Carbonatites: NIL-silicate igneous rocks

Magma Composition and Igneous Rocks

Introduction In the following chapters we will find that nearly all of the varied aspects of volcanism are either directly or indirectly controlled by the chemical composition of magma and the rate at which it cools and solidifies. If you are to understand and appreciate subsequent discussions of volcanoes and related volcanic phenomena, it is essential that you have a good grasp of the basic c...

Occurrence, Texture, and Classification of Igneous Rocks

A paleointensity technique for multidomain igneous rocks

[1] We developed a paleointensity technique to account for concave-up Arai diagrams due to multidomain (MD) contributions to determine unbiased paleointensities for 24 trial samples from site GA-X in Pleistocene lavas from Floreana Island, Galapagos Archipelago. The main magnetization carrier is fine-grained low-titanium magnetite of variable grain size. We used a comprehensive back-zero-forth ...

AGeochemical Classification for Feldspathic Igneous Rocks

In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeOþMgO) ratio or Fe-index, the modified alkali^lime index, the aluminum-saturation index, the alkalinity index, and the feldspathoid silica-saturation index.The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have ...

The Strontium Isotopic Composition and Origin of Carbonatites

The carbonatite problem can be stated as follows: Are carbonatites genetically related to sedimentary carbonate rocks, or are they derived from a silicate parent material? The isotopic composition of strontium in 21 carbonatites and in several related alkaline rocks has been determined. The mean Sr87/$r86 ratio for the 21 carbonatites is 0.7065 + 0.0003. The Sr87/sr86 ratios of sedimentary carb...