Making the Southern Margin of Laurentia themed issue
Composition of the mantle lithosphere beneath south-central Laurentia:
Evidence from peridotite xenoliths, Knippa, Texas
Urmidola Raye1,*, Elizabeth Y. Anthony2, Robert J. Stern1, Jun-Ichi Kimura3, Minghua Ren2, Chang Qing3, and
Kenichiro Tani3
Department of Geosciences, University of Texas at Dallas, 800 W. Campbell Road, Richardson, Texas 75080, USA
Department of Geological Sciences, University of Texas at El Paso, 500 W. University Avenue, El Paso, Texas 79968-0555, USA
3
Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15
Natsushima-cho, Yokosuka, 237-0061, Japan
1
2
ABSTRACT
Mantle xenoliths in ~83 Ma basanites
from south-central Texas provide a rare
opportunity to examine the lithospheric
mantle beneath southern Laurentia. These
peridotites represent lithosphere at the
boundary between Mesoproterozoic continental lithosphere and transitional Gulf
of Mexico passive margin. Here we report
petrographic, mineral, and major element
data for 29 spinel peridotite xenoliths from
Knippa and use these to characterize the
lithospheric mantle beneath south central
Texas. The xenoliths comprise spinel-bearing
lherzolites and harzburgites with coarse,
equigranular textures. Some peridotites contain veins of lizardite. There are no pyroxenites or eclogites. The peridotites contain
olivine (Fo89-92), orthopyroxene (En89-92), clinopyroxene (Wo40-45En45-49Fs3-5), and spinel.
Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzolites (Cr# = 0.14–0.21) and harzburgites
(Cr# = 0.25–0.36). Mineral and major element compositions indicate that the lherzolites are residues after <10% melt extraction
from primitive upper mantle and the harzburgites formed by <15% melt extraction.
Calculated oxygen fugacities indicate equilibration of the harzburgites at –1 to +0.61
and lherzolites at 0 to –2.6 log units with
respect to fayalite-magnetite-quartz (FMQ)
buffer, similar to lightly metasomatized
spinel peridotites elsewhere. The degree of
melt depletion and oxidation of the Knippa
peridotites are consistent with present data
sets for slightly metasomatized lithospheric
mantle and/or backarc samples rather than
forearc settings. Equilibration temperatures
range from 824 to 1058 °C (mean= 916 °C),
calculated at reference pressure of 2.0 GPa.
*[email protected]
Calculated mean seismic velocities Vs =
4.44 km/sec and Vp =7.87 km/sec show no
systematic difference between lherzolites
and harzburgites, and agree with present
geophysical measurements of upper mantle
velocity beneath Texas. The seismic velocities calculated for these samples will provide
important constraints for interpretation of
EarthScope and other geophysical data sets.
INTRODUCTION
Mantle xenoliths entrained in alkaline magmas are an important source of information
about the composition and physical state of
subcontinental mantle. Abundant localities
are found in western North America (Wilshire
et al., 1990). The majority of these samples
are located in the Basin and Range Province
where Mesozoic and Cenozoic tectonic elements have overprinted Precambrian lithospheric formation. Very few sample localities
are found in tectonic provinces that represent
the southern edge of Laurentia. The Knippa
locality, located in central Texas, is one such
example. The locality lies within the Balcones igneous province (Fig. 1) at the nexus of
Mesoproterozoic and transitional lithosphere
of the Gulf coastal plain. The lithosphere was
affected by the Mesoproterozoic accretion and
subsequent Paleozoic tectonism. Characterization of these samples therefore informs us
of a very different set of events and permits
comparison of subcontinental mantle across a
broad region.
This study reports petrographic descriptions,
major-element whole-rock analyses, and mineral compositions for these spinel-peridotite
xenoliths. We characterize the depletion history, thermometry, oxidation state, and seismic
velocities. These results complement a recent
trace-element study from the same locality by
Young and Lee (2009).
GEOLOGIC SETTING
The continental crust of Texas—and much
of southern Laurentia—was generated as part of
the ~1.37 Ga southern granite-rhyolite province
(Fig. 1) (Anthony, 2005; Barnes et al., 2002;
Bickford et al., 2000; Reese et al., 2000; Whitmeyer and Karlstrom, 2007). In the southeastern part of the Llano uplift (Fig. 1), dioritic and
tonalitic gneiss are inferred to represent a 1.33–
1.30 Ga allochthonous magmatic arc (Mosher,
1993; Roback, 1996). This arc is thought to
have accreted to Laurentia during the Grenville
orogeny at ~1.1 Ga due to N-dipping subduction
beneath Laurentia (Mosher, 1998). Young and
Lee (2009) studied trace-element compositions
of Knippa peridotites and found enrichments in
fluid-mobile trace elements (e.g., La) relative to
fluid immobile trace elements (e.g., Nb). They
concluded that these trace-element patterns
were caused by subduction-related fluid metasomatism that modified previously melt-depleted
continental lithosphere. They suggested that the
continental lithospheric mantle represented by
these xenoliths may have been the upper plate
during Mesoproterozoic subduction.
