Mueller, R. F.

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NASA.	 ;LsTM X 6 3 9
OXIDATIVE CAPACITY OF
MAGMATIC COMPONENTS
ROBERT F. MUELLER
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. MAY 1970 $ii f	
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GODDARD SPACE FLIGHT CENTER_
GREENBELT, MARYLAND
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IX-644-70-196
Preprint
OXIDATIVE CAPACITY OF MAGMATIC
COMPONENTS
Robert F. Mueller
May 1970
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
rKECEdING PAGE BLANK NOT H
O)UDATIVF. CAPACITY OF MAGMATIC
COMPONENTS
Robert F. Mueller
ABSTRACT
The capacity of magmatic components such as dissolved or precipitated
ferrous and ferric iron compounds and water to control the oxygen fugacity of
magmas is treated from a theoretical standpoint. Stoichiometric calculations
indicate that in relatively anhydrous magmas, such as those of the Skaergaard
	 j
complex, the oxygen fugacity is determined by the reaction of ferrous and ferric
iron if these are abundant relative to the quantity of dissolved water. However
if the quantity of water is high relative to iron, as in the calc-alkali batholiths,
the oxygen fugacity is likelyto be determined by the dissolved water of a given
H, D/H2
 ratio and by the tendency of H 20 to react with the Si-O-Si bridges of the
si_cate melt.
iii
t
PW
OXIDATIVE CAPACITY OF MAGMATIC
COMPONENTS
INTRODUCTION
In recent years it has been stated by a number of authors that the oxidation
states of certain magmas (including both the melt and crystalline precipitates)
can be attributed to a buffering effect of the iron components. Thus it was
suggested by Wones and Eugster (1965) that the magmas of the Southern California
batholith were buffered by "anhydrous mineral assemblages." More recently
Dodge, Smith and Mays (1969) interpreted the rocks of the Sierra Nevada botho-
lith in much the same way. Since this question has an important bearing on the
origin of batholiths and other planetary problems it is worthwhile to determine
the extent to which this mechanism is operative.
The application of the buffer mechanism in mineral chemistry (Eugster,
1957) depends on obtaining a univariant relation between the oxygen fuga.city
and the temperature at some fixed phase composition. A necessary requirement,
whether the buffer system is internal or externa.i ;o the system under study, is
that the quantity of buffer be large enough to prevent exhaustion. An analogous
effect may also be obtained through other techniques such as equilibrating gas
mixtures with the system under study. This was the technique employed by
Muan and Osborn (1956) in their important study of the system MgO - FeO -
Si0 2 - Fe2 03 . These authors deduced that if a melt were crystallized at
constant total composition of the condensed phases (melt and crystals), so that
no oxygen was added to or subtracted from the equilibrated. gas, the oxygen
r
i
fugacity would decrease with the temperature in a manner analogous to that of
a buffer system. Osborn suggested that a relatively anhydrous magma of basaltic
composition, which crystallized as a closed system, would also behave in this
manner and would give rise to the gabbroic series of differentiates such as
those of the Skaergaard complex (Wager and Deer, 1939). Furthermore in these
differentiates the steady decrease in oxygen fugacity with crystallization would
be accompanied by both absolute iron enrichment of the melt and by relative
iron enrichment (as expressed by the atomic ratio Fe/(Fe + Mg)) in both the
melt and the precipitating crystals. It is clear that if natural anhydrous magmas
behave in this manner their ferrous and ferric constituents act analogous to
internal buffers. The application of this idea to the Skaergaard complex is
entirely reasonable since it agrees with the thermochemical data of Fig. 1 and
is corroborated by the detailed mineral compositions as determined by Wager
and Deer (1939).
