Igneous Petrology and Magmatic Differentiation

Fractional Crystallization and Chemical Differentiation

• Definition and Process: Rocks frequently undergo differentiation as they crystallize. If a magma chamber crystallizes minerals such as olivine first, that olivine can remain at the base of the chamber. This process is known as fractional crystallization.
• The Mathematical Logic of Removal: By taking a fraction of the liquid and extracting specific chemical components to form olivine, the remaining liquid is physically altered.
  • Since olivine is a low-silica but high-magnesium ($Mg$) and high-iron ($Fe$) mineral, the liquid that remains after its removal contains less magnesium and iron and has a higher relative silica ($SiO_2$) content.
• Predictive Mineralogy: Knowledge of mineral composition allows for the prediction of subsequent liquid changes. As the liquid shifts from ultramafic to mafic, the silica content increases while magnesium decreases and iron increases.
• Trace Element Enrichment: Elements that do not fit into the crystal structure of early-crystallizing minerals like olivine (e.g., sodium ($Na$), potassium ($K$), calcium ($Ca$), phosphorus ($P$), and various trace elements) become increasingly concentrated in the residual liquid.

Mineralogical Transition from Mafic to Felsic

• Basalt to Diorite Transition: Crystallizing sub-equal amounts of pyroxene and plagioclase feldspar from a basaltic magma produces diorite.
  • Pyroxene Removal: Extracts iron, magnesium, and calcium.
  • Plagioclase Removal: Extracts silica, calcium, sodium, and aluminum.
• The Shift to Silica-Rich Liquids: As each new mineral crystallizes, different elements are removed, shifting the remaining melt to be more silica-rich and calcium-rich, and significantly less iron- and magnesium-rich.
• Diorite Composition: In a diorite, quartz begins to crystallize alongside sodium-rich plagioclase (rather than calcium-rich). Amphibole and biotite also appear.
  • Hydrous Minerals: Both amphibole and biotite incorporate water ($H_{2}O$) into their crystal structures, indicating that enough water has built up in the magma to produce hydrous minerals.
  • The Andes Connection: Andesites are the volcanic equivalents of diorites. They are named after the Andes Mountains because the volcanoes there produce high volumes of dioritic and andesitic magma changes.
• The Granite Stage: Moving from diorite to granite (via granodiorite and tonalite) involves the crystallization of quartz, plagioclase feldspar, orthoclase (potassium feldspar), and biotite.
  • In granite, the amount of plagioclase is quite low compared to orthoclase, which provides the potassium ($K$).

Bowen’s Differentiation Sequence and Numerical Content

• Silica Content Ranges:
  • Ultramafic rocks: Approximately the $40\%$ range.
  • Mafic rocks: Approximately the $50\%$ range.
  • Intermediate (Diorites): $60\%$ through the $69\%$ range.
  • Felsic (Granites): Roughly $70\%$ silica ($SiO_2$), resulting from high quartz and silica-rich potassium feldspar content.
• Thermal/Chemical Correlation: As silica increases, the temperatures of melting and crystallization both decrease.
• Co-occurrence Restrictions: Minerals that do not overlap on the vertical column of the differentiation diagram cannot occur together in the same rock.
  • Olivine and Quartz: These cannot coexist because quartz only crystallizes from high-silica magmas. If enough silica were present to form quartz, the magma would have produced pyroxene instead of olivine (as pyroxene has the same elements as olivine but requires more silica).
  • Orthoclase and Olivine: Orthoclase (potassium feldspar) occurs at the low-temperature differentiated end of the sequence and will not occur with high-temperature olivine.

