JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 5
PAGES 1027^1041
2008
doi:10.1093/petrology/egn015
Crystal^Melt Separation and the Development of
Isotopic Heterogeneities in Hybrid Magmas
JAMES S. BEARD*
VIRGINIA MUSEUM OF NATURAL HISTORY, 21 STARLING AVENUE, MARTINSVILLE, VA 24112, USA
RECEIVED FEBRUARY 21, 2007; ACCEPTED FEBRUARY 22, 2008
ADVANCE ACCESS PUBLICATION APRIL 4, 2008
isotopic heterogeneity; zoning; hybrid magma; crystal
separation; Sr isotopes; aplite; rhyolite
If a magma is a hybrid of two (or more) isotopically distinct endmembers, at least one of which is partially crystalline, separation of
melt and crystals after hybridization will lead to the development
of isotopic heterogeneities in the magma as long as some of the preexisting crystalline material (antecrysts) retains any of its original
isotopic composition.This holds true whether the hybridization event
is magma mixing as traditionally construed, bulk assimilation,
or melt assimilation. Once a magma-scale isotopic heterogeneity is
formed by crystal^melt separation, it is essentially permanent,
persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting
from crystal^melt separation can be as large as that resulting from
differential contamination, multiple isotopically distinct sources, or
in situ isotopic evolution. In one model, a redistribution of onethird of the antecryst cargo yielded a crystal-enriched sample with
87
Sr/86Sr of 07058, whereas the complementary crystal-poor sample
has 87Sr/86Sr of 07068. In other models, crystal-rich samples are
enriched in radiogenic Sr. Isotopic heterogeneities can be either
continuous (controlled by the modal distribution of crystals and
melt) or discontinuous (when there is complete separation of crystals
and liquid). The first case may be exemplified by some isotopically
zoned large-volume rhyolites, formed by the eruptive inversion of a
modally zoned magma chamber. In the latter case, the isotopic composition of any (for example) interstitial liquid will be distinct from
the isotopic composition of the bulk crystal fraction.The separation of
such an interstitial liquid may explain the presence of isotopically
distinct late-stage aplites in plutons. Crystal^melt separation provides an additional option for the interpretation of isotopically
zoned or heterogeneous magmas.This option is particularly attractive
for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. Non-isotopic chemical heterogeneities
can also develop in this fashion.
Most large-volume, continental magmas are complex
hybrids of mantle-derived basaltic melts and crustal rocks
(e.g. Lipman, 1984; DePaolo et al., 1992; Davidson et al.,
2005). Zoning and other heterogeneities both chemical
and isotopic, are common in, if not characteristic of, large
bodies of hybrid magma, both volcanic and plutonic
(Noble & Hedge, 1969; Moll, 1981; Halliday et al., 1984;
Kistler et al., 1986; Johnson, 1989; Johnson et al., 1990;
Hildreth et al., 1991; Verplanck et al., 1995; Reiners et al.,
1996; Chesner, 1998; Hildreth & Fierstein, 2000; Barbey
et al., 2001; Tsuboi & Suzukiba, 2003; Mikoshiba et al.,
2004; Dreher et al., 2005; Wilson et al., 2006). The most
common interpretations of isotopic variations in magma
bodies are differential contamination or magma mixing,
source variability, or isotopic evolution in long-lived
magma chambers with high Rb/Sr (Noble & Hedge,
1969; Moll, 1981; Johnson, 1989; Davies & Halliday, 1998;
Hildreth & Fierstein, 2000; Mikoshiba et al., 2004).
However, there is another mechanism by which isotopic
variability can develop in magmas. Given a hybrid
magma in which the end-members are isotopically distinct
and one or more of the end-members is partially crystalline,
isotopic variability can result from the simple separation of
solids from liquids. The end-members can be either multiple
magmas (hybridization mechanism ¼ magma mixing) or
a magma that entrains partially molten xenoliths (hybridization mechanism ¼ contamination/bulk assimilation).
*Corresponding author. E-mail: [email protected]
ß The Author 2008. Published by Oxford University Press. All
rights reserved. For Permissions, please e-mail: journals.permissions@
oxfordjournals.org
KEY WORDS:
I N T RO D U C T I O N
JOURNAL OF PETROLOGY
VOLUME 49
The isotopic variability can be large and may be a major
component of overall isotopic heterogeneity in hybrid
systems. It should be noted at the outset that this is not
isotope ‘fractionation’. It is rather, the preservation, propagation, and re-expression of extant isotopic and chemical
variability.
The principal assumption for the models proposed here
is that the solid components of the mixed system retain
their isotopic and chemical identity. In other words, crystals present at the time of mixing, including both phenocrysts present in magmas prior to mixing and any
xenocrystic crystalline material derived from an assimilant [referred to together as antecrysts, an expansion of
the term defined by Bacon & Lowenstern (2005) to include
both xenocrystic and cognate solids], do not equilibrate
with a thoroughly mixed melt. At the outset it is
acknowledged that this is unlikely to be strictly true.
However, the salient features modeled below will persist
to a greater or lesser degree as long as some crystals inherited from the pre-mixing end-members retain any of their
isotopic identity once the end-members are mixed.
This is clearly not an unreasonable assumption, as both
disparities in the isotopic compositions of phenocrysts
and melt and isotopic zoning within phenocrysts are
well-documented in both volcanic and plutonic rocks
(Johnson et al., 1990; Davidson & Tepley, 1997; Knesel
et al., 1999; Wolff et al., 1999; Baker et al., 2000; Waight
et al., 2000, 2001; Halama et al., 2002; Tepley & Davidson,
2003; Wolff & Ramos, 2003; Gagnevin et al., 2005; Ramos
& Reid, 2005; Wilson et al., 2006). In fact, it has been
argued that most arc magmas are mixtures of melts and
an unrelated, pre-existing ‘crystal cargo’ (Davidson et al.,
2005). A second assumption, important for several of
the models, is that, during partial melting of xenoliths,
the isotopic composition of residual minerals and melt
may be decoupled. Specifically, preferential melting or
retention of minerals of unlike isotopic composition
during (dehydration) melting will result in partial melts
having different isotopic composition from the coexisting
restite (e.g. Watson & Harrison, 1984; Hogan, 1995).
This decoupling has been verified by observation and
experiment for Sr isotopes and observed in migmatites
and inferred from granite compositions for Nd
(Hammouda et al., 1996; Tommasini & Davies, 1997; Ayres
& Harris, 1997; Knesel & Davidson, 1999; Zeng et al.,
2005a, 2005b).
A note of clarification is in order here. Obviously, hybridization is an open-system process. However, for the purposes of this paper, it will be generally assumed that once
hybridization has occurred, the system is closed to further
material input. It is in this context that ‘closed-system’
behavior is discussed from time to time.
