Geochemistry, Geophysics, Geosystems
Supporting Information for
Plume-cratonic lithosphere interaction recorded by water and other trace elements in
peridotite xenoliths from the Labait volcano, Tanzania
Hejiu Hui1,2,3, Anne H. Peslier4, Roberta L. Rudnick5, Antonio Simonetti2, Clive R. Neal2
1State Key
Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University,
Nanjing 210023, China
2Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN
46556, USA
3Lunar and Planetary Institute, USRA – Houston, Houston, TX 77058, USA
4Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, TX 77058, USA
5Department of Geology, University of Maryland, College Park, MD 20740, USA
Contents of this file
Text S1
Figures S1 and S2
Additional Supporting Information (Files uploaded separately)
Captions for Tables S1 to S4
Introduction
These supplementary materials provide additional information for analytical methods,
melting and melt-rock interaction models, and effective viscosity calculation. In addition, trace
element data are included in the supplementary tables. The authors consider the additional
information important as it clarifies the discussion in the main text.
1
Text S1.
1. Analytical Methods
Portions of the mantle xenoliths were gently crushed to keep grain integrity. Around 30
(or as many as we could obtain) representative mineral grains for olivine, orthopyroxene,
clinopyroxene, and garnet were handpicked in ethanol under a binocular microscope. Selected
mineral grains were washed with ethanol and dried at room temperature overnight. The dried
mineral grains were subsequently embedded in epoxy resin. Doubly polished sections 70 to 300
m thick were prepared using an Ingram grinder and an automatic polisher with diamond paste
(down to 1 m). The sections were cleaned with acetone and deionized water.
1.1. FTIR Analyses
Each mounted mineral grain was examined under a petrographic microscope. Two
criteria were used to select grains for Fourier transform infrared spectrometer (FTIR) analysis: (1)
the minerals should contain zones that are optically free of fractures and inclusions [Bell et al.,
1995], (2) for biaxial minerals (olivine, orthopyroxene and clinopyroxene), the polished surface
should be oriented perpendicular to the optic normal (ON), acute bisectrix (Bxa) or obtuse
bisectrix (Bxo) optical axes. The grain crystallographic orientation was identified with
interference figures and recorded. Garnet is uniaxial, and thus the crystal orientation does not
affect its infrared spectra. The thickness of each selected grain was measured using a Mitutoyo
digital micrometer. The grain mounts were stored overnight in a cabinet under a N 2 flux before
FTIR analyses.
Polarized infrared spectra of olivine and pyroxene crystals with wave numbers from
4000 to 650 cm-1 were collected using two different FTIRs at NASA-Johnson Space Center: A
Continuµm microscope attached to a Nicolet NEXUS 670FT FTIR spectrometer, and later a
newly installed Hyperion 3000 microscope attached to a Bruker Vertex 70 FTIR spectrometer.
The mercury cadmium telluride (MCT) detector and KBr beam splitter were used during FTIR
analyses. A Zn-Se wire grid polarizer rotated parallel to the optical indicatrix ,  or , was used
during analyses of olivine, orthopyroxene and clinopyroxene. Aperture size from 23  23 to 100
 100 µm2 during each analysis was chosen based on the grain size and the distribution of
fractures and inclusions in the grain. Each analysis typically consisted of 256 scans performed in
a N2 gas environment, thus minimizing interference from atmospheric water vapor. Up to four
different locations in each grain were analyzed to check the homogeneity of water within the
grain. Typically a new background spectrum was collected before a new grain was analyzed.
Cross-sectional analyses on two olivine grains were also carried out to demonstrate the
homogeneity of water within the grain using an AutoImage microscope on a Perkin Elmer
Spectrum GX FTIR at the University of Michigan using the same experimental procedure as
already described.
Each spectrum was normalized to 1 cm thickness and, if necessary, the baseline was
manually corrected using the Nicolet Omnic software. The overall absorption intensity in the
O-H region was determined by integrating the area beneath the OH absorbance bands
(between 3600 and 3000 cm-1 for olivine and garnet, 3700 and 2800 cm-1 for orthopyroxene, and
3700 and 3000 cm-1 for clinopyroxene). The integrated absorption intensity was converted to
water content (ppm H2O, and hereafter) using the Beer-Lambert law in the form CH2O = As/I,
where for pyroxene and olivine As = A + A + A is the sum of the integrated absorption
intensities for three principle directions (cm-2) (A, A and A correspond to the integrated
absorbance beneath the OH band with the vector of the incident infrared light E//, // and //,
respectively) while for garnet, it represents the integrated area beneath OH bands of
2
unpolarized spectra. I is the integral molar absorption coefficient (ppm-1cm-2) of 5.320.34 for
olivine, 14.840.59 for orthopyroxene, 7.090.32 for clinopyroxene, and 1.390.14 for garnet
[Bell et al., 1995, 2003].
