Petrology and thermal structure of the Hawaiian plume: a view from Mauna Kea
Claude Herzberg
Department of Geological Sciences
Rutgers University
Piscataway, New Jersey, U.S.A. 08854
Herzberg@rci.rutgers.edu
Supplementary Information
This material provides a detailed explanation of peridotite and pyroxenite modeling.
CaO Contents of Batch Melts of Fertile Peridotite
The CaO contents of experimental melts as functions of melt fraction (F) and
pressure1 are shown in Figure S1 for fertile peridotite KR-4003. These are compared with
computed liquids in equilibrium with olivine [L+Ol] and olivine+orthopyroxene
[L+Ol+Opx] from mass balance solutions to the equilibrium melting equation2. There is
excellent coverage of experimental data at 6, and 7 GPa1, but fewer data at 4 and 5 GPa,
especially at low melt fractions. Calculated and observed compositions at 3 GPa indicate
CaO = 10% where F = 0, but additional experimental data at low melt fractions is needed
for a more secure evaluation. Experimental data and computed liquids at 3, 6, and 7 GPa
have been regressed to F = 0, and the results illustrate the CaO content of liquidus on the
solidus is about 10%, and does not vary significantly with pressure in the 3-7 GPa (Figure
S1). Results at 4 and 5 GPa are interpolations.
CaO is now plotted with MgO for experimental data and computed liquids in
Figure S2. Also shown, for purposes of comparison, are the approximate CaO and MgO
contents of fertile peridotite at 1 and 2 GPa4-8. At pressures in the 1 to 3 GPa range,
dissolution of clinopyroxene with progressive melting above the solidus causes the CaO
contents of melts to increase; CaO reaches a maximum when all Cpx is melted out. CaO
then drops with continued progressive melting for the assemblages [L+Ol+Opx] and
[L+Ol]. However, the situation is different at pressures in the 4 to 7 GPa. CaO is at a
maximum for liquids on the solidus, and drops with continued progressive melting for all
assemblages above the solidus, which are typically [L+Ol+Cpx+Gt], [L+Ol+Opx+Gt],
[L+Ol+Opx], and [L+Ol] (see Herzberg and O’Hara2 for a complete description of these
assemblages).
CaO Contents of Accumulated Fractional Melts of Fertile Peridotite
A detailed discussion of the effects of perfect fractional melting on the FeO and
MgO contents of Kettle River Peridotite KR-4003 was given in Herzberg & O’Hara2.
Briefly, a computation of the liquid was made of 0.01 mass fractions of equilibrium
melting of the residue, and the liquid was perfectly removed from its residue. This
fractional melting process changes the composition of each successive residue, which is
melted again at 0.01 mass fractions of melting. Each liquid formed by 0.01 mass
fractions of melting is called an instantaneous melt. Each new melt was perfectly
extracted and mixed with the previously formed melt, and the mixture is the aggregate
fractional melt. Results of fractional melting on FeO and MgO1 were extended to include
Al2O3 and SiO2 2, and computation procedures used here for CaO are the same. That is,
1
the compositions of CaO in the instantaneous melt produced at 0.01 mass fractions of
melting were calculated from parameterizations of D for CaO as polybaric functions of
the MgO content of the residue. These instantaneous melts mix to yield an aggregate
melt, and the CaO contents for these are shown in Figure S3, and Figure 1b in the text.
CaO contents of Batch Melts of Fe-rich Peridotite
Melting experiments on Fe-rich peridotite with 10.3% FeOT indicate that CaO
might be lowered compared to those in Figures S1 and S29 for normal peridotite having
about 8% FeO. Humayun et al.10 argued that Hawaiian tholeiites might have been
produced from Fe-rich mantle peridotite. Could the low CaO parental lavas from Mauna
Kea be diagnostic of Fe-rich mantle peridotite? A close examination of Kushiro’s9
experiments indicates this is unlikely because, while CaO might be appropriately
lowered, the SiO2 contents are consistently too low by 2-4 wt% absolute, and would yield
alkalic lavas rather than the observed tholeiitic compositions.
CaO Contents of Peridotite Melts used in the Analysis of Stolper et al.11
Peridotite partial melt CaO contents at 30 kilobars used in the Stolper et al.11
analysis, adopted from a parameterization of Longhi12, appear to match for CaO content
of their Mauna Kea low SiO2 parental magma. However, these model peridotite melts are
significantly lower in SiO2 than the parental magma compositions, and they are alkalic
not tholeiitic.
There is some agreement in CaO between the model low SiO2 parental magma
Stolper et al.11 and experimental data at 28-32 kilobars from Salters et al.13 on liquids
equilibrated with “lherzolite assemblages” (i.e., L+OlOpxCpxGt). However, those
experimental melts are alkalic, not tholeiitic.
The parental Kea high SiO2 parental magma of Stolper et al.11 does not match up
with any experimental peridotite melts at ~ 30 kilobars. The match is improved with
Longhi’s12 model peridotite melts at 1.5 GPa; however, CaO in the parental magmas
remains about 1.5% lower than these peridotite melts, and garnet will not be stable at this
pressure.