This paper focuses on Sr isotope behavior, with a few
exemplars using Nd isotopes. However, the principles
NUMBER 5
MAY 2008
outlined here are, within the limits discussed above, applicable to any isotopic or, for that matter, chemical system.
M I X I N G O F I S O T O P I C A L LY
D I S T I N C T, PA RT I A L LY
C RY S TA L L I N E M AG M A S
The system chosen for modeling here is bulk assimilation
of a partially molten xenolith by a partially crystalline
basaltic magma. This system was chosen because it permits
construction of models applicable to hybridization by
either magma mixing or bulk assimilation. Models 1 and
2 (Figs 1 and 2), in which both end-members are in chemical and isotopic equilibrium prior to hybridization, are
equally applicable to bulk assimilation (‘contamination’)
or to magma mixing as traditionally construed
(e.g. Eichelberger, 1975). Model 3 (Fig. 3), in which one partially molten end-member is not isotopically or chemically
equilibrated, will apply, for the most part, to hybridization
by bulk assimilation. All percentages used in this paper are
weight percentages unless otherwise noted.
Mixing calculations
Two-component mixing lines are calculated using standard formulations (e.g. Faure, 1986). For components A
and B with isotopically distinct Sr,
ð87 Sr=86 SrÞmix ¼ ðfSrA SrB ½ð87 Sr=86 SrB Þ
ð87 Sr=86 SrA Þg=½Srmix ðSrA SrB ÞÞ
þ ðf½SrA ð87 Sr=86 SrA Þ
½SrB ð87 Sr=86 SrB Þg=ðSrA SrB ÞÞ
where Srmix ¼ SrA XA þ SrB XB
ð1Þ
ð1aÞ
and X is the weight fraction of component A or B. An identical formulation is used for Nd isotopes.
In these models, the bulk mixture of host magma and
xenolith lies along the bulk mixing line in Figs 1^3. The
bulk mixing line is the commonly used means for relating
the isotopic composition of a mixture to its end-member
components. However, in the models presented here (in
Fig. 1b, for example), four chemically and/or isotopically
distinct components are recognized: (1) the liquid in the
host magma; (2) the phenocrysts in the host magma; (3)
the liquid in the partially molten xenolith; (4) the residual
crystals in the partially molten xenolith. Thus, in addition
to the bulk mixing line, mixing lines can be calculated
between the liquid and solid components of the xenolith
(xenolith solid^liquid mix), and the liquid and solid components of the host (host solid^liquid mix), the liquid components of host and xenolith (mixed liquids), and the solid
components of host and xenolith (mixed solids) (Figs 1^3).
When the host and assimilant are thoroughly mixed, the
liquid in the mixed system will lie along the ‘mixed liquid’
line, the solids along the ‘mixed solids’ line.
1028
BEARD
CRYSTAL^MELT SEPARATION
Model 1: Xenolith plagioclase equilibrated during melting
(a)
Model 2: xenolith plagioclase consumed during melting
(a)
xenolith solid
xenolith solid-liquid mix
xenolith solid-liquid mix
0.711
xenolith solid
ho
ds
%
oli
ds
0.705
Bulk Kd for Sr in host magma = 1
0.703
80% host mixing line
host solid and liquid
0
(b)
100
200
300
0.703
500
0
(b)
xenolith solid-liquid mix
0.711
400
line
mi
xe
60
0.705
ixin
g
0.707
st m
Sr/86Sr
Sr/86Sr
87
Bulk Kd for Sr in
host magma = 1
87
80% host
mixing line
100
200
Sr/86Sr
87
Sr/86Sr
87
mix
ed
liqu
ids
so
s
host solid
500
600
80% host
mixing line
lid
0.705
host solid-liquid mix
400
line
d
e
ixing lin
300
60% host mixing line
ixe
m
200
ing
ds
oli
100
mix
ds
e
0
0.707
Bulk Kd for Sr in
host magma = 2
bulk
xe
g lin
line
st m
host liquid
0.703
0.709
mi
mixin
ids
60
80% ho
500
xenolith liquid
xenolith solid
bulk
d liqu
ing
t mix
os
%h
0.705
400
xenolith solid-liquid mix
0.711
Bulk Kd for Sr in
host magma = 2
mixe
0.707
host solid and liquid
300
xenolith solid
xenolith liquid
0.709
liquids
mixed
solids
ne
iqu
ids
0.709
ne
g li
60% host
mixing line
dl
g li
ixin
mixed
km
mi
xe
0.707
ixin
bul
0.709
xenolith liquid
km
xenolith liquid
bul
0.711
host liquid
0.703
700
Sr, ppm
Fig. 1. Sr mixing models for 50% crystalline basalt host magma and
partially molten gneissic assimilant. (See Table 1 for compositional
information and Table 2 for a description of the models in Figs 1^3.)
For Figs 1^3 labeled lines are as follows. ‘Xenolith solid^liquid mix’
and ‘host solid^liquid mix’ connect the compositions of coexisting
solids and liquids in the assimilant and host, respectively. There is no
‘host solid^liquid mix’ for Figs 1a, 2a, and 3a because the crystals and
liquid in the host have identical compositions (i.e. bulk Kd for Sr ¼1).
The ‘mixed solids’ and mixed liquids’ lines connect the compositions
of solids and liquids, respectively, in the host and assimilant. The bulk
mixing line gives the range of compositions for any mixture of host
and solid with end-points determined by the crystallinity of the host
(always 50%) and the assimilant (75% for models 1 and 3 and 25%
for model 2). The ‘60% host’and 80% host’ mixing lines represent the
range of possible compositions for a given mixture of host and assimilant (i.e. either 60 or 80% host) when crystals and liquids are
allowed to separate. Marks on lines are at 10% intervals unless
omitted for clarity. (See text for further discussion.) In Fig. 1, plagioclase is chemically and isotopically equilibrated during xenolith melting prior to assimilation. This model is akin to magma mixing with a
plagioclase-rich magma. In (a), bulk Kd for Sr in host basalt ¼1; in
(b), bulk Kd for Sr in host ¼ 2. Distribution of Sr between melt and
solid calculated for 50% Rayleigh fractionation. This effectively
assumes that the plagioclase is zoned in Sr.
Another set of solid^liquid mixing lines can be calculated for any binary mixture of xenolith and host.
This requires several sets of mixing calculations. First,
for a given mixture one must determine the fraction of
solids and liquids derived from the end-members.