Uncertainties are generated at each step of the data acquisition and reduction
processes. The uncertainty in grain orientation is estimated to be 5, which converts to 5%
H2O. The error from mineral mount thickness measurement is 3 µm. The largest uncertainty
comes from the manual correction of spectrum baseline using Omnic software. The
quantification of this uncertainty is somewhat ambiguous due to subjective characteristics of
identification of the baseline, which can be up to 10%. The overall uncertainty (1) of water
contents determined is typically up to 20%, calculated for each water content using an error
propagation formula.
1.2. LA-ICP-MS Trace Element Analyses
Trace- and minor-element concentrations of orthopyroxene, clinopyroxene and garnet
from the mantle xenoliths were measured using a 213 nm Nd-YAG New Wave laser ablation
system in conjunction with a Thermo-Finnigan Element 2 high resolution magnetic sector
inductively coupled plasma mass spectrometry (LA-ICP-MS) in the Midwest Isotope and Trace
Element Research Analytical Center (MITERAC) at the University of Notre Dame. The CaO
concentration of each mineral from the previous electron probe microanalysis [Lee and Rudnick,
1999] was used as an internal standard (CaO weight percent measured by the electron
microprobe, and 43Ca abundances measured by LA-ICP-MS). Silicate glass NIST 612 was used as
an external standard to correct for instrument drift and ensure reproducibility in the
measurements. Due to the low concentration of most trace elements in orthopyroxene, the
largest aperture (250 m) was used to ensure maximum material ablated for analyses in the
mass spectrometer. For clinopyroxene and garnet, different apertures (40, 80, 100 or 125 m)
were used in order to obtain accurate measurements of incompatible trace elements with low
concentrations, such as LREEs. The laser had a repetition rate of 5 Hz, and 100% power output.
Each spot analysis consisted of approximately 20 s collection of background (gas blank) and
approximately 40 s of laser ablation on the sample. The ablated particles were transported into
the ICP-MS by a helium-argon gas mixture with 0.7 l/min of He. The LA-ICP-MS data were
reduced using GLITTER (a software for the laser ablation microprobe, which provides the realtime interactive data reduction for LA-ICP-MS analysis). Furthermore, GLITTER can be used
to monitor whether incompatible-trace-element rich inclusions were ablated during analysis as
sudden change of concentrations of incompatible trace elements and Ca can be identified in
the time-resolved data. No inclusions appear to have been ablated during our analyses.
Typically, three grains, if possible, were analyzed to check the grain heterogeneity within each
mantle xenolith. The propagated standard errors (1) of 58 analyses of the external standard
during the measurement are better than 0.2 ppm for all the trace elements except Be (0.29
ppm), Ti (0.23 ppm), and Sr (0.36 ppm). Variability of orthopyroxene trace element contents
within one xenolith is less than an order of magnitude (mostly <50%), except for Cu, Mo, Sb, Yb,
and Pb in LB45, Ba in KAT17, Th in LB1, Pb, Ce, and Ba in LB11, Rb, Mo, Ba, and Lu in LB53, and
Rb, Ba, Dy, and Pb in LB24 (Table S1).
1.3. Trace Element Analysis Comparison with those of Aulbach et al. [2008]
Our new measurements of trace element abundances in clinopyroxenes from KAT17
and LB4 are consistent with those from Aulbach et al. [2008] (Fig. S1). However, trace element
contents of LB45 clinopyroxene from Aulbach et al. [2008] are slightly higher than those
3
obtained in this study (Fig. S1). This discrepancy could be due to sample heterogeneity,
perhaps linked to the presence of different generations of metasomatic clinopyroxene.
2. Melting and Melt-Rock Interaction Models
Three models are used in the present paper to quantify the behavior of water in mantle
processes (Fig. S2): equilibrium melting, fractional melting, and melt-rock interaction (MRI).
(1) Equilibrium batch melting [Shaw, 1979; Norman, 1998],
0
(S1a)
CS = (DS/L ´ CBR
) /(DS/L ´ XS ´ (1- F) + F),
BR/L
0
BR/L
(S1b)
CBR = (D
´ CBR ) /(D
´ (1- F) + F),
where CS and XS are the water content and mass fraction of olivine, orthopyroxene, or
clinopyroxene respectively, DS/L is the partition coefficient of water between mineral phase
(olivine, orthopyroxene, or clinopyroxene) and silicate melt, CWR0 is the initial whole-rock water
content, CWR is the whole-rock water content, DWR/L is the partition coefficient of water between
peridotite and silicate melt, and F is the fraction of silicate melt to total mass during partial
melting.
(2) Fractional melting [Shaw, 1979; Norman, 1998],
0
(S2a)
CS = (CWR
/ XS ) ´ (1- F)1/(D ´XS )-1,
0
WR/L 1/ P -1
(S2b)
CWR = CWR ´ (1- (P ´ F /D )) ,
i i/L
where P = å p D , i can be olivine, orthopyroxene, and clinopyroxene [Shaw, 1979]. Here pi
S/L
i
is the mass fraction of each mineral in silicate melt [Shaw, 1979] and assumed to be constant in
our calculation, 0.3 for clinopyroxene, 0.1 for orthopyroxene and 0 for olivine.
