基性岩、中性岩

•There is much geochemical and seismic evidence for mantle heterogeneity (at any one depth)
•Many reasons for mantle heterogeneity: melting history, metasomatism , contamination with subducted crust etc
•Isotopic ratios strongly suggest source heterogeneity
•Remember that these are insensitive to magma fractionation processes (and hence reflect source compositions) •OIB basalts have higher initial Sr-87/Sr-86 than MORB, implying that the mantle in the region underlying OIBs had higher Rb (an incompatible element)
•OIB and some continental WPB associated with highly evolved often alkaline magmas(even nephelinites, phonolites)
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Degree of Partial Melting and Basalt Types
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Alkalic, Tholeiitic and Calc-Alkaline
Sub-alkalic rocks can be subdivided into tholeiitic and calc-alkaline •Tholeiitic and calc-alkaline rocks plot as distinct fields on AFM plot
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Basalt Petrogenesis: Within-Plate Basalts
•Continental and oceanic within-plate magmatism have some similarities. They are richer in incompatibles than MORB, and (with exception of CFB) are dominantly alkalic •Dominantly alkali nature of OIB implies low degrees of partial melting •CFB volumes imply MUCH higher degrees of melting from mantle less depleted than MORB and are typically linked with episodes of continental rifting. •WPB basalts typically much more geochemically heterogeneous than MORB, implying significant mantle heterogeneity
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Basalt Petrogenesis: Convergent Margins
•Basalts are common in such a setting and very commonly occur with andesites. Most complex basalt petrogenesis to understand： •Tholeiitic basalts occur where subduction zone is shallowest •Alkalic basalts where subduction zone is deepest •IAB generally more enriched than MORB and have higher initial Sr87/Sr86 ratios •IAB most common in young arcs
•At least some of the variation in basalt types reflects different degrees of partial melting
Alkali basalts are of low degrees of melting (3-7%) and tholeiitic basalts are from higher degrees (20-30%) of melting (demonstrated experimentally and consistent with their relative abundances
Basalts: Basics
•WPB (Within-Plate Basalts) includes OIB, CFB and other within-continent basalts •MORB and OIB basalts must originate in mantle, but for CFB and arc basalts ，we must also consider crust, including melting of subducting oceanic crust •Arc basalts intimately associated with andesites
Tholeiitic refers to igneous rocks (usually basalts) that are low in K, low in incompatible elements, low Fe2O3/FeO ratio and displays pronounced Fe-enrichment on the AFM plot Calc-alkaline rocks do not display Fe enrichment on AFM plot and have higher incompatible elements
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Major Element Characteristics of Basalt Types
•MORB basalts are of typical tholeiite character: ie very low in incompatible elements(K, Ba, P, Sr, U, Th, Zr), low Fe2O3/FeO etc •IAB and continental arc basalts: tholeiite to calcalkaline, but some alkaline also. Compared to MORB all of these basalts are high in K2O and low in MgO and CaO
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Source Heterogeneity and Basalt Types
•This is especially important when considering basalts associated with convergent margins (where crust is subducted)
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Alkalic, Tholeiitic and Calc-Alkaline
•Common three-fold geochemical classification of basalts (and other igneous rocks)
•Igneous rocks can be alkalic (alkaline) or sub-alkalic, depending on ratio of total alkalis to silica •OIB and some within-continent basalts are commonly alkalic. May have nepheline crystals •Sub-alkalic rocks are much more common than alkalic
橄榄玄武岩 Ol＋Cpx＋Pl
（MacKenzie等，1982）
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Basalts: Basics
•Basalts are most common primary melt of mantle and are very common on Earth and terrestrial planets, hence its important to have a good grasp on basalts •Basalts occur in every tectonic environment •5 tectonic settings commonly used to subdivide basalts. Each has a distinct chemistry： •MORB (Mid-Ocean Ridge Basalt), • IAB (Island Arc Basalt), •CAB (Continental ArcBasalts, but CAB also used for CalcAlkaline Basalts), •OIB (Ocean Island Basalt) •CFB (Continental Flood Basalt).
•OIB basalts: Most commonly alkalic, but tholeiitic (not as poor in K and incompatibles as MORB) types occur •CFB basalts: Voluminous tholeiitic basalts. Not as poor in K and incompatibles 6as MORB