Osborn (1959) also proposed an origin for the talc-alkali series of dif-
ferentiates which characterize the batholiths and large segments of continental
rocks in general. This hypothesis again draws on the system MgO - FeO -
Fe 203 - Si02 , but depends now on open system behavior in which the oxygen
fugacity is constrained to remain constant or even to increase as crystallization
occurs. As a result there is a precipitation of magnetite and a corresponding
enrichment of the melt in silica. Osborn therefore argued that the silica en-
richment, which also characterizes talc-alkali differentiation, is attributable
to the maintainence of relatively high oxygen fugacities by oxidizing constituents
such as water which are dissolved in the melt. However he made no attempt to
show that Poe did in fact remain high during talc-alkaline differentation or that
2
M
Ithe dissolved water in these magmas could act as an oxid;%ing agent. Un-
fortunately subsequent experimental work has virtually been confined to
anhydrous systems so that it has not been possible to determine the oxidative
characteristics of hydrous plutonic magmas from direct experiments.
REPRESENTATIVE MAGMATIC MINERAL REAC'T'IONS
There are a number of well known reactions which may be used to define
the oxidation and hydration states of magmas and which illustrate the point we
wish to make here. Some of these are as follows:
olivine	 fluid	 magnetite quartz
2 Fe2 SiO4 + 2 02 ^- Fe304 + 2 Si02	(a)
pyroxene	 fluid magnetite quartz
--	 3 Fe SiO3
 + 2 02 ^: Fe304 + 3 Si02	(b)
olivine	 fluid	 pyroxene	 magnetite
Fe2 SiO4 + 6 02 1. Fe Si O3 + 3 Fe304	(c)
biotite	 fluid	 feldspar magnetite fluid
	K Fe3 5i 3 Al lo (OH) 2 + 2 02 IV—	 308+ Fe304 + H2O	 (d)
Reaction (a) is well known through its utilization in experimental buffer
systems. However its use in natural rocks is restricted to quite reduced
assemblages in which the olivine is sufficiently iron-rich to coexist with free
silica. For this reason reactions (b) and (c) are more useful since they are
applicable to a wider range of conditions.
It may be shown that for the pure system FeO , - 02 - Si02 the oxygen
fugacity temperature curves for all three reactions virtually coincide with curve
WI
(a) as shown in Fig. 1. The applicability of reactions (b) and (c) to the pyroxenes
will of course depend on the stability of FeSiO 3
 with respect to pressure
(Lindsley, Speidel and Nafziger, 1968). However this stability field is probably
expanded to much lower pressures than that of pure FeSiO 3
 by only modest
amounts of MgSiO 3
 (Olsen and Mueller 1966). It appears that in at least some
batholiths these minimal pressure requirements were met since quite iron-rich
pyroxenes are present (Fig. 2).
Wones and Eugster (1965) have given an extensive discussion of the stability
relations in the system phlogopite-annite and reaction (d). Although the biotites
of batholiths deviate somewhat in composition from the phlogopite-annite series,
it is likely that their oxidation relations are similar. Fig. 2 indeed shows that
all the ferromagnesian minerals follow similar variation trends with respect
to Fe'+ /(Mg + Fe 2 +) and that their Fe 2+ - Mg distribu`ion cofficients do not
differ from unity by much more than a factor of 2. "onseiVently it appears in
each mineral Fe e+ /(Mg + Fe 2+) reflects the oxygen fugacity in approximately
the same way.
Reference to Fig. 1 shows that in the case of the anhydrous reactions (a),
(b)and (c), Poe must decrease substantially to maintain a constant composition
of the silicates as the temperature decreases during crystallization-differentia-
tion. It also shows that if Fe e+ /(Mg + Fe 2+ ) increases during differentiation,
as is implied by Fig. 2, Poe
 must decrease even more than for a constant
Feet /(Mg + Fe 2+ ) ratio.
EVIDENCE FOR THE OXIDATION STATES OF CALC-
ALKALINE MAGMAS
Some time ago it was shown by Larsen and Draisin (1948) and by Larsen
and Schmidt (1958) that in the Southern California and Idaho botholiths the
4
atomic fraction Fe'+/(Fe2,+
 
Mg) of the pyroxenes, biotites and hornblendes
all increased systematically in the apparent differentiation sequence gabbro-
granite (Fi:;. 2). More recent data such as those of Best and Mercy (1967)
confirm this effect in other plutons.