For components A and B the weight fraction of solids
0
100
200
host solid-liquid mix
300
400
Sr, ppm
500
host solid
600
700
Fig. 2. Models where plagioclase is consumed during xenolith
melting; akin to mixing with a plagioclase-poor magma. It should
be noted that Fig. 1 is one melting^mixing end member whereas
Fig. 2 is another. (a) Bulk Kd for Sr in host basalt ¼1; (b) bulk Kd
for Sr in host ¼ 2. Distribution of Sr between melt and solid
calculated for 50% Rayleigh fractionation. This effectively assumes
that the plagioclase is zoned in Sr. Lines and other information as
for Fig. 1.
derived from component A in a given mixture (SA,m) is
given by
SA ,m ¼ ½ðXs,A ÞðXm,A Þ=½ðXs,A ÞðXm,A Þ þ ðXs,B ÞðXm,B Þ
ð2Þ
where Xs,A,B is the weight fraction solid in component A or
B and Xm,A,B is the weight fraction of component A or B in
the mixture. The weight fraction of liquid derived from a
given component (LA,m) may be derived by an identical
formulation:
LA,m ¼ ½ðXl,A ÞðXm,A Þ=½ðXl,A ÞðXm,A Þ þ ðXl,B ÞðXm,B Þ:
ð3Þ
For an 80^20 (80% host, 20% assimilant) mixture of a
host magma that is 50% solid and an assimilant that is
75% solid, 73% of the total solids and 89% of the total
melt will derive from the host magma. Thus, in Fig. 1b
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JOURNAL OF PETROLOGY
VOLUME 49
Model 3: Xenolith (restite) plagioclase non-reactive, non-equilibrated
(a) 0.715
Bulk Kd for Sr in host magma = 1
xenolith
liquid
bu
lk
liq
d-
m
ixi
uid
87
0.8
0.7
s
uid
liq
li
so
th
0.710
d
ixe
m
li
no
xe
Sr/86Sr
0.9
x
mi
ng
0
200
400
e
60% host mixing line
xenolith solid
200
300
m
Sr/86Sr
0
g
in
ne
e
g li
lin
ixin
host liquid xenolith solid
0.700
200
xenolith-host solid mixing line
omitted for clarity
ix
tm
200 400 600
k
e
hos
0.75
l
bu
lin
87
ng
ixi
m
0.705
ix
id m
-liqu
st
ho
ds
mixed liqui
80%
400
Bulk Kd for Sr in host magma = 2
0.95
xenolith
liquid
0.85
d
soli
lith
%
0.710
60
0.715
o
xen
(b) 0.720
host solid
and liquid
mixed solids
300
host solid-liquid mix
400
500
MAY 2008
Alternatively, if no separation occurs, the mixture will lie
on the bulk mixing line and have the isotopic and chemical
composition of the bulk mixture. However, the general
case will involve incomplete separation of liquids and
crystals. The chemical and isotopic compositions of these
partially separated mixtures lie along the 80% host
mixing line.
Models for mixing of two solid^liquid
mixtures
lin
80% host mixing line
0.705
NUMBER 5
host solid
600
Sr, ppm
Fig. 3. Models where plagioclase is indifferent (no reaction, melting,
equilibration, or crystallization) during xenolith melting. As a consequence of this, isotopic compositions in the melt and restite
are decoupled. Insets show full range of variability in the system.
The area in boxes in the insets is the area of the main figure. Dashed
lines continue off the diagram. (a) Bulk Kd for Sr in host basalt ¼1;
(b) bulk Kd for Sr in host ¼ 2. Distribution of Sr between
melt and solid calculated for 50% Rayleigh fractionation. This effectively assumes that the plagioclase is zoned in Sr. The ‘mixed
solids’ line in (b) is omitted for clarity. Other lines and information
as for Fig. 1.
(for example) the end-points of the 80% host mixing line
are the 73% host point on the ‘mixed solids’ line and the
89% host point on the ‘mixed liquids’ line. It should be
noted, of course, that the line passes through the 80^20
mixture on the bulk mixing line. The line itself is calculated using equation (1) with A and B from equation (1)
defined as the end-points of the line calculated using equations (2) and (3).
Looking again at the 80% host mixing line in Fig. 1b,
one can now calculate the chemical and isotopic variation
along this line related to the relative proportions of crystals
and liquids. If the liquid is extracted entirely from the
solids, it will have the chemistry and isotopic composition
of the end-point on the ‘mixed liquids’ line whereas the
solid remainder will have the chemical and isotopic composition of the end-point on the ‘mixed solids’ line.
The two end-members chosen for the mixing models are a
600 Ma biotite gneiss (assimilant or xenolith) which is
assimilated by a mantle-derived basalt (host). For all
models, the basalt host is taken to be 50% crystalline
with liquid and crystals in chemical and isotopic equilibrium. The isotopic and appropriate chemical and mineralogical compositions of the two end-members are given
in Table 1.
Sr is strongly partitioned into plagioclase in comparison
with other solid phases considered in the models below.
Because of this, the behavior of plagioclase during melting
will control the distribution of Sr between the solid and
liquid fractions.
The six models for Sr isotopes in Figs 1^3 represent three
different behaviors of plagioclase during xenolith melting
(models 1^3) and two different bulk Kd values for Sr in
the host magma (a and b). In models 1a, 2a, and 3a, the
bulk Kd for Sr in the host basalt is assumed to equal unity,
hence the solid and liquid in the host have identical Sr concentration. In models 1b, 2b, and 3b the bulk Kd for Sr
(modeled for Rayleigh fractionation) in the host is taken
to be two, hence the host solids are enriched in Sr relative
to the host liquid.
For model 1 (Fig. 1) plagioclase and melt in the xenolith
are in chemical and isotopic equilibrium prior to mixing.
The melting reaction,
25bio þ 15qtz þ 60plagioclase1 ¼ 25melt þ
ð4Þ
12opx þ 3FeTiox þ 60plagioclase2
is modified after Patin‹o Douce & Beard (1995) for the melting of a biotite gneiss at 9508C and 5 kbar (Table 2). This is
an end-member case in which it is assumed that all plagioclase in the system re-equilibrates during partial melting
(bulk Kd for Sr ¼ 2) and takes on the Sr isotopic composition of the bulk xenolith (87Sr/86Sr ¼ 071136). Most Sr
resides in plagioclase (and, thus, the solids) and the
87
Sr/86Sr is equal for solids and liquids in both the host
and xenolith, Hence, as is clear from inspection of Fig. 1,
the solid component of the both the 60^40 and 80^20 mixtures is enriched in both total Sr and radiogenic Sr relative
to the liquids in those mixtures (Fig. 1a and b). This assimilation model is akin to a magma mixing model where
plagioclase is an abundant phenocryst phase in a magma
that is then mixed into the basalt host.
