesite samples suggests complex differentiation and interaction with pre-existing rocks.

Q & A

What are AFM diagrams and their significance in studying magmas?

AFM diagrams are used to illustrate the chemical composition trends of magmas, particularly in igneous and metamorphic contexts. They help visualize the relationships between sodium, magnesium, and iron content, revealing important trends in basaltic compositions.

How do mid-ocean ridge basalts differ from arc basalts in their chemical trends?

Mid-ocean ridge basalts typically follow an iron enrichment trend on the AFM diagram, moving up and then down in composition. In contrast, arc basalts, especially continental arc basalts, do not exhibit this trend but move directly towards rhyolite compositions through different crystallization processes.

What is the alkaline trend observed in certain basalts?

The alkaline trend refers to a compositional shift in certain basalts characterized by higher aluminum content and an increase in sodium and potassium while decreasing iron and magnesium content. This trend is observed in high alumina basalts.

What are the key minerals involved in the foliation and calcalan trends?

In the foliation trend, high magnesium minerals like olivine and pyroxene precipitate, while the calcalan trend involves the co-precipitation of high iron phases, such as magnetite, along with high magnesium materials.

Why do arc magmas typically have higher water content compared to mid-ocean ridge basalts?

Arc magmas often originate from subduction zones where water-rich sediments are introduced into the mantle, leading to higher water content in the resultant magmas. In contrast, mid-ocean ridge basalts have lower water content due to their formation processes.

What role does beryllium-10 play in understanding subduction processes?

Beryllium-10 is produced in the atmosphere and can be transported into subduction zones. Its presence in volcanic rocks indicates rapid processes, as it decays within a relatively short timeframe (1.5 million years), providing insights into the speed of material movement through geological systems.

How does the composition of the crust affect volcanic activity in subduction zones?

The age and thickness of the crust influence volcanic activity; for example, in the Andean subduction zone, the varying crustal ages and slab dip angles affect magma generation and volcanic arc formation. Shallow slab dips can lead to a lack of volcanism, known as flat slab subduction.

What is the significance of mixing arrays observed in volcanic compositions?

Mixing arrays represent the isotopic and compositional variations in volcanic rocks, indicating that these materials result from the interaction between mantle-derived magmas and continental or sedimentary components. This helps geologists understand the origins and evolution of volcanic systems.

What are some of the primary rock types found in the Andean volcanic zones?

In the Andean volcanic zones, common rock types include basalt, andesite, tonalite, and granodiorite, with tonalites and granodiorites being the most volumetrically significant.

How does the crystallization sequence influence magma evolution?

The sequence of crystallization affects magma composition by determining which minerals solidify first. This process leads to variations in chemical composition and can create diverse rock types within a volcanic system, reflecting the history of magma mixing and differentiation.