Recently also thermochemical data for the assemblage orthopyroxene-mag-
netite-quartz (Reaction (b)) were titilized to calculate the oxygen fugacities as
a function of the mineral compositions (Mueller 1967, 1969). It was concluded
from this study that in the Southern California batholith the systemwtic change
in Fe e+ /(Fe2+ + Mg) shown by Fig. 2 might correspond to as much as a ten
order of magnitude decrease in oxygen fugacity between the gabbros and granites.
A large part of this decrease in oxygen fugacity is attributable to the normal
variation with temperature at a constant activity ratio d H) /dH2 in the magma.
However Fig. 1 indicates that such a constant ratio might correspond to a decrease
in Fee+ /(Fe2+ + Mg) of orthopyroxene (and olivine) since curve (a) has a steeper
slope than the curves of constant H 2 O /H2 . Therefore the increase in Fee+
(Fe 2+ + Mg) observed may well cu,-respond to a decrement in Po e in addition
to that alraady implied by a constant d H20 /dH2 ratio. Consequently it is
possible that d H20/dH, actually decreased during differentiation, and that this
decrease is attributable to the strong reaction of H 2 O with the Si-O-Si bridges
of the silicate melt (Mueller, 1969). Interestingly enough the same reaction
can also account for the very existence of the talc-alkali series since it
greatly enhances the solubilities of the quartzo-feldspathic constituents relative
to those of the ferromagnesian constituents (Mueller, 1969). As a consequence
enrichment of the melt in quartz and feldspar occurs during differentiation. As
far as the oxygen %gacity is concerned in this scheme it bears little relation
5
to any possible buffering effect of the ferromagnesian minerals since their
abundances are usually too low, as we shall show presently. Indeed the ferro-
magnesian mineral compositions may be regarded as simply reflecting the
oxidation state engendered by the fluid interaction within the melt.
STOICHIOMETRY OF OXIDATION
The control of oxygen fugacity through buffer systems depends not only on
the proper choice of reactions but also on a sufficient quantity of buffer tieing
presented to prevent exhaustion by the reactions. Consequently the oxidative
capacity of magmatic components such as crystalline precipitates or dissolved
iron depends greatly on their quantity relative to the quantity of the competing
oxidant such as dissolved water or carbon dioxide. In the case of relatively
anhydrous magmas such as those of the Skaergaard complex the ferromagnesian
components were adequate to provide something approaching a true buffer
capacity except that we have little reason to believe that the system remained
univariant since the silicates increased in iron content. However as the water
content is increased the capacity of the ferromagnesian constituents to control
the oxygen fugacity is diminished.
To illustrate tho oxidative capacity of the components of a typical hydrous
calk-alkali magma we may consider a system of the composition of granodiorite
with the following characteristics:
3 wt percent H20
T ti 1000°K
10 wt percent ferromagnesian minerals
H2O 
_ 100
Hz	1
6
For purposes of the calculation it is helpfui to convert the components to
molecular values. According to Shaw (1965) 3 weight per cent H 2O is approxi-
mately equivalent to 10 mole percent H z
 O in a melt with a composition corres-
ponding to the region of the temperature minima in the system Ab-Or-;°.'.J2
 -H 20.
We shall assume the same data apply to the granodiorite composition. Fig. 2
shows that in this composition range, with An - 30, the parameter Fe e* /
(Fe 2+ + Mg)., 0.5 in the ferromagnesian silicates. However in a complex
silicate such as mica or hornblende the quantity of FeSiO 3 is even more
diluted by constituents such as A1, 0 3 . Now it is apparent that the quantity of
the Niggli formula unit 1/2 Fe SiO 3 is less than the weight percent of the same
constituent so that an assui,iption of a 6 mole percent value of 1/2 Fe SiO 3 in the
granodiorite represents a safe upper limit. We may now rewrite reaction (b)
as a prototype of the limiting reaction for oxidation:
fluid	 iron silicate magnetite ailIca	 fluid
1120 + 3 FeSiO3	Fe 	 + 3 Si02
 + Hz	 (e)
We assume that this reaction represents the general oxidation of ferrous silicate
in all the ferromagnesian minerals. Then for each unit of 1/2 FeSiO 3 that reacts,
one unit of H 2 O is destroyed and one unit of Hz is form .z^d. '.onsequently if 6
mole percent 1/2 FeS1O 3 reacts it will use up ten percerc of the H2 O present
and the final ratio Is
H20 90
H2	11
Thus to decrease H 2O/H 2 by one order of magnitude it is necessary to oxidize
all the ferrous silicate present in the granodiorite magma.
