Volcanic SuitesFrancis 2014

PacayaAgua

Acatenango

Liquidus Projections for haplo-basaltsThe Basalt Tetrahedron at 1 atm:

The olivine - clinopyroxene - plagioclase plane is a thermal divide in the haplo-basalt system at low pressures and separates natural magmas into two fundamentally different magmatic series. Sub-alkaline basaltic magmas with compositions to the Qtz-rich side of the plane fractionate towards Qtz-saturated residual liquids, such as rhyolite. Alkaline basaltic magmas with compositions to the Qtz-poor side of the plane fractionate towards residual liquids saturated in a feldspathoid, such as nepheline phonolite.

Alkaline basalts fall to the Foid-side of the olivine-clinopyroxene-plagioclase plane (1 atm thermal divide) and fractionate to foid-saturated residual liquids. Sub-alkaline basalts fall to the Quartz-side and fractionate towards quartz-saturated residual liquids.

Since the dominant mineral in the mantle source of basaltic magmas is olivine, we can achieve a further simplification by projecting the liquidus of basaltic systems from the perspective of olivine:

The Effect of Pressure

Increasing pressure shifts the oliv-cpx-opx peritectic point towards less Si-rich compositions. At approximately 10 kbs this invariant point moves into the oliv - cpx- opx compositional volume, and the first melt of the mantle has an olivine basalt composition. The invariant point is still a peritectic point, however, because of the extensive solid solution of cpx towards opx. At pressures exceeding 15-20 kbs, this invariant point moves outside the simple olivine - cpx - qtz system, into the Neph-normative volume of the basalt tetrahedron. The first melt of mantle peridotite is an alkaline olivine basalt at these high pressures.

Under dry conditions, The invariant point determining the composition of the first melt moves from the silica-saturated field to the silica-undersaturated field with increasing pressure.

Projection schemes, such as the olivine projection, have gone beyond the limits of accurately portraying phase relations in multi-component systems. Furthermore, this projection scheme does not account for the variation in Fe/Mg, nor the effect of Fe-Ti oxide phases, which commonly appear during crystal fractionation. However, most basaltic magmas become saturated in olivine, clinopyroxene, and plagioclase after a relatively short temperature drop below their liquidii, and thereafter crystallize a “gabbroic” mineral assemblage as the residual liquid evolves in composition from basalt to andesite and then dacite. This is difficult to portray in phase diagrams because the plagioclase, olivine, cpx, and opx are all solid solutions, which change as the composition of the residual liquid evolves. These complications force us to abandon the phase diagram approach and turn to variation diagrams to study the liquid lines of descent in volcanic suites.

Parent Magma Crystal Cumulate + Residual Magma

Volcanic RocksPlutonic or

Intrusive Rocks

Mafic Magma Gabbroic Cumulates + Felsic Magma

Volcanic rocks approximate the compositions of magmatic liquids. They represent aliquots of liquid that have escaped to the surface. The compositional variation observed in the liquids that the volcanic rocks represent is produced by varying degrees of crystal fractionation of a largely “gabbroic” mineral assemblage that now comprises plutonic intrusions.

Crystal Fractionation

Whole = Σ Parts

Lever Rule: e/C = x/y

y

amount of granitein basalt = x/(x+ y)

C

Si variation diagrams (Harker Diagrams):

Si variation diagrams are commonly used in suites dominated by relatively evolved lavas, such as the andesites, dacites and rhyolites of calc-alkaline volcanic suites. The official classification scheme for volcanic rocks is based on a total alkalis versus SiO2 diagram.

Mg variation diagrams (Bowen Diagrams):

Mg is an analogue for temperature, so that plotting other elements against Mg, gives one an idea of how these elements change as temperature drops during crystal fractionation. This type of diagram is most commonly used in suites with relatively primitive Mg-rich lavas, and is less useful for volcanic suites dominated by relatively felsic lavas.

Appearance of Cpx

Sub-Alkaline Mafic Magmas:

Sub-alkaline basalts dominate mafic terrestrial magmatism, erupting in three distinct tectonic environments, each associated with its own trace element characteristics and liquid line of descent:

MORB: 20 km3/yrat mid-ocean ridges

Hot-spot: 2 km3/yrabove mantle plumes

Arc: 6 km3/yr behind subduction zones

MORB and Hot Spot Subduction Zones

Although sub-alkaline volcanic suites are easily distinguished from alkaline volcanic suites in a plot of total alkalis versus silica, tholeiitc volcanic suites are similar to calc-alkaline volcanic suites.

Tholeiitic

Calc-alkaline

Tholeiitic versus Calc-Alkaline Fractionation Trends

Calc-alkaline volcanic suites are characterized by decreasing Fe and with decreasing Mg, in the range from basalt to andesite

Tholeiitic volcanic suites are characterized by increasing Fe and decreasing Al with decreasing Mg in the range from basalt to andesite, while Si rises quite slowly with fractionation.

These differences are greatest at the boundary between basalt and andesite (SiO2 = 55 wt.%), where tholeiitic andesites typically have Al2O3 content less

than 15%, in contrast to the higher Al2O3 contents of calc-alkaline andesites.

Tholeiitic volcanic suites are characterized by decreasing Al with slowly increasing Si in the range from basalt to andesite.

Calc-alkaline volcanic suites are characterized by increasing Al with increasing Si in the range from basalt to andesite.

Ti is typically low (< 1.2 wt.% TiO2) in calc-alkaline suites and

remains relatively constant and then decreases slowly with increasing Si.

In tholeiitic suites, Ti first increases by a factor of 2 or more at almost constant Si and then decreases rapidly with increasing Si in the range 50 to 55 wt.% SiO2.