Erroneous Peridotite Source Primary Magmas in the Models of Herzberg and
Coauthors2,14
A new way to obtain model primary magma solutions was described in Herzberg
& O’Hara2 and applied to the low and high SiO2 whole rocks from the HSDP2 project14.
The method is described in the footnote to Table 1. It is a coupled forward and inverse
model using assumed peridotite compositions. The forward component consisted of
primary magma compositions in FeO-MgO and CMAS projection space, constrained by
experiment and calculation on assumed peridotite source compositions. A comprehensive
model for peridotite-source melting must satisfy all major elements in simple plots and in
projection, a requirement of Duhem’s Theorem. However, I did not parameterize CaO at
that time. I was pleased to find that the Herzberg and O’Hara2 method now agrees with
the new CaO parameterization for the high CaO HSDP2 lavas, consistent with peridotite
source melting at Mauna Kea. There is also internal consistency for other picritic
occurrences such as Gorgona, and Baffin Island (Table 1, text). However, this
consistency breaks down for most Hawaiian tholeiites because they are too low in CaO to
2
be peridotite-source melts (Figure 1b). The Herzberg2,14 models for the Hawaiian
tholeiites are now wrong because our assumed peridotite source composition was wrong.
A pyroxenite source is preferred.
Phase Diagrams for Pyroxenite Sources
Reviews of phase equilibria pertinent to this discussion are given by Milholland
and Presnall15 for the system CaO-MgO-Al2O3-SiO2 and Kogiso et al.16 for natural
pyroxenitic compositions. Addition of TiO2, Cr2O3, FeO, Na2O, K2O to the system CaOMgO-Al2O3-SiO2 can affect the phase diagrams in complex ways that have yet to
undergo experimental calibration. Consequently, only compositions in natural complex
systems comparable to potential Hawaiian source and primary magma compositions will
be considered here.
The pyroxene-garnet plane is a thermal divide that separates liquids on each side
from being derived from each other either by partial melting or crystallization15-17. It is
defined by pyroxene and garnet compositions that plot within the plane DiopsideEnstatite-Pyrope-Grossular (Figure S4). To visualize this as a divide, we will use the
projection Diopside into the plane Olivine-Quartz-CATS (i.e., calcium Tschermak’s
pyroxene) as recommended by Kogiso et al.16, using the projection code of O’Hara18; all
pyroxene and garnet compositions project along the line Opx-CATS. All source
compositions on the olivine-rich side of the thermal divide will form partial melt
compositions that remain on the olivine-rich side of the divide. Similarly, all source
compositions on the SiO2-rich side of the thermal divide will form partial melt
compositions that remain on the SiO2-rich side of the divide (Figure S4).
We are particularly interested in liquid compositions that are in equilibrium with
orthopyroxene + clinopyroxene + garnet, and will use the notation [L+Opx+Cpx+Gt].
Experimental data19-23 on compositions close to those of the divide permit the liquidus
crystallization fields to be mapped out at 3 GPa (Figure S4b). Data by Keshav et al.23 are
at 2.5 GPa rather than 3.0 GPa, and provide a bound for liquids coexisting with Cpx+Gt.
For the more olivine-rich equilibrium [L+Ol+Opx] at 3 GPa we use experimental data
from Walter1 and its parameterization2. Likewise, data from Walter1 are used for the
equilibrium [L+Ol+Opx+Cpx]. The cotectics [L+Ol+Opx+Cpx] and [L+Opx+Cpx+Gt]
intersect at a pseudoinvariant point [L+Ol+Opx+Cpx+Gt] (Figure S4a,c). This is a wellknown peritectic reaction point defined by L+Opx=Ol+Cpx+Gt2,17. At lower
temperatures, Opx disappears and liquids evolve along the cotectic [L+Ol+Cpx+Gt].
Data of Keshav et al.23 and Hirschmann et al.24 for [L+Cpx+Gt] at 2.5 GPa are used
because they provide a bound on [L+Ol+Cpx+Gt] at 3.0 GPa. Although this equilibrium
is not particularly important for most melt generation below Hawaii, it is considered here
for purposes of completeness. And although the 2.5 GPa experimental liquids must be
slightly deficient in MgO and enriched in Al2O3 compared to [L+Ol+Cpx+Gt] at 3.0 GPa,
they provide a good visual sense of the projected distribution of this cotectic.
To the Si-rich side of the pyroxene –garnet thermal divide, no experimental data
exist on [L+Opx+Cpx+Gt] that are relevant to Hawaii. However, data of Pertermann and
Hirschmann25 on a MORB-like eclogite provide important constraints. Shown in Figure
S5 are their data for liquids for the equilibrium [L+Qz+Cpx+Gt], and its likely
intersection with [L+Opx+Cpx+Gt]. This intersection defines a pseudoinvariant point
involving [L+Qz+Opx+Cpx+Gt], and it is drawn as a eutectic. The Pertermann and
3
Hirschmann data are important because they indicate the cotectic for [L+Opx+Cpx+Gt]
projects linearly on each side of the thermal divide as shown in Figure S5, and it is also
likely to be orthogonal to the thermal divide. This [L+Opx+Cpx+Gt] cotectic is indicated
as a broken line to emphasize it is an interpolation that requires testing by experiment.
