Rebuttal
The Case against the Null Hypothesis of
Marsh
Jim Miller
Department of Geological Sciences
Precambrian Research Center
University of Minnesota Duluth
Mineralogical
Magazine
(1996)
Bruce Marsh
Johns Hopkins U.
Elements
(2006)
Contributions to
Mineralogy and Petrology
(2013)
Contributions
to Mineralogy
and Petrology
(in press)
With Rais Latypov, 2005, Finland
Marsh’s Arguments against In Situ
Crystallization Differentiation
1) Massive, undifferentiated mafic-ultramafic intrusions are
common; indeed, they are the rule.
2) The abundance of homogeneous intrusions is due to
crystallization from phenocryst –free basaltic magmas that
are not able to differentiate in crustal chambers because
evolved melts are trapped in solidificaton fronts. (Examples:
Sudbury Complex and Ferrar Peneplain Sill).
3) Layered intrusions with pronounced phase and cryptic
layering are attributed to the repeated emplacement of
batches of magma with phenocrysts in different
combinations and with varied chemical compositions
(Examples – Hawaiian lava lakes, Ferrar Basement Sill,
Palisade Sill)
Postulate 1
Massive, undifferentiated mafic-ultramafic
intrusions are common
• By Marsh’s definition, differentiated intrusion must have produce a
grantitic distillate. Bowen showed that even perfect fractional
crystallization will produce a high silica liquid fractionate – and even the, it
will only comprise about 5% of the total magma volume.
• Marsh uses whole rock SiO2, MgO, and CaO and indicators of
differentiation but ignores other important indicators - phase layering,
cryptic layering of solid solution minerals, enrichment in incompatible
trace elements and depletion of compatible elements.
• Marsh ignores or reinterprets units of the Sudbury Igneous Complex that
clearly indicate internal differentiation - lower mafic norite and upper
quartz gabbro, and considers only whole rock MgO rather than the mg#
of pyroxene.
Marsh’s evidence of undifferentiated mafic intrusions with late
small granite segregations filling delaminated chill in the roof zone
Marsh (1996)
Not Differentiaton ??
Even the Sudbury Complex shows Cryptic Variation
Postulate 2
Phenocryst –free basaltic magmas are not able to
differentiate in crustal chambers because evolved
melts are trapped in solidification fronts
• In mafic intrusions of all sizes, shapes and depths of emplacement, it is
clear that these intrusion show systematic differentiation whether as
shown dramatically in the closed systems of the Skaergaard and Sonju
Lake, or in large thick sequences like Stillwater, Bushveld and the Layered
Series at Duluth.
• Many MLI lack an upper border series as would be expected in Marsh’s
model of collapsing solidification fronts. Sonju Lake in particular can be
modeled by bottom up crystallization differentiation of single evolved
tholeiitic magma.
Bulk Intrusion Composition
= Parent Magma
SiO2
TiO2
Al2O3
FeOt
MnO
MgO
CaO
Na2O
K2O
P2O5
Volatiles
Total
mg#
47.6
2.28
14.0
14.7
0.21
8.3
9.4
2.47
0.55
0.30
0.20
100.0
50.2
Sc
V
Cr
Co
Ni
Rb
Sr
Ba
Y
Zr
Nb
Hf
La
Ce
Sm
Eu
Tb
Yb
Lu
34
192
111
75
185
20
233
171
20
114
17
3.1
14.7
33.4
4.1
1.6
0.8
2.1
.32
Liquid Line of Descent
Calculated by summing composition of
rock column above a specific horizon
= moderately evolved olivine tholeiitic basalt
From Miller and Chandler (1998)
and Miller and Ripley (1997)
Fractional Crystallization Modelling
CHAOS 2 (NIELSEN, 1990)
Model Parameters : fO2 = -2 log QFM; trapped liquid = 20%
Postulate 3
Layered intrusions with pronounced phase and cryptic
layering are attributed to the repeated emplacement of
batches of magma with phenocrysts in different
combinations and with varied chemical compositions
• How did these crystal slurries of varied composition acquired their variability if
not by differentiation in some ficticous magma chamber at depth? It is
paradoxical that even in the case of the largest known intrusion of largely basaltic
composition, most of the fractional crystallization reflected in the cryptic
compositional trends of the layered sequence has to be relegated to conjectural
processes that took place in an underlying (and possibly even more voluminous)
chamber
• How can the random input of varied crystal slurry compositions create a
systematic phase stratigraphy commonly observed in both closed and open
system intrusions?
•Why aren’t porphyritic basalts more common in LIPs hosting many
differentiated layered intrusions – e.g. the MCR? And why are they typically
dominated by only Pl phenocrysts?
From Latypov (2009)
The High Priest of Magmatology
“An attempt is made at the outset to provide a list of inviolate
Magmatic First Principles that are relevant to analyzing most
magmatic problems. These involve: initial conditions; critical
crystallinity; solidification fronts; transport and emplacement
fluxes; phenocrysts, xenocrysts, primocrysts; crystal size;
layering and crystal sorting; thermal convection; magmatic
processes are physical”.