1030
BEARD
CRYSTAL^MELT SEPARATION
Table 1: Compositions of starting materials
Mineral
Age (Ma)
Mode
Rb (ppm)
Sr (ppm)
Rb/Sr
87/86init
87/86now
Sm
Nd
(ppm)
(ppm)
Sm/Nd
144/143init
144/143now
10
64
016
0512
512371
Xenolith
biotite
600
25
300
36
83
0704
0914512
plagioclase
600
50
4
510
00078
0704
0704194
25
quartz
600
all minerals except biotitey
600
75
86
32
027
0512
0512634
bulk
600
100
77
264
029
0704
071136
895
40
021
0512
0512499
bulk
0
100
50
400
025
0704
0704
2
10
02
05127
05127
melt, Sr bulk D ¼ 1
0
50
50
400
025
0704
0704
325
1625
02
05127
05127
solids, Sr bulk D ¼ 1
0
50
50
400
025
0704
0704
075
375
02
05127
05127
melt, Sr bulk D ¼ 2
0
50
50
200
025
0704
0704
325
1625
02
05127
05127
solids, Sr bulk D ¼ 2
0
50
50
600
025
0704
0704
075
375
02
05127
05127
Host magmaz
Quartz is assumed to contain insignificant Sr, Rb, and REE (1 ppm).
y
Includes REE-rich trace phases (e.g. apatite, sphene, zircon).
z
Plagioclase is assumed to contain 51 ppm Nd.
Host bulk D for Nd, Sm ¼ 03.
For model 2 (Fig 2a and b) it is assumed that all of the
plagioclase is consumed during melting of the xenolith via
the reaction
25bio þ 25qtz þ 50plag ¼ 75melt þ 15opx
þ 5cpx þ 5FeTiox:
ð5Þ
Because the melting reaction involves the entire rock, solid
and liquid components of the xenolith are again in chemical and isotopic equilibrium at the time of mixing.
However, because melting of plagioclase has released
most of the Sr to the melt phase (bulk Kd for Sr in a
plagioclase-free system is 5003), liquid-rich compositions
are enriched in Sr and that Sr is highly radiogenic. It
should be noted that melting of the xenolith in model 2 is
much more extensive than for models 1 and 3. This assimilation model is akin to a magma mixing model where plagioclase is not present as a phenocryst phase in a magma
that is then mixed into the basalt host.
For model 3 (Fig. 3a and b), plagioclase is indifferent to
melting in the xenolith, neither reacting with, contributing
to, consuming, nor equilibrating with the melt. The melting reaction
25bio þ 15qtz ¼ 25melt þ 12opx þ 3FeTiox
ð6Þ
is modified after Patin‹o Douce & Beard (1995) for the melting of a biotite gneiss at 9508C and 5 kbar (Table 2). In this
model, all of the Sr in the partial melt of the xenolith is
derived from the biotite, whereas essentially all of the
Sr in the xenolith residua resides in plagioclase.
Thus the liquid and solid share, respectively, the isotopic
Table 2: Models for Sr during xenolith melting
Sr (ppm)
87/86now
Model 1
Melting reaction (plagioclase equilibrated, bulk D ¼ 2)
25bio þ 15qtz þ 60plag1 ¼ 25melt þ 12opx þ 3FeTiox þ 60plag2
xenolith melt
66
071136
xenolith restite
330
071136
bulk xenolith
264
071136
Model 2
Melting reaction (plagioclase-out)
25bio þ 25 qtz þ 50plag ¼ 75melt þ 15opx þ 5cpx þ 5FeTiox
xenolith melt
xenolith restite
bulk xenolith
349
071136
9
071136
264
071136
Model 3
Melting reaction (plagioclase indifferent)
25bio þ 15qtz ¼ 25melt þ 12opx þ 3FeTiox
xenolith melt
36
0914512
xenolith restite
340
0704194
bulk xenolith
264
071136
characteristics of biotite and plagioclase. The striking
feature of Fig. 3a and b is the extremely radiogenic
character of the xenolith melt (87Sr/86Sr ¼ 09145), a
consequence of high Rb/Sr in the biotite (Table 1;
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JOURNAL OF PETROLOGY
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MAY 2008
For the equilibrium models (1 and 2), the isotopic composition of the solids and liquids does not vary between endmember models; only the Sr content of the solids and
liquids is affected. However, even partial disequilibrium
during xenolith melting will result in deviation from isotopic and chemical equilibrium. In particular, even small
amounts of disequilibrium (e.g. 10%) during melting can
have large effects on liquid compositions in a crystal-rich
system and solid compositions in a melt-rich system
(Fig. 4a and b).
For model 3, if plagioclase is actually formed as a consequence of the melting reaction (e.g. amph ¼ plag þ melt),
the composition of the solids will vary along the line in
Fig. 4c. The consequences of neoblastic plagioclase formation on mixing relations are seen in Fig. 4d. It should be
noted, in particular, that the solids in this case will have
Knesel & Davidson, 1999). Thus, even though this melt
contains only 36 ppm Sr (as opposed to 400 ppm in the
host magma) it has a strong influence on the isotopic composition of the system. The xenolith solid, in contrast, is
nearly as non-radiogenic as the host magma. In fact, it
was necessary to omit the ‘mixed solids’ line in Fig. 3b
for clarity. It should be noted that if the initial 87Sr/86Sr
of the xenolith is elevated, 87Sr/86Sr in the plagioclase will
be as well. In such cases, 87Sr/86Sr of a mixed solid in a
mixed system could conceivably be as high as or higher
than the mixed liquid (analogous to the behavior seen
in Fig. 1).
The range of xenolith solid and liquid compositions that
can be produced by combinations of the three models lies
within the areas outlined by the mixing lines in Fig. 4a (for
liquid compositions) and Fig. 4b (for solid compositions).