Following Mesoproterozoic subduction and
the Grenville orogeny, the lithosphere of southern
Laurentia was affected by three major tectonic
events during Phanerozoic time—two episodes
of rifting and ocean opening separated by continental collision (Thomas, 2006). The first rifting episode in Early Cambrian time (~530 Ma)
was associated with opening of the Iapetus
Ocean. During or shortly after this, a continental
sliver that ultimately became the Precordillera
of Argentina rifted away (Thomas and Astini,
1996). Associated with early Paleozoic ocean
formation, a passive continental margin formed
and persisted throughout most of Paleozoic time.
Collision of Laurentia with Gondwana during
the final stages of Pangea assembly resulted in
the Ouachita orogeny during Pennsylvanian
Geosphere; June 2011; v. 7; no. 3; p. 710–723; doi:10.1130/GES00618.1; 13 figures; 3 tables; 1 supplemental table file.
710
For permission to copy, contact [email protected]
© 2011 Geological Society of America
Composition of the mantle lithosphere beneath south-central Laurentia
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SAMPLE DESCRIPTION
200 km
Gulf of Mexico
26°
0.5 s
1.0 s
1.5 s
Jurassic oceanic
lithosphere
Figure 1. Location of Knippa mantle xenolith locality in south-central Texas, showing simplified crustal provinces. Knippa peridotite xenoliths are hosted by ~83 Ma basanites that
erupted along the lithospheric discontinuity separating Mesoproterozoic lithosphere of the
Texas craton and the Jurassic transitional lithosphere of the NW Gulf of Mexico passive
margin. The Ouachita orogen approximates the boundary between the North American
craton to the north and west and transitional crust to the east and south. Geophysical studies show that orientation and magnitude of splits correlate to crustal provinces (Gao et al.,
2008). The rapid variation in splitting delay times from Llano uplift to southeastward might
be either due to different degree of alignment of the crystals’ fast axes or to difference in
thickness of the anisotropic layer (Satsukawa et al., 2010). APM—apparent plate motion;
BIP—Balcones Igneous Province.
time (~350 Ma). Late Triassic uplift (~225 Ma;
Dickinson et al., 2010) heralded rifting in what
was then the interior of Pangea. This culminated
in Late Jurassic seafloor spreading to form the
Gulf of Mexico (165–140 Ma; Bird et al., 2005;
Stern et al., 2011). The Gulf of Mexico opening
also established the present passive continental
margin of SE Texas (Fig. 1).
The Balcones Igneous Province (Fig. 1) lies
above the transition that separates Mesoproterozoic crust from the Pennsylvanian Ouachita
orogenic lithosphere. The Balcones Igneous
Province trend also approximates the southern
limit of cratonic North America and its boundary with the southern and western limit of attenuated transitional crust beneath the Texas Gulf
mafic crust of Jurassic age (Mickus et al., 2009);
and (4) thinned crust of southern Laurentia. The
underlying lithospheric mantle may have a similarly complex history.
Knippa peridotite xenoliths are hosted by
basanites of the Balcones Igneous Province,
which is characterized by isolated, monogenetic
igneous centers that formed numerous small
plugs, laccoliths, sills, tuff rings, and lava lakes
(Barker et al., 1987; Spencer, 1969). Balcones
Igneous Province basanites give 40Ar/39Ar ages
of ~81.5–83.5 Ma (Griffin et al., 2010). The
xenoliths are spinel peridotites. The absence
of garnet- and plagioclase-peridotite indicates
derivation from a depth range of 40 to 85 km,
i.e., within the upper part of the subcontinental
lithospheric mantle.
coastal plain, a result of the Jurassic opening of
the Gulf of Mexico (Mickus et al., 2009; Sawyer
et al., 1991). This lithospheric transition was
most recently reactivated by Miocene faulting
(~25–10 Ma) (Galloway et al., 1991).