However, this linear behavior can be viewed with some confidence because it is very
similar to low-pressure phase equilibrium studies. For example, at 1 atmosphere the
cotectic [L+Forsterite +Anorthite +Augite] in the system CaO-MgO-Al2O3-SiO2 contains
a thermal divide which separates liquids with low and high SiO2 contents26,27. This
cotectic also projects orthogonal to and linearly on each side of its divide, as does the
cotectic in more complex systems28; crystallization along it is responsible for much of the
petrological variation in mid-ocean ridge basalts29,30.
Experimental data at 4 GPa are few. Only the data of Walter1 and its
parameterization2 exist, and these characterize the compositions of liquids for the olivinerich equilibria [L+Ol+Opx], [L+Ol+Cpx+Gt], and [L+Ol+Opx+Cpx+Gt]. These are
shown in Figure S6. The effect of increasing pressure from 3 to 4 GPa is to expand the
garnet and orthopyroxene liquidus crystallization fields at the expense of olivine,
observations that have been described in considerable detail elsewhere2.
Figure S7 is a summary of the essential phase diagram at 3 and 4 GPa. Liquid in
[L+Opx+Cpx+Gt] within the garnet-pyroxene plane at 3 GPa is well-constrained, and has
the composition 3a indicated in Figure S7 and Table S1. Liquid at the olivine-rich
termination of this 3 GPa cotectic [L+Ol+Opx+Cpx+Gt] is also well-constrained, and has
the composition 3b in Figure S7 and Table S1. At 4 GPa, liquid [L+Ol+Opx+Cpx+Gt] is
known (i.e., 4b), but the composition of liquid for [L+Opx+Cpx+Gt] within the garnetpyroxene plane is not. A linear exptrapolation of the 4 GPa cotectic [L+Opx+Cpx+Gt]
constrains the composition 4a in Table S1. Future experimental studies are required to
calibrate these 4 GPa compositions. However, the linear behavior of [L+Opx+Cpx+Gt] at
3 GPa gives us some confidence in this extrapolation. Experimental data of Yaxley and
Green31 at 3.5 GPa have not been plotted; however, they are in good agreement with
bounds imposed by the cotectics in Figure S7.
HSDP Whole Rock Fractionation
Variations in whole rock SiO2, MgO, and CaO displayed in Figures 1 and 2 of the text
are mostly the products of olivine phenocryst addition and subtraction. However, there is
a small component of plagioclase and augite addition and subraction that is not obvious
in these plots, and it compromises the model primary magma compositions that have
been computed by projection. This is clearly evident in Figure S8, where the model
pyroxenite primary magmas for whole rocks are compared with those of the glass
analyses in a diagram that depicts liquids that crystallize olivine+plagioclase+diopside
[L+Ol+Plag+Aug]30. Many whole rocks are coincident with glasses, but others scatter as
outliers towards vectors defined by plagioclase and plagioclase+augite. The outliers were
filtered out of the whole rock model pyroxenite melt compositions, and the remaining
ones used in Figures 3 and 4 of the text. This is the most simple model-independent
procedure for correcting for minor plagioclase + augite fractionation, and facilitates a
direct comparison of model primary magma compositions derived from whole rocks and
glasses.
4
Second-stage Pyroxenite Sources
Direct partial melting an original basaltic crustal protolith yields partial melts in
equilibrium with eclogite or quartz/coesite eclogite at 3.0-3.5 GPa25,31. These melts are
too low in NiO, MgO, and high in SiO2 to match Hawaiian tholeiites25,31. They are also
too enriched in Al2O3 to be in equilibrium with orthopyroxene (Figure S9). Likewise, the
primitive Mauna Kea magmas in equilibrium with Fo90.5 olivine11 are too deficient in
Al2O3 to be in equilibrium with a bimineralic eclogite source (Figure 2, text). How does
the source develop orthopyroxene? Sobolev et al.32 proposed a second-stage melt-rock
reaction process, and this is illustrated in Figure S9.
At 3 GPa, quartz eclogite begins to melt at about 1315oC, and advanced melts can
leave behind a residue of bimineralic eclogite25,31. Reaction of the SiO2-rich melts with
the peridotite host31-33 is illustrated in Figure S9. Yaxley and Green31 reported normal
peridotite and eclogite separated by an orthopyroxene-rich reaction layer of with minor
Ol+Cpx+Gt. The compositions of the 2 pyroxenes and garnets at this Opx-rich layer are
shown in Figure S9b. High degree melts of eclogitic crust react with peridotite to produce
Cpx+Gt. Low degree melts of eclogitic crust react with peridotite to produce
Opx+Cpx+Gt, the pyroxenite source for high SiO2 melts at Mauna Kea. Sobolev et al.32
report a second stage reaction produce of Cpx+Gt, but Opx is likely to be important.
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