Calc-alkaline versus Tholeitiic Volcanic Suites

Oceanic versus Continental Volcanic Arcs

The modal composition of calc-alkaline volcanic suites on continents is shifted to higher Si contents (andesites dominate) than those of oceanic suites (basalts dominate), and the mafic to intermediate lavas that build the strato-volcanoes of the continental arcs are typically accompanied by the eruption of voluminous felsic ignimbrite sheets, along with the intrusion of coeval granitoids whose “dacitic” compositions occupy a population minimum in the volcanic suite, between rhyolite and evolved andesite.

Melt Inclusion Data

Are andesite magmas mixtures of

mantle-derived basaltic magmas

and rhyolitic partial melts of the

continental crust?

M A S HMixing - Assimilation - Storage - Homogenization

Oceanic versus Continental Tholeiitic Suites

Continental tholeiitic volcanic suites are commonly bi-modal, consisting a dominant basalt to basaltic andesite mode and a lesser rhyolite mode. In most cases, the rhyolites are thought to represent melts of the continental crust, induced by the basaltic magmas. Mature tholeiitic volcanic suites in the oceans, such as Iceland, are also bi-modal, with the rhyolites being produced by second stage melting of amphibolitized basalts at the base of the basaltic pile.

Wet Melting:

The presence of water has a dramatic effect in lowering the solidus temperature of peridotite, such that the introduction of water may cause it to melt, if its ambient temperature is between that of the dry and wet solidus. This process is thought to be important in the mantle wedge above and behind subduction zones.

Wet Melting:

The presence of water also has a dramatic effect on the composition of the invariant point representing the first melt of the mantle, shifting it to relatively Si-rich compositions more similar to basaltic andesite to andesite rather than olivine basalt. The presence of water during melting may in part explain the abundance of andesites with respect to basalts in subduction zone magmatism.

dry

Many of these differences can be understood in terms of the effect of water pressure on phase equilibria. In tholeiitic suites, dry low-pressure conditions favour the early appearance and fractionation of plagioclase, which induces Fe-enrichment and Al depletion in the derived residual liquids. In calc-alkaline suites, however, fractionation at elevated water pressures suppresses the crystallization of plagioclase, as a result there is an absence of Fe enrichment and Al depletion during fractionation from basalt to andesite.

The Effect of Water

The presence of water dramatically lowers the stability of plagioclase in basaltic magmas, inhibiting its crystallization to temperatures well below the liquidus. The absence of plagioclase in the early fractionating mineral assemblage prevents the build up of Fe and leads intermediate residual liquids that are rich in Al and Si.

The Effect of Water

Early plagioclase fractionation drives tholeiitic basalts to very Fe-rich basaltic compositions to the point at which an Fe-Ti oxide begins to crystallize Mg.

The presence of water inhibits the crystallization of feldspar in calc-alkaline magmas resulting in no Fe build-up and leading to residual liquids that are poor in Fe.

The Effect of Water

Early plagioclase fractionation drives tholeiitic basalts to Al-poor compositions and reduces the increase in Si with decreasing Mg in residual liquids.

The presence of water inhibits plagioclase crystallization to temperatures well below the liquidus. The absence of plagioclase in the early fractionating mineral assemblage results in a continued increase in Al with more rapidly increasing Si with decreasing Mg into the andesite range

The Effect of Water

The appearance of an Fe-Ti oxide on the liquidus of Fe-rich tholeiitic basalts is clearly visible in the tholeiitic suite. Note the nearly 3 fold increase in Ti over a very limited increase in Si.

Calc-alkaline suites are more oxidized, Fe-Ti oxides crystallize relatively early keeping Ti low with increasing Si in the residual liquids.

The Effect of Water

Ironically, this leads to over saturation in plagioclase at low pressures, and the development of the strongly plagioclase–phyric character typical of basalts and andesites in most calc-alkaline suites. Upon rising to the surface, calc-alkaline magmas lose their dissolved water and become supersaturated in plagioclase because of their composition with respect to the position of the one atmosphere cpx – plag cotectic. Further, when plagioclase does come on the liquidus at high water pressures, the modal proportion of plagioclase in the cumulate assemblage is much higher than that at low pressures.

The presence of water dramatically lowers the stability of plagioclase in basaltic magmas, inhibiting its crystallization to temperatures well below the liquidus.

Under wet conditions, the invariant point determining the composition of the first melt of the mantle moves towards more Si-rich compositions (andesitic) with increasing pressure.

Summary of the differences between Calc-alkaline and

Tholeiitic fractionation trends

Calc-alkaline volcanic suites are characterized by decreasing Fe and increasing Al with decreasing Mg, in the range from basalt to andesite, while Si rises relatively rapidly with fractionation. Ti is typically low (< 1.2 wt.% TiO2) and remains relatively constant and

then decreases slowly with increasing Si. Calc-alkaline volcanic suites are commonly dominated by lavas of intermediate composition, like andesite. The magmas of calc-alkaline suites are also characterized by relatively high oxidation states compared to the tholeiitic basalts of MORB or OIB suites. Tholeiitic volcanic suites are characterized by increasing Fe and decreasing Al with decreasing Mg, in the range from basalt to andesite, while Si rises quite slowly with fractionation. These differences are greatest at the boundary between basalt and andesite (SiO2 ~ 55 wt.%),

where tholeiitic andesites typically have Al2O3 content

less than 15%, in contrast to the higher Al2O3 contents of

calc-alkaline andesites. Ti first increases by a factor of 2 or more and then decreases rapidly with increasing Si in the range 50 to 55 wt.% SiO2. Tholeiitic volcanic suites

are typically bimodal, with large volumes of basalt, smaller volumes of rhyolite, and a relative paucity of lavas with intermediate andesitic compositions.