We do not have a clear picture of the nature
of transitional crust, largely because it is buried
beneath thick sediments of the Gulf of Mexico
coastal plain. This crust could be composed
of one or more of the following components:
(1) metamorphosed sediments and crust of the
Paleozoic passive margin, deformed during the
Ouachita orogeny (Mickus and Keller, 1992);
(2) fragments of Gondwana, left behind when
Gondwana separated from Laurentia in Jurassic
time (Rowley and Pindell, 1989); (3) juvenile
Geosphere, June 2011
Spinel-peridotite xenoliths were collected
from the Vulcan quarry, situated in Knippa,
Uvalde County, Texas (29.278°N, 99.657°W).
Host basanites are dense, black, and fresh.
These lavas are aphyric to sparsely phyric in
hand specimen, and in thin section have scattered phenocrysts (90% olivine and 10% clinopyroxene). The black groundmass contains finely
dispersed clinopyroxene, plagioclase, olivine,
nepheline, titaniferous magnetite, melilite, zeolite, amphibole, phlogopite, and apatite. Mg#
( = Mg/Mg + Fe) of the basanites range from
0.67 to 0.75, have low Ni and Cr abundances,
and show strong, light rare-earth element
(LREE) enrichment, with chondrite-normalized
(La/Yb)N = 19–24 (Griffin et al., 2010).
The ultramafic xenoliths are dispersed in
the basanite, where they comprise 5%–10% of
the total rock volume. They have subspherical
or ellipsoidal forms, with long axes ranging
from 1 to 6 cm. They are usually well rounded,
although some are polygonal. Host rock-xenolith contacts are sharp in most cases, but some
show reaction rims suggesting interaction with
basanite magma.
PETROGRAPHY
Knippa xenoliths are Type 1 peridotites (Frey
and Prinz, 1978). They consist of olivine,
orthopyroxene, clinopyroxene, and the aluminous phase is spinel (Fig. 2). All constituent
phases are homogeneous, without chemical
zoning. Spinels are brown, elongate, and are
mostly in contact with clinopyroxene. Mineral
boundaries vary from straight to gently curved,
and commonly form 120° triple junctions, indicating recrystallization under equilibrium conditions. Olivines are typically fractured, and
some grains display kink banding and undulose
711
Raye et al.
B
A
Opx
Sp
Opx
Cpx
1 mm
Ol
Triple
junction
Opx
Lizardite
Veins
Ol
Ol
Cpx
1 mm
C
Opx
D
Ol
Opx
Ol
Ol
Lizardite
veins
Ol
200 µm
1 mm
(~0.1 mm) than those in lherzolite. Table 1
lists modal analyses of Knippa peridotites,
70% of which are lherzolites, with up to 12%
clinopyroxene. Modes were also calculated
using the method of Lee (2003) (Calculated
Mode, Table 1). Bulk chemical compositions
and mineral compositions were used to determine mineral mass proportions. Calcium oxide,
MgO, FeO, Al2O3, and SiO2 were used for the
inversion. In these calculations, homogeneous
four-phase mineral compositions (i.e., orthopyroxene, clinopyroxene, olivine, and spinel) were
assumed. Accessory phases were not considered.
Mineral mass proportions (Xi) were determined
by matrix inversion via X = (CTC)–1 CTB, where
X was the column matrix consisting of mineral
mass proportions (Xi), C was the mineral composition matrix, CT was the transpose of C, and
B was the bulk composition column matrix. The
calculated modes were also checked by MINSQ
program of Herrmann and Berry (2002), which
produced similar results. Agreement between
visual and calculated modes is excellent.
ANALYTICAL PROCEDURES
E
F
Lizardite
veins
Opx
Ol
Sp
Ol
Ol
Cpx
500 µm
1 mm
Figure 2. Photomicrograph of Knippa peridotites showing (A) elongated spinel (Sp) grain
surrounded by orthopyroxene (Opx) and clinopyroxene (Cpx); (B) olivine (Ol) grains showing triple junctions along with Opx and Cpx; (C) lizardite veins along grain boundaries
and within cracks of olivine grains; (D) backscattered-electron image of thick lizardite
veins within Ol grains and thin veins within Opx grains; (E) zoom-in image of lizardite veins
within Ol grains; (F) X-ray scan image (including Ca, Fe, Si, and Cr elements) showing
grain-size variations between Ol, Opx, Cpx, and Sp, respectively.
extinction. Sparse spinel grains are distributed
around margins of large silicate minerals. Some
clinopyroxene and spinel show reaction rims
along common grain boundaries. None of the
Knippa xenoliths contain accessory amphibole,
phlogopite, or silicate glass. Serpentine veins
(Fig. 3) are dominated by lizardite (Satsukawa
et al., 2010); these are found in some of the
peridotite xenoliths. These veins follow grain
boundaries as well as cracks in olivine grains.