(3) Melt-rock interaction [DePaolo, 1981],
(S3a)
Ca = (CDM ´ DWR/L ) /(DWR/L ´ (1- F) + F),
0
-1
0
(S3b)
Cm = Cm ´ f (1- (Ca /Cm ) ´ (r /(r -1))),
(S3c)
CBR = Cm ´ Mm + Ca ´ (1- Mm ),
Ol, Opx
CCpx = (CWR - CWR /D
WR/L
´
åM
i
´ Di / L ) / M Cpx,
(S3d)
åM
i
´ Di / L ) / M Opx ,
(S3e)
´ Di / L ) / M Ol ,
(S3f)
i
Ol, Cpx
COpx = (CWR - CWR /DWR/L ´
i
Opx, Cpx
COl = (CWR - CWR /DWR/L ´
åM
i
i
where CDM is the water concentration in the depleted mantle, Cm0 is the initial concentration of
water in the melt that interacted with the depleted mantle (0.5 wt% and 0.25 wt%), f (assumed
to be 0.95) is the mass ratio of the interacted magma to the initial magma; r (assumed to be
0.97 [Kelemen et al., 1998]) is the mass ratio of the assimilated magma to the crystallized
magma, Mm is the proportion of the interacted magma added into the peridotite (1 and 0.5 wt%
of magma containing 0.5 and 0.25 wt% H2O respectively), and Mi (i = Ol, Opx, Cpx) is the modal
proportion of each mineral phase in peridotite.
Water contents of both depleted mantle (DM = 150 ppm H2O, near upper limit of MORB
source [Michael, 1988; Dixon et al., 2002]) and primitive mantle (PM = 1100 ppm H2O [Palme
and O’Neill, 2007]) were used in our modeling (Fig. S2). The modal proportion used for
clinopyroxene, orthopyroxene, and olivine in the equilibrium and fractional melting models are
15%, 25%, and 60% respectively, while those in the melt-rock interaction modeling are 10%,
4
25%, and 65% respectively. The partition coefficients of water between pyroxenes and melt
were calculated using the equations of O’Leary et al. [2010] using the chemical compositions of
LB45 pyroxenes, which is the most fertile sample [Chesley et al., 1999]. A constant value of
0.0006 [Tenner et al., 2009] for water partitioning between olivine and melt is used. The water
partition coefficient between bulk peridotite and melt was assumed to be 0.008 (at 2 GPa
[Tenner et al., 2009]).
3. Effective Viscosity Calculation
Effective viscosity (eff; in Pas) is defined as the ratio of the shear stress (; in Pa) to the
strain rate ( e˙ ; in s-1):
(S4)
heff = t /e˙.
The shear stress in our calculation is assumed to be 0.3 MPa. The strain rate of an olivine
aggregate can be calculated using following equation [Mei and Kohlstedt, 2000]:
e˙ = Acret n1(exp(c 0 + c1 lnCOH + c 2 ln 2 COH + c 3 ln3 COH )) r exp(-
Q + PVcre
),
RT
(S5)
where Acre = 90MPa-(n1+r)s-1 with the experimentally derived stress exponent n1 = 3.5 and the
water fugacity exponent r = 1.2 [Hirth and Kohlstedt, 2004], Q is the activation energy (480
kJ/mol [Hirth and Kohlstedt, 2004]), Vcre is the activation volume (10.610-6 m3/mol [Li et al.,
2008]), and COH is the water content in H per 106 Si converted from that in ppm using a SiO2
content of 40.27 wt% in LB45 olivine [Lee and Rudnick, 1999]. The temperature T in K varies
with the depth in km and was calculated using two different cratonic geotherms. One based on
a typical Archean cratonic geotherm with a surface heat flow of 41 mWm-2 [Rudnick et al., 1998]
is:
(S6)
T = 88.304 + 7.057P - 0.0096P 2 + 5.7 ´10-6 P 3.
The other one using a geotherm passing through the xenolith equilibration temperatures and
pressures with a surface heat flow of 50 mWm-2 [Lee and Rudnick, 1999] is:
(S7)
T = -22.192 +16.495P - 0.0752P 2 + 20.4 ´10-5 P 3.
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6
Figure S1. Comparison of trace element abundances in clinopyroxenes from KAT17, LB45, and
LB4 obtained in this study and by Aulbach et al. [2008].
7
Figure S2. Equilibrium, fractional melting and melt-rock interaction (MRI) models for the
Labait peridotites. See Fig. 1 caption for details on the modeling.
8
Table S1. Trace elements (ppm) of orthopyroxene from Labait mantle xenoliths.
Table S2. Trace elements (ppm) of clinopyroxene from Labait mantle xenoliths.
Table S3. Trace elements (ppm) of garnet and kelyphite from Labait mantle xenolith LB24.
Table S4. Water contents in melt equilibriated with olivine, orthopyroxene, and clinopyroxene
of Labait mantle xenoliths.
9