Download VolcSuite14

Volcanic Suites
Francis 2014
Agua
Pacaya
Acatenango
Liquidus Projections for haplo-basalts
The 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.
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:
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 quartzsaturated residual liquids.
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.
Crystal Fractionation
Parent Magma
C
Crystal Cumulate + Residual Magma
Plutonic
or
Intrusive Rocks
Volcanic Rocks
Whole = Σ Parts
Lever Rule: e/C = x/y
Mafic Magma
Gabbroic Cumulates + Felsic Magma
amount of granite
in basalt = x/(x+ y)
y
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.
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/yr
at mid-ocean ridges
Hot-spot:
2 km3/yr
above mantle plumes
Arc:
behind subduction zones
6 km3/yr
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 calcalkaline 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.
Calc-alkaline volcanic suites
are
characterized
by
increasing Al with increasing
Si in the range from basalt to
andesite.
Tholeiitic volcanic suites are
characterized by decreasing Al
with slowly increasing Si in
the range from basalt to
andesite.
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 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 calcalkaline 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?
MASH
Mixing - Assimilation - Storage - Homogenization
Oceanic versus Continental
Tholeiitic Suites
Continental tholeiitic volcanic suites are commonly bimodal, 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 bimodal, 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.
dry
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.
The Effect of Water
Many of these differences can be understood in terms of the effect
of water pressure on phase equilibria. In tholeiitic suites, dry lowpressure 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 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
The presence of water dramatically lowers the stability
of plagioclase in basaltic magmas, inhibiting its
crystallization to temperatures well below the liquidus.
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.
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.