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Basalts and Related Mafic Volcanics

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Basalts and Related Mafic Volcanics Basalts and related mafic volcanics Basalt: Simple petrographic description: Fine-grained to porphyritic volcanic rock composed predominantly of subequal amounts of plagioclase and clinopyroxene (augite). Simple chemical definition: Volcanic rock containing between 45 and 53 % SiO2. Restriction of our discussion of mafic volcanic rocks to basalt as described above excludes a variety of volcanic rock types, many present in small amounts, which are important members of the family of mafic volcanics. So I will extend this important group to include any fine-grained to porphyritic mafic volcanic (or subvolcanic) rock that formed primarily by partial melting of the mantle. It is also appropriate to include in this group of volcanics the variety of daughter magma types formed by differentiation of a parental basalt, e.g., phonolites, trachytes, rhyolites, etc. Many mafic volcanics so defined would appear to have little in common, e.g., MORB and nephelinite, but it is now clear that both formed by partial melting of the mantle and that differences in composition, mineralogy, eruptive style, tectonic association, volatile content, etc. reflect differences in the mantle source regions, particularly differences in P, T, melt fraction, volatile fugacities, and, of course, composition and mineralogy of the source region. Primary magmas, as distinct from parental or primitive magmas, are the “holy grail” of petrology but are very rare. Why? How might we recognize a primary magma? “Melodrama of geochemical adventures” (Dave Walker) have occurred during the segregation from source and subsequent transport and emplacement in or on the crust.. Melodrama: fractional crystallization, assimilation, magma mixing + minor processes such as flow differentiation, compaction, Soret diffusion, liquid immiscibility, volatile transfer… Polybaric fractionation of primary magmas: important concept championed by O’Hara Partial melting of mantle peridotite 1100 1200 1300 1400 1500 (TºC) Melting begins when upwelling Plag lherzolite mantle intersects the peridotite (ol-opx-cpx-pl) solidus. With decreasing P above solidus the solidus, extent of melting 10 increases. The amount of melting Spinel lherzolite 20% is limited by the heat available since (Ol-opx-cpx-sp) 1% the heat of fusion is large. Extent of 50 P (kbars) melting can vary from ~1% to 20% 20 ~20%. The T, P, % melting, compn 10% and mineralogy of source region, and presence and types of volatiles Garnet lherzolite 1% (Ol-opx-cpx-gar) present determine the composition 30 of the basaltic magma produced. Depth (km) Basalt 100 15 g tin el 40 103 adiabat l m O a 2 rti Gra Pa phite Diam 5 lherzolite ond 50 Wt% Al harzburgite 150 0 dunite 0 .4 .8 1.2 1.6 Wt%TiO2 60 Partial melting (~15%) of fertile lherzolite produces basalt leaving depleted residue of harzburgite + dunite Polybaric Fractionation (what is it?) Hypothetical polybaric cooling path (yellow) 1. Segregation of magma from source rock followed by stalling and cooling at ~25 kb during which cpx and garnet 1 (eclogite) crystallized. Fractionation would occur if the crystals were removed from the system, even partially. 2. Stalling and fractionation at 12 kb during which Al-rich cpx and plagioclase fractionated 3. Stalling and fractionation of ol + plag + cpx at 1-2 kb (shallow magma chamber) 4. Post-eruptive fractionation of ol + cpx + plag + FeTi oxides (probably minor because 2 of rapid cooling following eruption) This, of course, is only part of the melodrama. What about assimilation, magma mixing,…? PT projection of phase stability fields determined experimentally in a basalt from Snake River Plains (after Thompson (1972). 