Bruce Marsh, 2013
Wager and Brown actually considered the
Sequential Slurry Model, but found it wanting
because of chemical evidence and too much
coincidence.
“It could be argued that regular, cryptically varying series of
crystal mushes were successively injected, rather than liquids,
which is equivalent to agreeing that layered intrusions existed,
but with the unusual reservation that they always lay below the
levels of present-day exposures. The regularity of the cryptic
variation of many layered intrusions is particularly difficult to
explain according to the idea of continuous tapping of a deeper
magma reservoir, for it implies that the complete crystallization
record of a deep-seated intrusion was transferred, stage by
stage, to a higher level intrusion”.
Lawrence Wager and Malcolm Brown (1968, p. 546)
Other Issues with Marsh’s (2013) Claims
The views of Wager and Brown (1968)
Marsh (2013, p. 669) attributes the improbable scenario of
instantaneous emplacement of crystal-free magma for the
origin of the Skaergaard, Stillwater and Bushveld intrusions
to Wager and Brown.
Although they ascribed emplacement of the Skaergaard to
be a geologically rapid event of one magma batch, in the
case of the Bushveld intrusion, they state that it was
“unlikely that the act of magma injection was a single
incident” (p. 404) and they encompassed the idea of
“stages when fresh supplies of basaltic magma entered the
chamber from below”
Other Issues with Marsh’s (2013) Claims
Mineral-Melt Reactions
Marsh (2013) makes the curious statement (on p. 685) that
“For magma to differentiate by fractional crystallization
crystals must be available to react with the melt, the
ultimate origin of the crystals is almost immaterial.”
This is diametrically at odds with conventional wisdom:
fractional crystallization is a consequence of the physical
and/or kinetic isolation of crystals from their parent melt
so that such a reaction is inhibited
Other Issues with Marsh’s (2013) Claims
Misinterpretation of Bowen’s Reaction Series
Marsh’s view (on p. 666) that Bowen’s “reaction series
became synonymous with fractional crystallization”. As
Bowen (1928) was at pains to point out, the portrayal of
“reaction series” incorporated into Figure 9 of Marsh
(2013) principally expresses the relationship between
mineral phases and generalized crystallization
temperature. Mineral-melt reactions are in fact part and
parcel of equilibrium crystallization whereas in perfect
fractional crystallization they are prohibited.
Other Issues with Marsh’s (2013) Claims
Size of Crystals in Sills and Layered Intrusions
Marsh (1996) points out that many mafic sills contain
dense basal accumulations of large (2-10 mm) olivine or
orthopyroxene phenocrysts and suggest that the size of
these crystals alone preclude them from having grown in
situ. In his new paper, Marsh (2013) makes the additional
claim that “The crystals in Skaergaard…..are too large to
have grown in a body of this size”. In particular, Marsh
(2013) proposes a simple model of kinetic crystal growth
that does not take into account the role of equilibration
(coarsening) that is so important in the postcumulus
textural development of most plutonic rocks (Higgins
2011).
Other Issues with Marsh’s (2013) Claims
Size Distribution of Crystals in Sills and MLI
Skaergaard Intrusion and Palisades Sill both have upper
border sequences that are compositionally mirror images of
their extensively differentiated, and significantly thicker,
lower sequences. In both the Skaergaard (Naslund 1984) and
the Palisades (Hristov and Naslund 1994), rocks in the upper
border sequences are 2 to 3 times coarser-grained than the
time-equivalent lower sequences. If in situ crystallization is
disallowed, then the formation of these two intrusions by
multiple injections of crystal slurries must have involved
sinking of the smaller crystals and floating of the larger
crystals, which is exactly the opposite of what would be
predicted by the crystallization front capture model of Marsh
(2013).
Other Issues with Marsh’s (2013) Claims
Kilauea Lava Lake
Marsh (2013) presents the Kilauea Iki lava lake as an ‘endmember’ example of differentiation by simple accumulation of
crystals transported in the lava. Recent research suggests,
however, that filling and solidification of the lake may be
complex (Vinet and Higgins 2011). Olivine CSDs are almost
uniform throughout the lake and show no significant variation in
slope that would indicate that settling has occurred.
Furthermore, a detailed inventory of the zoning and deformation
of olivine crystals suggests that the lake was partly filled from
the base and that bulk compositional variations may be a relic of
this process (Vinet and Higgins 2011).
Other Issues with Marsh’s (2013) Claims
Evidence for In Situ Fractional Crystallization
Marsh (2013) presents the Kilauea Iki lava lake as an ‘endmember’ example of differentiation by simple accumulation of
crystals transported in the lava. Recent research suggests,
however, that filling and solidification of the lake may be
complex (Vinet and Higgins 2011). Olivine CSDs are almost
uniform throughout the lake and show no significant variation in
slope that would indicate that settling has occurred.
Furthermore, a detailed inventory of the zoning and deformation
of olivine crystals suggests that the lake was partly filled from
the base and that bulk compositional variations may be a relic of
this process (Vinet and Higgins 2011).