Serpentine-rich veins also contain apatite, pentlandite, and pyrrhotite (Fig. 3), but most olivines
and orthopyroxenes are unaffected away from
the reaction boundaries.
712
Following the International Union of Geological Sciences (IUGS) classification for ultramafic rocks (LeBas and Streckeisen, 1991),
Knippa xenoliths are lherzolites (modal clinopyroxene >5%) and harzburgites (<5% clinopyroxene) as shown in Figure 4. Lherzolites are
more abundant than harzburgites. Lherzolites
are predominantly equigranular (Fig. 2), and
olivine is 1–4 mm, orthopyroxene is 1–2 mm,
clinopyroxene is 0.25–1 mm, and spinel is
~0.15 mm. This variation in mineral grain size
is also shown by thin-section X-ray scan map
(Fig. 2F). Harzburgites are moderately fractured and have clinopyroxenes that are smaller
Geosphere, June 2011
The xenoliths were trimmed to remove all
adhering basanite and cut into two similar
halves. One part was cut into rectangular slabs
0.5 cm thick and sent for thin-section preparation. Visual modes (Table 1) are estimates
from thin-section examination. The other half
was crushed into rock chips to be pulverized.
Considering xenolith grain size, texture, and
homogeneity, 5–10 g of the crushed rock provided a representative whole-rock sample of
each xenolith. Aliquots of the crushed material were ground to a fine powder in agate jars
using a shatter box. Mineral analyses were
carried out on 29 polished thin sections using
the wavelength-dispersive Cameca SX-100
electron microprobe at the University of
Texas at El Paso. The machine was operated
using an accelerating voltage of 15kV, a beam
current of 20 nA, a focused beam diameter of
5–10 µm, and a counting time of 10 s. Natural
standards from the Smithsonian Institute were
used for the analysis of all phases. Analytical
results reported in Tables S1–S4 in the Supplemental Table File1 generally represent the
1
Supplemental Table File. Excel file of four
tables: Table S1: Representative Microprobe Analysis
of Olivine Compositions; Table S2: Representative
Microprobe Analysis of Orthopyroxene Compositions;
Table S3: Representative Microprobe Analysis of Clinopyroxene Compositions; and Table S4: Representative Microprobe Analysis of Spinel Compositions.
If you are viewing the PDF of this paper or reading
it offline, please visit http://dx.doi.org/10.1130/
GES00618.S1 or the full-text article on www.gsapubs
.org to view the supplemental table file.
Composition of the mantle lithosphere beneath south-central Laurentia
Lizardite
2
Apatite
1
2
3
1
Lizardite
3
2
Figure 3. Backscattered-electron image
of accessory phases within lizardite veins.
Common accessory phases are apatite
and pentlandite. Numbers show representative microprobe analyses listed in
the table below.
Pentlandite
1
50 µm
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Pentlandite
Lizardite
3
2
2 Avg(n=5) Std.dev At% 1
1
0.56 Fe 26.94 26.88 24.31
37.73
37.09 37.95
0.00 Ni 40.40 38.85 40.35
0.00
0.00 0.00
0.17 S 33.56 34.89 31.78
0.14
0.10 0.00
0.10
0.06
0.00 0.00
0.49
7.62
7.28 7.41
0.10
0.06
0.01 0.00
0.09
0.24
0.27 0.14
2.17
37.82
39.66 38.39
0.27
0.15
0.00 0.00
0.01
0.01
0.01 0.00
0.00
0.00
0.00 0.00
0.59
83.85
84.42 83.89
Avg Std.dev
26.05 1.50 SiO2
39.87 0.88 TiO2
33.41 1.56 Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
F
Cl
Total
Apatite
1
2
0.46 0.46
0.00 0.00
0.02 0.00
0.00 0.00
0.09 0.06
0.00 0.00
2.66 0.45
53.48 53.66
0.22 0.23
0.00 0.00
38.95 39.46
2.67 3.00
0.40 0.37
98.95 97.69
3
0.46
0.08
0.00
0.12
0.55
0.04
0.44
54.34
0.16
0.02
38.18
2.51
1.26
98.16
Ol
Ol
Dunite
10
20
80
Harzburgite
30
Opx
Pyroxenite
Wehrlite
60
Lherzolite
Cpx
Opx
70
40
50
Peridotite
90
50
Cpx
Hzb
Serp Hzb
Lhz
Serp Lhz
Young and Lee (2009)
Geosphere, June 2011
Figure 4. Modal mineralogy of Knippa
peridotites obtained by visual estimates. Following International Union
of Geological Sciences classification
for peridotites (LeBas and Streckeisen,
1991), Knippa xenolith suite consists
of lherzolites (Lhz; modal Cpx >5%)
and harzburgites (Hzb; <5% Cpx).
