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 peridotiteMelting begins when upwelling mantle intersects the peridotitesolidus. With decreasing P above the solidus, extent of melting increases. The amount of melting is limited by the heat available since the heat of fusion is large. Extent of melting can vary from ~1% to ~20%. The T, P, % melting, compn

and mineralogy of source region, and presence and types of volatiles present determine the composition of the basaltic magma produced.D

epth

(km

)

GraphiteDiamond

solidus

Spinel lherzolite(Ol-opx-cpx-sp)

Garnet lherzolite(Ol-opx-cpx-gar)

20%

1%

20%

10%

1%

10

20

30

40

50

60

P (k

bars

)

50

100

150

1100 1200 1300 1400 1500 (TºC)Plag lherzolite(ol-opx-cpx-pl)

adiabat

Partial melting (~15%) of fertile lherzolite produces basalt leaving depleted residue of harzburgite + dunite

Basalt

0

5

10

15

0 .4 .8 1.2 1.6

Wt%

Al 2O

3

Wt%TiO2

lherzoliteharzburgite

dunite

Partial m

elting

PT projection of phase stability fields determined experimentally in a basalt from Snake River Plains (after Thompson (1972).Carnegie Inst. Wash Yb. 71)

1

2

34

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 (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 + cpxat 1-2 kb (shallow magma chamber)

4. Post-eruptive fractionation of ol + cpx + plag + FeTi oxides (probably minor because of rapid cooling following eruption)

This, of course, is only part of the melodrama. What about assimilation, magma mixing,…?

Polybaric Fractionation (what is it?)

Common petrographic differences between tholeiitic and alkaline basalts

Subalkaline (Tholeiitic) Basalt and Alkalic Basalt2 principal types of basalt

Tholeiitic Basalt Alkalic Basalt

Olivine (slightly zoned) commonly resorbed Olivine (commonly zoned)Phenocrysts± reaction rims of opx

Opx: uncommonMicrophenocrysts of Fe-Ti oxides (Ilm, Mt)

Opx absent

Plagioclase commonPlagioclase usually follows olivine

Cpx: pale brown augite; Pigeonite: Variable Clinopyroxene is titaniferous (zoning common)

Usually fine-grained, intersertal, ophitic Commonly fairly coarse, intergranular, ophiticGroundmass No olivine and rare/no alkali feldspar Olivine common

Cpx = augite (± pigeonite) Titanaugite (faint violet color)Opx (hypersthene) may rim olivine Opx and pigeonite absentFine-grained FeTi oxides Interstitial sanidine or feldspathoid may occurGlass (if present) is usually dacitic/rhyolitic Glass is rare, quartz absent

Microphenocrysts of Fe-Ti oxides

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 featuresIn 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.

304 feldspar analysis in a single section of Picture Gorge basalt (An84Or0.5 to An0.3Or60) illustrating extreme fractional crystallization during fairly rapid cooling. After Lindsley and Smith (1971) Carn. Inst. Wash. Yb., 69, 274.

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 magnetite-ulvospinel solid solutions) are widespread in most basalts and are widely used as geothermometers and oxybarometers. The oxygen fugacity of the basaltic magmas exercises an important control on crystallization trends.

Ternary feldspars

Pyroxene quadrilateral

Mineral chemistry (cont.)

Groundmass plagioclase and 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

Ternary feldspar compositions

Backscattered electron images of phenocrysts in mafic magmas

Microphenocrysts of cpx, opx, plag, ilmenite, magnetite, apatite in a rhyolitic glass matrix (Mt. Baker andesite)

Phenocryst of twinned amphibole in alkali basalt from Canary Islands. Note the breakdown rim

Melt inclusion in cpx phenocryst. Note also ilm-mt, apatite and glass, Mt. Baker andesite

Zoned amphibole phenocryst, Mt. Baker andesite

Mafic magma types (chemical distinction)Alkalis vs silica plot

Basalt tetrahedron

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

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)

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

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.