(b) 0.714
(a)
model 3
pla
gm
ilibrates
0.708
s
elt
0.8
plag melts
0.710
plag equilibrates
plag equ
xenolith liquid compositions
model 1
model 2
0.712
0.9
plag
0.706
87
Sr/86Sr
NUMBER 5
mel
ts
0.704
model 2
plag melts
xenolith solid compositions
model 1
0.7
0
0.702
100
200
300
400
0
100
200
Sr, ppm
(d) 0.716
xenolith solid composition:
variation as new plag forms during
melting, plagioclase/melt Kd = 2
Sr/86Sr
soli
d-liq
bu
lk
solid
25% product plag
340
342
344
0.704
dl
0.7
lin
e
200
400
iqu
ids
s
lid
so
solid, model 3
(0% product plagioclase)
xe
m
ixi
ng
0.8
mix
d
0.705
mi
uid
0.9 xenolith liquid
ixe
0.708
0.704
400
m
87
olith
0.712
0.706
0.703
338
25% plagioclase formed during
xenolith melting
xen
0.708
0.707
300
Sr, ppm
(c) 0.710
0.709
model 3
60% host
80% host
host solid and liquid
346
348
350
Sr, ppm
200
300
400
Sr, ppm
Fig. 4. Summary of the effects of plagioclase behavior on the compositions of melt and crystals in the ‘xenolith’end member. (a) Effects on melt
compositions. Melting of plagioclase drives melt composition towards the model 2 (plagioclase-free) composition. Equilibration of plagioclase
without melting drives the unequilibrated melt from model 3 towards the equilibrated melt of model 1. The area outlined by the mixing lines
defines the range of melt compositions that may form during melting of the xenolith. For an equilibrated system, the range of melt compositions
is restricted to the mixing line connecting model 1 (no plagioclase in the melt) and model 2 (all plagioclase in the melt). (b) Effects on solid
compositions. The area outlined by the mixing lines defines the range of solid compositions formed during the melting of the xenolith. (c) Effect
on the bulk solid of the formation of additional modal plagioclase as a product of the melting reaction. This is an extreme (and unrealistic)
example, intended only to demonstrate the effect. (d) Model 3a, recalculated assuming 25% product plagioclase.
1032
BEARD
CRYSTAL^MELT SEPARATION
a higher Sr content and 87Sr/86Sr than the liquids.
These diagrams are shown for demonstration purposes
only. Large amounts of new plagioclase will not generally
form during dehydration melting of biotite gneiss (e.g.
Patin‹o Douce & Beard, 1995).
The two models for Nd isotopes (Table 3; Fig. 5a and b)
are essentially equivalent to models 1 and 3 for Sr isotopes
(Table 2). For both models, a bulk Kd for Nd of 03 is
assumed for the host magma. In the first model, based on
melting reaction (4) (Fig. 5a), the liquid and solid phases in
the partially melted xenolith are in chemical and isotopic
equilibrium. For this model, 143Nd/144Nd is lower in mixed
solids than in the coexisting mixed liquids. In the second
model, based on melting reaction (6) (Fig. 5b), all Nd in
the partial melt of the xenolith is derived from biotite,
which is modeled as having Sm/Nd of 016 (Yang et al.,
1999). REE in the restite (Sm/Nd ¼ 027) are largely contained in trace phases such as apatite and sphene (Condie
et al., 1995; Ayres & Harris, 1997; Bea & Montero, 1999). For
this model, 143Nd/144Nd is lower in the mixed liquids than
in the coexisting mixed solids.
Mixing of liquids with a partially
crystalline host magma
10556
051253
xenolith restite
1815
051253
bulk xenolith
4000
051253
In general, assimilation of a liquid by a partially crystalline magma will increase isotopic inequalities in the
system. In the simplest case, where the crystals and liquid
in the host magma have the same chemical and isotopic
composition, the mixture appears to devolve to a simple
mixing line (Fig. 6a). However, there will be significant
differences in the chemical and isotopic compositions of
the bulk mixture on the one hand and the mixed liquid
on the other (Fig. 6a). Mixing relations become more
apparent in Fig. 6b and c. These models are equivalent,
the only difference being in the composition of the assimilant liquid. A noteworthy feature is the large potential isotopic heterogeneities manifest in Fig. 6c and d, where the
melt modeled in Fig. 3a [derived from reaction (6)] is
mixed directly with the host magma.
Development of isotopic heterogeneity in
homogenized binary mixtures of
assimilant and host
Table 3: Models for Nd during xenolith melting
Nd (ppm)
144/143now
Model 1
Melting reaction (restite equilibrated, bulk D ¼ 03)
25bio þ 15qtz ¼ 25melt þ 12opx þ 3FeTiox
xenolith melt
Model 2
Melting reaction (restite indifferent)
25bio þ 15qtz ¼ 25melt þ 12opx þ 3FeTiox
xenolith melt
64
0512371
xenolith restite
32
051264
bulk xenolith
40
051253
Of particular interest for all models are the 60^40 and
80^20 host^assimilant mixing lines (labeled ‘60% host
mixing line’ and ‘80% host mixing line’ in Figs 1^4).
(a)
(b)
host solid-liquid mix
mixed solids
60% host mixing line
80% host
mixing line
80% host mixing line
mixe
lids
0.5125
bulk
mix
xe
no
lith
ids
so
lid
0.5125
d liqu
d so
144
Nd/143Nd
ix
bulk m
0.5127
mixe
0.5126
host solid-liquid mix
Bulk Kd for Nd = 0.3
0.5127
60% host
mixing line
mix
ed
-liq
uid
mi
x
liqu
ids
xenolith solid-liquid mix
0.5123
0
50
100
150
0
20
40
60
Nd, ppm
Nd, ppm
Fig. 5. Nd mixing models. (See Table 1 for compositions and Table 3 for descriptions of models.) Mixing lines as described in Fig. 1. Bulk Kd for
Nd in host ¼ 03 for both models and for xenolith in (a). Mineral and melt compositions calculated for Rayleigh fractionation. (a) Solid and
liquid in xenolith are chemically and isotopically equilibrated prior to assimilation. Melting reaction as in Fig. 1b. (b) Xenolith Nd derived from
biotite (Sm/Nd ¼ 016), restite Nd controlled by trace phases (Sm/Nd ¼ 027). Melting reaction as for Fig. 3a. It should be noted that the ‘60%
host’ mix line lies very close to the liquid mix line and the ‘80% host’ mix line lies close to the bulk mix line. This is coincidental.
1033
JOURNAL OF PETROLOGY
(a)
VOLUME 49
(b)
assimilant = 100% melt of xenolith
assimilant = 100% melt of xenolith
bul k
liquid
0.705
80% host {
0.707
all solids
80%
0.705
60%
hos
300
400
host liquid
0.703
100
200
0.77
500
300
400
600
700
(d)
host
0.5127
0.72
60%
80% host
0.71
Nd/143Nd
144
ids
700
80% host
0.5125
60%
hos
host solid
assimilant = 25% melt of xenolith
0.70
100
200
300
t
host
host liquid
0
iqu
500
dl
300
host 50% solid
Sr bulk Kd host = 2
ix
100
host 50% solid
Nd bulk Kd host = 0.3
xe
line
0.7
0.73
km
0.8
mi
bul
ing
mix
uids
0.74
liquid
solid
0.9
bulk
mixed liq
0.75
Sr/86Sr
500
Sr, ppm
assimilant = 25% melt of xenolith
0.76
87
t
host solids
Sr, ppm
(c)
hos
t
bulk
host magma
0.703
200
mixed
li
} 60% host
bulk
e
e
lin
liquid
0.707
host 50% solid
Sr bulk Kd host = 2
g lin
ng
0.709
quids
ix i
m
host 50% solid
Sr bulk Kd host = 1
in
mix
lk
bu
0.709
Sr/86Sr
MAY 2008
0.711
0.711
87
NUMBER 5
400
500
600
0.5123
700
0
10
20
30
40
50
60
70
Nd, ppm
Sr, ppm
Fig. 6. Assimilation of a liquid by a partially crystalline host magma. (a) Assimilant is a 100% melt of the gneiss, bulk Kd for Sr in the host ¼1.
