Igneous Chemical Composition PDF

GEOL 102 PETROLOGY Chemical Components of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Rocks Molten material from which igneous rocks form are rich in Silica (SiO2) Chemical Composition of Igneous Rocks ...

GEOL 102 PETROLOGY Chemical Components of Igneous Rocks 2nd Year 2nd Semester | DGEOL - Patrick Sam M. Buenavista  Igneous Rocks Molten material from which igneous rocks form are rich in Silica (SiO2) Chemical Composition of Igneous Rocks  Igneous Rocks Major Elements Elements with concentration of > 1 wt.% in Earth’s crust. 7 ions bonded with Oxygen = SiO2, Al2O3, FeO, CaO, Na2O, K2O, MgO Minor Elements Elements with concentration of 1.0 wt.% - 0.1 wt.% in crust Cr, Mn, P, H, Ti; Concentration is related to SiO 2 concentration Trace Elements Elements with concentration of < 0.1 wt.%; in ppm (1000ppm) Provide information on genesis and history of igneous rocks. REEs, HFSEs, LILs  Compatibility Measure of the ease by which an element fits into a crystal structure. Compatible Elements Incompatible Elements Fit easily into crystal structures and Does not easily fit into crystal structure form long-lasting bonds. and forms easily-breakable bonds. Immobile; do not readily migrate Mobile; readily migrate into melt Tend to remain in Residual Rock (Restite) Tend to migrate into Anatectic Melts Fe, Mn, Zn, Ti, V, Cr, Co, Ni, Cu; HFS REE, LIL Anders & Grevesse (1989)  Trace Elements Rare Earth Elements (REEs) Elements with atomic numbers from 57 (La) to 71 (Lu). Odd-number REE are more abundant, so crustal REE / chondrite REE = Chondrite-normalized Pattern Light REE La, Cs, Pr, Nd, Sm; more incompatible than HREE Heavy REE Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; less incompatible than LREE  Trace Elements High Field Strength (HFS) elements Have relatively high ionic charge (+3, +4) for a given radius. Ionic radius:Valence charge ratio is < 0.2; immobile elements Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, Y; useful for tracing mantle processes Large Ion Lithophile (LIL) elements Elements with radius:valence ratio of > 0.2 for a given radius. Mobile elements; Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th, Ta Determine role of hydrous fluid interaction and parent melt. Mineral Composition of Igneous Rocks Primary Minerals Minerals that crystallize directly from magma at elevated T. Secondary Minerals Minerals that form later due to chemical reactions between melt and country rock. These replace primary minerals or fill in voids through alterations.  Major Minerals Constituents that occur in > 5% abundance. Quartz Quartz, Tridymite, Cristobalite K-feldspar Microcline, Orthoclase, Sanidine Plagioclase Anorthite to Albite Feldspathoid Leucite, Nepheline, Sodalite Mica Muscovite, Biotite, Phlogopite, Lepidolite Amphibole Hornblende, Riebeckite, Richterite Pyroxene Augite, Diopside, Pigeonite, Aegerine, Hypersthene, Enstatite, Bronzite Olivine Forsterite to Fayalite  Accessory Minerals Constituents that occur in < 5% abundance. Magnetite Corundum Pyrope Hematite Apatite Melilite Ilmenite Cassiterite Monazite Spinel Pyrite Epidote Sphene Chalcopyrite Allanite Rutile Molybdenite Tourmaline Fluorite Pentlandite Topaz Chromite Pyrrhotite Columbite Zircon Uvarovite Uraninite Terminologies on Igneous Chemical Composition Color Index / Mafic Index Proportion of mafic minerals in the total abundance of felsic and mafic minerals in an igneous rock. Does not include dark-colored non-crystalline solids Color Index / Mafic Index [ 𝐶𝐼 = %𝑚𝑎𝑓𝑖𝑐 %𝑚𝑎𝑓𝑖𝑐 +% 𝑓𝑒𝑙𝑠𝑖𝑐 ] x 100 Color Index / Mafic Index Color Index / Mafic Index Modal Classification / Mode Actual identification of mineral and its percentage by volume by visual inspection Hand lens for coarse-grain Petrographic microscope for fine- grain  Modal Classification / Mode Quartz Orthoclase (Alkali Feldspar) Biotite Plagioclase Modal Minerals Quartz Plagioclase Alkali Feldspar Mafic Minerals Feldspathoid  Point Count Analysis More accurate modal classification using petrographic microscope. A thin section is moved incrementally on a grid system and tabulates atleast 400 mineral points. Point Count Analysis Point Count Analysis Normative Mineralogy / CIPW Norm Indirect identification scheme using data derived from chemical analysis of the rock sample. Cross, Iddings, Pirsson, & Washington (1902) Normative Mineralogy / CIPW Norm 1. Chosen hypothetical mineral set closely resembles the actual minerals. 2. Magma crystallized at low-pressure, anhydrous condition near the surface, forming volcanic rocks.  Normative Minerals Quartz Magnetite Olivine Orthoclase Ilmenite Enstatite Albite Apatite Hypersthene Anorthite Corundum Nepheline Diopside Descriptive Terms based on Chemical Composition Abundance of Silica Based on SiO2 content  Silica Saturation Based on quartz, feldspars, and feldspathoids  Silica Saturation Presence/absence of quartz, feldspars, and feldspathoids All available oxides were used, and additional SiO2 Oversaturated remains to form normative quartz (free quartz) All available oxides and SiO2 were used. Saturated Indicated by no quartz or with forsterite/feldspathoid SiO2 is depleted before all oxides are used; insufficient Undersaturated SiO2 to form quartz, feldspars, or orthopyroxenes. Indicated by presence of forsterite or feldspathoids.  Abundance of Aluminum oxide Based on Al2O3 content compared to CaO, Na2O, K2O Peraluminous Minerals with unusually high Al2O3 Muscovite, Corundum, Topaz, Kyanite, Sillimanite, Andalusite Peralkaline Normative or modal minerals with high K2O and/or Na2O Na-rich minerals e.g., Aegerine, Riebeckite, Arvedsonite, Aenigmatite Mafic minerals with average Al2O3 content Metaluminous Lack of Al2O3-rich mineral, Na-pyroxene, Na-amphibole Subaluminous Mafic minerals with low Al2O3 content Abundance of Aluminum oxide Based on Al2O3 content compared to CaO, Na2O, K2O Questions?