The Chemistry of Igneous Rocks 1- Some important terms a- Modal mineralogy versus normative mineralogy: - Modal mineralogy: is the abundance of minerals expressed in volume percent. - Normative mineralogy: is the calculated mineralogical compositions based on the major elements oxides abundances of the rock. It is used to classify the fine-grained rocks. b- Major, minor and trace elements: - Major elements: are those make up more than 1weight percent (wt.%) of the rock. They include (SiO2, Al2O3, FeO and Fe2O3, MgO, CaO, Na2O, and K2O). - Minor elements: typically make up 0.1 to 1.0 weight percent of a rock. These include TiO2, MnO, and P2O5. - Trace elements: Trace elements compose less than 0.1 percent of a rock; they are expressed as elements as opposed to oxides and their concentrations are reported in parts per million by weight. The Chemistry of Igneous Rocks 1- Some important terms c- Compatible and incompatible elements: - Compatible elements: are those preferentially incorporated into crystallizing phases from a melt and decrease in abundance in the magma as differentiation proceeds. - Incompatible elements: are those not compatible with crystallizing phases from a melt and increase in abundance in the magma during differentiation because they are preferentially retained in the melt rather than incorporated into crystallizing phases. 2- Major elements indices of differentiation a. Alkali-Lime Index: - An important chemical control on the differentiation history of a magma is the relative abundances of CaO, Na2O, and K2O because these elements are important constituents of feldspars. - For most igneous rock series, CaO decreases with increasing silica on a binary diagram, whereas Na2O and K2O increase. Alkali-Lime Classification for Igneous Rocks Alkali-Lime Index (wt % SiO2 where CaO = Na2O + K2O) Name < 51% Alkalic 51–56% Alkali-calcic 56–61% Calc-alkalic >61% Calcic b- Iron Enrichment Index (Fe-index): - Magmatic differentiation in some suites (settings) leads to distinctive iron enrichment, whereas in other suites this iron enrichment is lacking. - The iron enrichment index [(FeO + 0.9Fe2O3) / (FeO + 0.9Fe2O3 + MgO)] measures the extent to which total iron became enriched during the differentiation of a magma. - It is important to note that the “iron enrichment,” measured by the Fe-index, is relative to magnesia. - Although the absolute abundance of FeO, Fe2O3, and TiO2 increase with differentiation during the early fractionation of most basalts, once Ti-magnetite and ilmenite begin to crystallize these oxides become compatible and decrease with differentiation. - Thus iron enrichment could take place in some suites even as the absolute abundance of iron decreases provided that iron abundance decreases at a slower rate than the depletion of MgO from the melt. - A number of differentiation processes may cause iron enrichment, including the fractional crystallization of olivine from a magma. In contrast, fractional crystallization of magnetite will deplete the melt in iron and enrich it in silica. d- Alkalinity Index (AI) - The alkalinity index (AI) (Al-(Na+K)): measures the relative abundances of aluminum and alkalis. - Alkaline rocks are defined as those that have higher sodium and potassium contents than can be accommodated in feldspar alone. - Peralkaline rocks have AI <0, whereas metaluminous and peraluminous rocks have AI >0.