Although this appears to be a simple mixing line, the bulk and liquid compositions for the mixtures differ such that removal of crystals will
produce chemical and isotopic heterogeneities. (b) As for (a), except bulk Kd for Sr in host ¼ 2. (c) Assimilant liquid is a 25% melt of the
xenolith (compare Fig. 3a). Bulk Kd for Sr in host is 2. Inset shows complete range of variation in model. Box in inset outlines area of main
figure. Dashed lines continue off diagram. (d) Nd isotopes modeled with a 50% crystalline host magma (bulk Kd for Nd ¼ 03) mixed with
a liquid derived from 25% melting of the xenolith (compare Fig. 3a). The potential for extreme isotopic heterogeneity in (c) and (d) should
be noted.
0.712 xenolith, bulk
bu
lk
0.710
xenolith liquid, 36 ppm Sr, 87Sr/86Sr = 0.9145
liquid in 60–40 mix
m
ix
A
60% host mixing line
t-r
ich
0.708
liq
87
Sr/86Sr
el
m
C
0.706
uid
m
ix
bulk composition, 60-40 mix
x'l
h
c
-ri
These binary mixtures can be used to represent crystal^
liquid separation in physically homogenized mixed
magmas in which the antecrysts retain their isotopic identity. As an example, Fig. 7 shows an annotated expansion of
part of Fig. 3a. The 60% host mixing line connects a point
on the host^assimilant liquid mixing line (A) to a point on
the host^assimilant solid mixing line (B). These two points
are the compositions of, respectively, the liquid and the
solid fractions of a bulk mixture consisting of 60% host
magma and 40% assimilated xenolith. The intersection
(C) of the 60% host liquid^solid mixing line with the
bulk mixing line gives the relative proportions of solid
and liquid in the mixed system, in this case (coincidently)
60% solid and 40% liquid. Thus for this model, a homogenized bulk mixture of 60% host and 40% assimilant
will be 60% crystalline and have 87Sr/86Sr ¼ 070625. If
we now allow solids and liquids in this homogenized
0.704
B
xenolith solid
solid in 60–40 mix
0.702
250
300
350
Sr, ppm
Fig. 7. Expansion of Fig. 3a. (See text for discussion.)
1034
host solid,
liquid & bulk
400
BEARD
CRYSTAL^MELT SEPARATION
(a)
(b)
xenolith
0.711
0.711
liquids
liquid, 60% host mix
bu
ng
lin
e
0.709
ix
0.707
C
Bulk Kd = 2
mixture = 60% host, 40% xenolith
solid, 60% host mix
B
0.705
300
350
Y
solid-liquid tie lines,
equilibrium crystallization
0.703
100
400
Sr, ppm
solids
mixture = 60% host, 40% xenolith
host
0.703
250
100% crystallized
(=bulk)
0.707
solid com
sitions
po
fractional crystallization
equilibrium crystallization
0.705
0 % crystallized
m
st
Sr/86Sr
ixi
liquids
50
ho
87
m
%
0.709
A
60
lk
100 Z
0% crystallized
200
300
400
500
Sr, ppm
Fig. 8. Crystallization of the 60% host mixed magma from Fig. 3. Bulk Kd for Sr during crystallization ¼ 2. (a) Chemical and isotopic evolution of solids and liquids during fractional (Rayleigh) and equilibrium (Berthelot^Nernst) crystallization. The isotopic composition of the liquid
remains unchanged during crystallization. 87Sr/86Sr in the bulk solid increases as crystals are mantled with newly crystallized material in isotopic and chemical equilibrium with the liquid. A, initial liquid composition; B, initial solid composition; C, bulk composition. At 100% crystallization, the solid composition ¼ the bulk composition. (b) Solid^liquid tie-lines (tick marks at 10% intervals) during equilibrium
crystallization. Yand Z mark the solid and liquid compositions, respectively, at 90% crystallization. (See text for discussion.)
system to separate without further crystallization, separation of the magma into solid- and liquid-rich regions will
yield crystal-rich regions with relatively low 87Sr/86Sr and
complementary melt-rich regions with higher 87Sr/86Sr
(Fig. 7). Such separation occurring on the scale of a
magma chamber will produce an isotopically zoned body.
As an example, let us start with the homogenized bulk
mixture with 60% crystals 87Sr/86Sr ¼ 070625. If onethird of the crystals are removed from one region of the
magma [leaving it with 50% crystals (60^20 ¼ 40 mass
units crystal, 40 mass units melt)] and added to another
region of magma [which would now have 67% crystals
(60 þ 20 ¼ 80 mass units crystal, 40 mass units melt)]
an isotopic heterogeneity will be created. The volume
of magma containing 50% crystals will have
87
Sr/86Sr ¼ 07068, and the complementary volume containing 67% crystals will have 87Sr/86Sr ¼ 07058. If the
modal variation in antecryst content is continuous, zoning
will be continuous as well.
Crystallization of the homogenized
hybrid magma
The mixing behaviors shown in Fig. 8 reflect the
assumption that pre-existing crystals (antecrysts) in the
end-members retain their isotopic characteristics after
hybridization (see the Introduction). However, once the
system is mixed, any new solids that crystallize from the
hybrid magma are assumed to be in chemical equilibrium
with the hybrid melt phase, either in their entirety
(equilibrium crystallization) or instantaneously (fractional
crystallization). In either case, isotopic equilibrium
between new solids and extant melt is to be expected. This
will lead to isotopic zoning in the crystals, with a relict,
disequilibrium antecrystic core and an isotopically equilibrated, neoblastic rim.