7
1-=
If the ratio H2 0/H 3 is decreased during tale-alkaline differentiatfzii uy
the oxidation of ferrous silicate there is no evidence for i*.. Indeed lve have al-
ready seen evidence (Fig. 2) that FeSiO3 arc;Amulates in the melt and tL > ; • ^ ,e-
'	 cipitating crystals as differentiation proceeds just as it did in the Skaergaard
complex. Thus it a«,ears that as the water content of a magma rises the iron
compounds have less effect and vomething akin to the role of a buffer is played
by the fluid constituents of the magma. To a first approximation the oxygen
fugacity of the magma is at each stage of differentiation controlled by d H 0 Af H
s	 z
ratio of the dissolved fluid. This ratio is in turn determined by the initial
condition and by the differential loss of Hz and H2 O during differentiation.
However, it is also determined by the differential interaction of H2 O and HZ
with the melt components (Mueller, 1969) and more parti^ularly with the Si-O-Si
bridges whose number steadily increases as differentiation proceeds.
I
8
REFERENCES
Best, M. G., and Mercy, E. L. P., 1967, Composition and crystallization of
mafic minerals in the Guadalupe igneous complex, California. Am.
Mineral V. 52, p. 436-474.
Dodge, F. C. W., Smith, V. C., and Mays, R. E., 1969, Biotites from granitic
rocks of the central Sierra Nevada batholith, California. Jour. Petrol.
V. 10, p. 250-271
Eugster, H. P., 1957, Heterogeneous reactions involving oxidation at high press-
ures and temperatures. Jour. Chem. Physics V. 26, p. 1160.
Larsen, E. S. Jr., and Draisin, W. M., 1948, Composition of the minerals in the
rocks of the Southern California batholith. Internat. Geol. Cong. Report
18th Session, Great Britain, pt. 2, p. 66-79.
Larsen, E. J. Jr., and Schmidt, R. G., 1958, A reconnaissance of the Idaho
batholith and comparison with the Southern California batholith. U. S.
Geol. Survey Bull. 1070-A.
Lindsley, D. H., Speidel, D. H. and Nafziger, R. H. 1968, P-T4O 2 relations for
the system Fe-O-S'0 2 , Am. Jour. Sci. v. 266, p. 342-360.
Muan, A., and Osborn, E. F., 1956, Phase equilibrium at liquidus temperatures
in the system MgO-FeO-Fe 2 O3 -S'02 . Am. Ceram. Soc. Journ. V. 39, p.
121-140.
Mueller, R. F., 1967, New evidence on the oxidation states of plutonic magmas.
Geol Soc. Am. Program: Ann. Meeting New Orleans. p. 157
Mueller, R. F., 1969, Hydration, oxidation and the origin of the tole-alkali series.
NASA TN D-5400 and Abstr Am. Geophys. Union V. 50 #4, p. 333.
I
9
10
Olsen, E. J., and Mueller, R. F., 1966, Stability of orthopyroxenes with respect
to pressure, temperature and composition. Jour. Geol. V. 74, p. 620-625.
ROsborn, E. F., 1959, Role of oxygen pressure in the crystallization and dif-
ferentiation of basaltic magma. Am. Journ. Sci. V. 257, p. 609-647.
Shaw, H. R., 1965 Comments on viscosity, crystal settling, and convection in
granite magmas. Am. Jour. Sci. V. 263 p. 120-152.
Wager, L. R., and Deer, W. 4., 1939, Geological investigations in East
Greenland, part III. The petrology of the Skaergaard intrusion; Kangerd-
lugssvag, East Greenland: Medd. om Gronlard, V. 105, no. 4 p. 1-335.
Wones, D. R., and Eugster, 1965, Stability of biotite: experiment, theory, and
application. Am. Mineralogist V. 50, p. 1228-1272.
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and Draisin (1948).
12
10
4


Oxidative capacity of magmatic components

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