Basalts and Related Rocks

1. Elements based on Concentration 

   1. Major >1.0 wt% 

      1. Present in high concentrations 

      2. Expressed as weight 

      3. Control a large extent the mineralogy and crystallization/melting behaviour in igneous systems 

   2. Minor 0.1 - 1.0 wt% 

      1. Commonly substitute for an element in a principal mineral 

      2. Ex. substitutes for Fe or Mg in most mafic minerals 

      3. If they reach a sufficient concentration, they may form a separate mineral phase present in minor amounts 

   3. Trace <0.1 wt% 

      1. Commonly substitute for an element in a principal mineral 

      2. Too dilute to form a separate phase 

      3. Ex. 

         1. Lanthanides (rare earth elements REEs), Platinum group of elements (PGEs), Transition metals 

2. Incompatible and compatible elements 

   1. Incompatible elements: usually reside in the melt phase and are excluded from crystalline minerals. Subgroups: 

      1. HFS High field strength elements: smaller, highly change 

      2. LIL Large ion lithophile elements: have low field strength, and are more mobile, particularly if a fluid phase is involved. 

   2. Compatible elements: more readily accommodated in one or more minerals than in the melt phase. 

Basalt 

1. Basalt: 

   1. fine-grained mafic igneous rock consisting essentially of augite + calcic plagioclase 

   2. Aphyric or with phenocrysts 

   3. The most voluminous type of rock on the Earth’s surface

   4. Mineralogy 

2. Olivine-rich basalts 

   1. Picrite:visibly enriched in olivine crystals as phenocrysts  

   2. Ankaramite: rich in olivine and augite phenocrysts

3. Common Classification Schemes for Basalts 

   1. Alkaline vs. subalkaline 

   2. On basis of tectonic setting 

4. K₂O 

   1. Most significant variable in major element composition 

5. On the basis of tectonic settings 

   1. Mid-oceanic ridges (MORBs) 

      1. Olivine tholeiitic basalts 

      2. Aphyric or most commonly contain phenocrysts 

      3. Crystallization sequence: olivine±Chromite±plagioclase±augite

      4. Glassy matrix - quenching by water 

      5. Vary little in composition 

      6. Characterized by low contents of K₂O and other incompatible elements 

      7. N-MORB: Normal MORB

      8. E-MORB: Enriched MORB

   2. Hotspots (OIBs):

      1. Ocean island basalts 

      2. Alkali basalts 

      3. Enriched in incompatible elements

   3. Subduction zones (IABs) 

SUMMARY 

• Typically formed in tectonic settings where oceanic crust is subducted beneath continental crust.

• Characterized by low silica content and high levels of sodium and potassium.