Both fractional (Rayleigh) and equilibrium (Bertholot^
Nernst) crystallization paths were calculated using
Cm ¼ Ci f ðD 1Þ ðRayleighÞ
ð7Þ
and
Cm ¼ Ci =½D þ f ð1 DÞ ðBertholot NernstÞ
ð8Þ
where Cm is the concentration of Sr in the evolved melt,
Ci is the the initial concentration of Sr in the melt, f is the
melt fraction and D is the bulk partition coefficient for
Sr. These formulae were used to calculate the composition
of the newly crystallized solids. The newly crystallized
solids are assumed to be in chemical and isotopic equilibrium with the melt (Figs 7 and 8a, point A). However,
because the system is 60% solid (i.e. point B, Figs 7
and 8a) at the outset, the solids plotted along the crystallization paths are mixtures of pre-existing (disequilibrium,
antecryst) and newly crystallized (equilibrium, neoblastic)
solids. The isotopic characteristics of these mixed solids are
calculated using equation (1) and the isotopic and chemical
compositions at points A and B (Figs 7 and 8a).
Now, let us close the system to further material input.
During closed-system crystallization of the homogenized
hybrid magma, the isotopic composition of the bulk solid
evolves on curved paths towards the bulk isotopic composition of the system (point C, Fig. 8a) as it incorporates
radiogenic Sr from the liquid. The isotopic composition of
the liquid will not change during crystallization unless
there is diffusive exchange with the antecrysts. However,
the very act of crystallization will further isolate the
1035
JOURNAL OF PETROLOGY
VOLUME 49
antecrystic cores and help prevent isotopic exchange
between the liquid and relict antecrysts sequestered in the
cores of crystals. In Fig. 8, the enrichment in Sr in the solid
seen during the early stages of crystallization reflects a
high bulk Kd for Sr (2) used in the crystallization models.
If the bulk Kd ¼1, the solid evolution line will follow the
mixing line toward the bulk composition, whereas the
liquid composition will remain constant.
Figure 8b shows tie-lines [actually, they are calculated
as solid^liquid mixing lines using equation (1)] connecting
coexisting solid and liquid compositions at various points
during equilibrium crystallization of the homogenized
hybrid magma. This emphasizes the continuing isotopic
inequality (a consequence of the presence of antecrystic
cores in the growing crystals) between solid and liquid in
the system even as it crystallizes. In short, even though the
neoblastic crystal rims are in isotopic equilibrium with the
liquid, the bulk solid and bulk liquid will not be in equilibrium. This is shown diagrammatically in Fig. 9.
It should be noted that at any point along the crystallization path, separation of crystals from liquid will, perforce, yield an isotopic heterogeneity in the magma whose
magnitude can be calculated (as for Fig. 7) by reference to
the 10% tick marks in Fig. 8b. This holds for both equilibrium and fractional crystallization paths.
Partial equilibration of antecrysts during postmixing crystallization will move the liquid composition
toward the bulk composition along the tie-lines in Fig. 8b.
In a completely equilibrated system, of course, the final
solid and liquid will be isotopically identical. In many or
most hybrid systems, however, it is likely that some sort of
isotopic heterogeneity will be preserved, even if it is not as
extreme as that posited by Fig. 8.
DISCUSSION
Once isotopic inequality between the solid and liquid components of a magma is established, it tends to persist
regardless of subsequent fractionation, crystallization, or
mixing events (Fig. 8). This is because isotopic homogenization between the liquid and crystal fractions of the
mixed magma can occur only via diffusion or recrystallization and, especially, only if isotopic homogenization
occurs before crystal-enriched volumes of magma form. Sr
in feldspar, in particular, preserves records of isotopic heterogeneity during the lifetime of many magma chambers
(e.g. Davidson & Tepley, 1997), and, indeed for millions
and even billions of years beyond that time in felsic and
mafic plutons (Waight et al., 2000; Halama et al., 2002;
Tepley & Davidson, 2003). If antecrysts do not fully
equilibrate, the mixing behavior of isotopically dissimilar,
partially crystalline magmas combined with normal
(gravitational separation, sidewall accumulation, filterpressing, etc.) crystal^liquid separation processes will
result, perforce, in the development of a magma-scale
NUMBER 5
MAY 2008
isotopic heterogeneity. Although diffusive or other equilibration early in the mixing history can mitigate the development of large-scale isotopic heterogeneity in hybrid
magmas, other factors, especially separation of melt from
xenolith (Fig. 6), will tend to exacerbate it. Once magmascale heterogeneities form, they will be unaffected by any
subsequent crystallization, mixing, or equilibration event.
The heterogeneity can be mitigated only by diffusion at
the scale of the magma chamber and, hence, is essentially
permanent.
Aplites and pegmatites
From a mechanistic point of view, a logical interpretation
of late, differentiated intrusions (e.g. aplites and pegmatites) that cut across many plutons is that of interstitial
melt expressed during the last stages of crystallization.
However, the isotopic composition of many late aplitic or
pegmatitic intrusions differs from the bulk isotopic composition of the host pluton (e.g. Kistler et al., 1986; Johnson
et al., 1990; Barbey et al., 2001; Ernst et al., 2003), leading to
their interpretation, in many cases, as local injections of
unrelated magma. The behavior of isotopes in partially
crystalline mixtures provides a means whereby, in some
cases, the chemical and mechanical interpretations can be
reconciled (Figs 8 and 9).
Of particular importance here is the idea that once crystal^liquid isotopic heterogeneities are established in a
mixed magma, they persist. Let us take, for example, a
liquid separated from solids after 90% crystallization
(point Z, Fig. 8b). This liquid will have an isotopic composition inherited and unchanged from the original mixture.
The bulk solid composition will have evolved along the
line labeled ‘solids’ to point Y (Fig. 8b). Separation of
liquid from solid at that point will yield a chemically differentiated liquid in apparent isotopic disequilibrium with
its host as shown diagrammatically in Fig. 9. However, it is
clear that the isotopic heterogeneity is, in this case, inherited from the original mixing event. There is no need to
call upon a separate and unrelated aplite magma.
Isotopic zonation in high-silica rhyolites
The behavior of isotopes in mixed systems may have implications for a much more significant (volumetrically, at
least) geological problem; the origin of large-volume rhyolite tuffs. Recent studies and syntheses across a variety of
disciplines are now concluding [and confirming; see, for
example, Buddington (1959) or Lipman (1984)] that caldera-forming rhyolites are the surface manifestation of the
same large-volume magmatic events that produce granitic
batholiths. Rhyolites represent the evolved, melt-rich part
of the system, and granitic plutons represent the complementary crystal-rich portion (Halliday et al., 1991;
Bachmann & Bergantz, 2004; Lipman, 2008). Implicit
in this interpretation is that crystal^melt separation is
important in the petrogenesis of large-volume rhyolites.