The Skaergaard Intrusion Case Study

• Location and Context: One of the world's best-studied layered mafic intrusions is the Skaergaard Intrusion in East Greenland.
• Geological History: It is $65,000,000$ years old. It was intruded during the rifting of the northern Atlantic (East Greenland, Iceland, Scotland). Magnetism from East Greenland is identical to that of West Scotland, proving they were once attached.
• Scale and Dimensions:
  • Depth/Height: Roughly $6\,km$.
  • Width/Length: Approximately $8\,km \times 10\,km$.
  • Total Volume: Roughly $480\,km^3$ ($8 \times 10 \times 6$) of magma in a single chamber.
• Compositional Spectrum: The base consists of olivine cumulates, while the top contains granitic pegmatites, capturing the full temperature and compositional spectrum.
• Thermal Dynamics: Magma chambers this large can remain liquid for millions of years. Rocks are poor conductors of heat, so cooling rates through hot wall rocks are very slow. Even once solidified, the rock may remain at temperatures like $600^{\circ}\text{C}$.

Mechanics of Melting and Magma Components

• Melting Points of Pure Minerals: Pure minerals have extremely high melting temperatures (e.g., micas can reach $1,200\,^{\circ}\text{C}$ to $1,400\,^{\circ}\text{C}$).
• Impure/Mixed Melting: A mixture of minerals (an impure material) will melt at a much lower temperature than a pure mineral. Melting typically begins at grain boundaries where two or three minerals meet.
• The Critical Melt Fraction: Melt starts as isolated pools. Only when the melt fraction reaches about $3\%$, $4\%$, or $5\%$ does it become a connected network that can be squeezed out by rock pressure and migrate into dikes or conduits.
• Magma Definition: Magma is not just liquid. It consists of three parts:
  1. Melt: The liquid component.
  2. Crystals: The solid fragments.
  3. Bubbles: The gas phase.
• Magma Mixing and Assimilation:
  • Assimilation: Magma can digest and melt pieces of the surrounding wall rock, changing its composition.
  • Mixing: Two different magmas (e.g., a mantle-derived basalt and a crust-derived rhyolite) can mix or erupt simultaneously. Mount Etna in Italy has erupted rhyolite from a central caldera and basalt from the base at the same time.

Cumulate Textures and Layering

• Cumulus Minerals: Crystals that form in the magma and settle to the floor because they are denser than the liquid. They are often well-formed (euhedral) because they crystallized in an open liquid.
• Adcumulus: Continued growth of the cumulus mineral on top of the original crystal after it has settled.
• Intercumulus: Minerals that grow from the final liquid trapped in the angular spaces between settled crystals.
• Density Separation:
  • Sinking: Mafic minerals like olivine and pyroxene (iron/magnesium silicates) sink.
  • Floating: Minerals like plagioclase feldspar can sometimes be less dense than the basaltic liquid and float to the roof, a process called "underplating."
• Igneous Stratigraphy: This process creates sedimentary-like bedding in igneous rocks, known as cumulate layering. It allows geologists to identify the wall, floor, or roof of a magma chamber.

Economic Geology and Newfoundland Examples

• Economic Enrichment: Processes in mafic magma chambers can concentrate valuable minerals. Platinum and chromium are enriched in basaltic magmas.
• Chromite and Platinum: Dense minerals like chromite ($FeCr_2O_4$) or sperrylite (a platinum sulfide) settle into discrete, high-density layers on the chamber floor.
• Newfoundland Occurrences:
  • Lewis Hills: Contains mantle rocks with complex chromite layers.
  • Falconbridge Discovery: A mining company drilled a layer in the eighties containing a quarter of a million ($250,000$) tons of chromite.
  • Labrador: Contains some of the world's largest anorthosite magma chambers, which are greater than $90\%$ plagioclase feldspar.

Grandiose Examples: Sierra Nevada and Yosemite

• Yosemite National Park: Features spectacular ridges of layered granites that show the same features as mafic intrusions.
• Flow Meters: In these granites, feldspar crystals (some half as long as a human finger) are aligned. They lay flat on the depositional surfaces and align with the flow direction of the magma currents, acting as paleocurrent indicators for the molten rock.