3 Carnegie Inst. Wash Yb. 71) 4 2 principal types of basalt Subalkaline (Tholeiitic) Basalt and Alkalic Basalt Common petrographic differences between tholeiitic and alkaline basalts Tholeiitic Basalt Alkalic Basalt Usually fine-grained, intersertal, ophitic Commonly fairly coarse, intergranular, ophitic Groundmass No olivine and rare/no alkali feldspar Olivine common Cpx = augite (± pigeonite) Titanaugite (faint violet color) Opx (hypersthene) may rim olivine Opx and pigeonite absent Fine-grained FeTi oxides Interstitial sanidine or feldspathoid may occur Glass (if present) is usually dacitic/rhyolitic Glass is rare, quartz absent Phenocrysts Olivine (slightly zoned) commonly resorbed Olivine (commonly zoned) ± reaction rims of opx Plagioclase usually follows olivine Plagioclase common Clinopyroxene is titaniferous (zoning common) Cpx: pale brown augite; Pigeonite: Variable Opx absent Opx: uncommon Microphenocrysts of Fe-Ti oxides Microphenocrysts of Fe-Ti oxides (Ilm, Mt) It is difficult to distinguish subalkaline basalts from alkalic basalts petrographically, even in thin section. If a chemical composition is available, it is possible to be much more precise in classification. Simple chemical classification [LeBas et al. (1986) J. Pet., 27, 745] and a more complex classification [Irvine and Barager (1971) Can. J. Earth Sci, 8, 523] Additional petrographic features In a sense, single basalt samples may represent microcosms of the fractionation process because they have cooled sufficiently quickly that fractional crystallization was the dominant process. Clear evidence is the occurrence of rhyolitic glass in the groundmass of many tholeiitic basalts and strongly zoned phenocrysts that reflect changing melt compositions. Melt inclusions in phenocrysts also preserve melt comps at various stages of crystallization. Pyroxene quadrilateral Ternary feldspars Composition of pyroxenes in olivine tholeiites and icelandites from Iceland showing extensive zonation of augite and pigeonite. From Carmichael (1967) Am. Min. 52, 1815. Fe/(Fe+Mg) increases with fractn Coexisting Fe-Ti oxides (ilm-hem solid solutions and 304 feldspar analysis in a single section of Picture magnetite-ulvospinel solid solutions) are widespread in most basalts and are widely used as geothermometers and Gorge basalt (An84Or0.5 to An0.3Or60) illustrating extreme fractional crystallization during fairly rapid oxybarometers. The oxygen fugacity of the basaltic magmas cooling. After Lindsley and Smith (1971) Carn. exercises an important control on crystallization trends. Inst. Wash. Yb., 69, 274. Mineral chemistry (cont.) Ternary feldspar Groundmass plagioclase and compositions sanidine from (a) potassic basalt and (b) trachyte. Alkali-rich lavas tend to produce more alkali feldspar and less plagioclase. Note the two separate fractionation trends (cores to margins) for the coexisting plagioclase and sanidine. Alkalic magmas are significantly richer in K2O and Na2O relative to tholeiitic basalts Back scattered electron (BSE) image of zoned plagioclase phenocryst Backscattered electron images of phenocrysts in mafic magmas Microphenocrysts of cpx, opx, plag, ilmenite, magnetite, Melt inclusion in cpx phenocryst. Note also ilm-mt, apatite in a rhyolitic glass matrix (Mt. Baker andesite) apatite and glass, Mt. Baker andesite Phenocryst of twinned amphibole in alkali basalt Zoned amphibole phenocryst, Mt. Baker andesite from Canary Islands. Note the breakdown rim Mafic magma types (chemical distinction) Alkalis vs silica plot Alkali versus Silica plot was originally proposed to distinguish different types of Hawaiian basalts. Can be applied to other provinces. Data from McDonald (1968) GSA Memoir 116 Below right: Projection from Di (cpx) shows data from a variety of basalts classified on petrographic criteria as tholeiitic (black) or alkalic (orange). Note the revised dividing line. From Irvine and Barager (1971) Can J. Earth Sci, 8, 523 Basalt tetrahedron Basalt tetrahedron is based on normative minerals computed from chemical analysis Norm incompatibilities: Ne and En (hyp) Ne and Q, Fo (ol) and Q Basalts will plot in one of the three sub- tetrahedra: Qz-normative (Quartz tholeiites), Ol+hyp normative (olivine tholeiites); Ne normative (alkalic basalts) From: Yoder and Tilley (1962) J. Pet., 3, 342 Simple chemical classification of Volcanic Rocks After Le Bas et al. (1986) J. Petrol., 27, 745-750. .
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