1036
BEARD
CRYSTAL^MELT SEPARATION
pre-mix
87Sr/86Sr
≠ 87Sr/86Sr
homogenization
(a)
(b)
crystallization of homogenized magma
end-stage expulsion of interstitial melt
(d)
APLITE
DIKE
(c)
0.710
0.706
0.704
0.710
end crystal
(to scale for 60% mixture)
0.706
87Sr/86Sr
≠ 87Sr/86Sr
Fig. 9. Formation of a discontinuous isotopic heterogeneity (e.g. aplite) by crystal melt separation. (a) Pre-mixing configuration. (b) Hybrid
magma immediately after homogenization. Gray and black rectangles represent antecrysts from the two end-members. Gray background is the
chemically and isotopically homogenized mixed melt phase. (c) Crystallization of the hybrid magma forms neoblasts (open rectangles) and
crystal overgrowths (open rims) in chemical / isotopic equilibrium with the liquid. Note that isotopic heterogeneity is carried only by the
antecrysts. (d) Late aplite dike expressed from the largely crystalline system. The aplite will retain the isotopic composition of the original
mixed melt phase. Coexisting solids carry a mixed antecryst^neoblast isotopic signature.
Given that these rocks are exemplars of hybrid magmas,
they would seem to provide an important test of the relationship between crystal^melt separation and the development of isotopic zoning in hybrid magmas
Many zoned ash or ignimbrite eruptions are characterized by the early eruption of chemically and isotopically
evolved, crystal-poor magma, followed by magma that is
increasingly crystal-rich and less evolved. The process typically envisioned for this is the ‘inversion’of a zoned magma
chamber (Smith & Bailey, 1966; Hildreth, 1979; Smith,
1979; Duffield et al, 1995; Brown et al., 1998; Hildreth &
Fierstein, 2000; Dreher et al., 2005; Bindeman et al., 2006).
Figure 10 is a diagrammatic illustration of the development
of such a magma chamber by crystal^melt separation in a
hybrid magma and its subsequent eruption as a zoned tuff.
In magmas where recognizable, isotopically distinct
antecryst populations are present and/or isotopic contrast
is correlative with apparent fractionation relationships
amongst similar (e.g. dacitic to rhyolitic) magmas,
crystal^melt separation must be considered as a strong
candidate for the source of isotopic variation. On the
other hand, if isotopic zonation is manifest in glass separates, at least some of the zoning must be related to
differential contamination or mixing.
1037
JOURNAL OF PETROLOGY
VOLUME 49
NUMBER 5
pre-mix
(a)
(d)
87Sr/86Sr
≠ 87Sr/86Sr
(b)
homogenization
(c)
MAY 2008
crystallization of
homogenized magma
(e) eruption/inversion
crystal-melt separation
0.7058
(f)
0.7058
0.7068
87Sr/86Sr
≠ 87Sr/86Sr
0.7068
Vertical Section
Horizontal Section
Fig. 10. Formation of a nominally continuous isotopic heterogeneity (e.g. zoned magma chamber) by crystal^melt separation. Mixing, homogenization and crystallization as in Fig. 9. Separation of crystals and melt to any degree will result in isotopic inequalities in the magma chamber. If variation in antecryst content is continuous, isotopic variation will be continuous as well. Eruption (and consequent inversion) of the
magma chamber can result in a tuff zoned in crystal content and isotopic composition. Crystallization of the magma chamber can yield an
isotopically zoned pluton. (a)^(c) as in Fig. 9. Symbols as in Fig. 9. (d) Crystals accumulate along the walls, roof, and floor of the magma
chamber. (e) Inversion of the magma chamber (e.g. large-volume rhyolite eruption). (f) Depictions of a zoned magma chamber. Pluton edge
and core isotopic compositions are as modeled for Fig. 7 (see text). Bright areas have the highest 87Sr/86Sr. If modal variation in antecryst content
is continuous, isotopic zoning will be as well.
Isotopic zoning in plutons
An isotopically zoned magma chamber could, of course,
freeze in place without erupting, yielding an isotopically
zoned pluton (e.g. Fig. 10e and f). A note of caution,
however, is in order. Large-volume rhyolites, however
complex their petrogenesis, represent short-lived eruptive
events rooted in a single, contemporaneously active
magma system, perhaps even a single magma chamber.
Large plutons on the other hand, may represent longerlived composites that, furthermore, are easily remobilized
and reworked by subsequent magmatic events (Bindeman
& Valley, 2003; Glazner et al., 2004; Bacon & Lowenstern,
2005; Bindeman et al., 2006; Lipman, 2008). The character
of the zoning might provide a clue to its origin. If the
modal abundance of isotopically distinct antecrysts correlates with the zoning pattern, not only might this explain
1038
BEARD
CRYSTAL^MELT SEPARATION
the origin of the zoning, but it may be an indication that
the pluton represents a single-stage magma chamber.
Obviously, if the zoning correlates with other features,
such as chilled intrusive contacts, a composite origin may
be indicated.
CONC LUSIONS
The solid and liquid components produced by the mixing
of isotopically distinct, partially crystalline end-members
will themselves be isotopically distinct. Thus crystal
separation in hybrid magmas can be responsible for a significant component of the overall isotopic heterogeneity of
the magma. The magnitude of the isotopic variability
resulting from crystal^melt separation subsequent to hybridization may be as large as that resulting from differential
contamination or multiple isotopically distinct sources. In
effect, crystal^melt separation provides another option for
generating isotopic heterogeneities in magmas. This option
is particularly attractive for systems (e.g. Hildreth &
Fierstein, 2000; Wilson et al., 2006) whose chemical variation is otherwise explicable by fractionation-dominated
processes.
AC K N O W L E D G E M E N T S
I would like to thank John Hogan, Kurt Knesel, Frank
Ramos, and Associate Editor Wendy Bohrson for insightful
and thorough reviews. This work is an outgrowth of pluton
studies supported by NSF grant EAR0000719 with continuing support provided by the Virginia Museum of
Natural History. This contribution is dedicated to the
memory of Paul Ragland.
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Bachmann, O. & Bergantz, G. W. (2004). On the origin of crystal-poor
rhyolites extracted from batholithic crystal mushes. Journal of
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Bacon, C. R. & Lowenstern, J. B. (2005). Late Pleistocene granodiorite
source for recycled zircon and phenocrysts in rhyodacite lava at
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Baker, J. A., McPherson, C. G., Menzies, M. A., Thirlwall, M. F.,
Al-Kadasi, M. & Mattey, D. P. (2000). Resolving crustal and
mantle contributions to continental flood volcanism, Yemen; constraints from mineral oxygen isotope data. Journal of Petrology 41,
1805^1820.
Barbey, P., Nachit, H. & Pons, J. (2001). Magma^host interactions
during differentiation and emplacement of a shallow-level, zoned
granitic pluton, Tarcouate pluton, Morocco: implications for
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