Data from Young and Lee (2009) also
shown. Ol—olivine; Opx—orthopyroxene; Cpx—clinopyroxene; Serp—
serpentine.
713
Raye et al.
average of five or more analyses of each grain
and of several grains from different parts
of the same sample; these results are summarized in Table 2. In order to examine the
equilibrium among mineral phases, attention
was paid to evaluating phase homogeneity.
Rock type
Sample
Kn1
Kn3
Kn4
43.70
42.05
42.65
SiO2
TiO2
0.08
0.03
0.02
3.21
3.63
4.06
Al2O3
FeO
7.48
7.86
7.45
MnO
0.12
0.12
0.11
MgO
40.54
42.64
42.29
CaO
2.66
1.70
1.53
0.15
0.11
0.10
Na2O
0.02
0.01
0.01
K2O
0.03
0.02
0.02
P2O5
LOI
0.00
0.00
0.00
Total
97.98
98.16
98.24
VISUAL MODE ESTIMATE
Rock type
Sample
Kn1
Kn3
Kn4
Ol
63
69
60
Opx
25
20
28
Cpx
10
8
9
Sp
2
3
3
CALCULATED MODE ESTIMATE
Rock type
Sample
Kn1
Kn3
Kn4
Ol
61
67
63
Opx
25
21
26
12
8
7
Cpx
Sp
2
4
4
Minerals in Knippa peridotite xenoliths are
homogeneous with little grain-to-grain chemical variation except near contacts with host
basalt where some reaction is evident. All of
the compositions we report were measured far
from these contacts.
Major-element composition of Knippa xenoliths (Table 1) was determined in two separate
labs. Nine samples were sent to Activation Laboratories Ltd., Lancaster, Ontario, and analyzed
by tetraborate fusion–inductively coupled plasma
(ICP) method, while 20 samples were analyzed by
TABLE 1. WHOLE-ROCK COMPOSITION OF KNIPPA PERIDOTITES
Lherzolites
Kn6
43.15
0.03
3.27
7.61
0.12
41.85
2.17
0.13
0.02
0.02
0.00
98.35
Kn7
42.76
0.07
3.24
8.04
0.12
41.88
1.91
0.16
0.04
0.02
0.00
98.24
Kn12
42.40
0.04
2.39
7.73
0.11
44.04
1.81
0.11
0.02
0.02
0.00
98.67
Kn13
42.59
0.07
2.83
8.21
0.12
42.10
2.12
0.13
0.04
0.04
0.00
98.26
K2F3
41.60
0.03
1.49
7.50
0.12
45.47
1.05
0.06
0.01
0.01
2.69
100.02
Kn6
65
23
10
2
Kn7
69
20
9
2
Kn12
76
13
9
2
Kn13
70
20
8
2
Kn6
64
24
10
3
Kn7
66
23
9
2
Kn12
76
15
7
1
Kn13
68
20
9
3
RR1
43.33
0.03
3.83
7.56
0.12
41.13
2.11
0.10
0.04
0.01
0.00
98.26
RR2
44.12
0.05
1.91
8.04
0.12
42.83
1.34
0.11
0.02
0.02
0.00
98.56
K2R3
42.09
0.02
0.93
7.18
0.11
45.91
0.66
0.03
0.01
0.02
1.97
98.93
K2G
45.12
0.02
1.99
5.40
0.12
44.82
1.86
0.09
0.03
0.01
0.75
100.22
K2R5
40.84
0.04
1.55
7.60
0.12
44.71
1.32
0.09
0.01
0.02
3.58
99.87
K2B
42.77
0.04
2.53
7.66
0.12
43.17
2.49
0.17
0.03
0.03
0.87
99.88
Kn31
44.01
0.05
2.34
7.95
0.13
40.83
2.53
0.16
0.01
0.07
0.00
98.07
Kn34
42.76
0.05
1.98
7.93
0.12
42.86
2.04
0.11
0.02
0.02
0.00
97.89
Kn36
44.56
0.05
1.91
7.22
0.11
42.29
1.70
0.08
0.01
0.01
0.00
97.93
K2F3
68
21
10
1
Lherzolites
RR1
RR2
65
70
23
22
10
7
2
1
K2R3
76
15
7
2
K2G
75
12
10
3
K2R5
80
12
7
1
K2B
70
22
6
2
Kn31
66
22
10
2
Kn34
65
25
8
2
Kn36
69
20
9
2
K2F3
68
17
12
3
Lherzolites
RR1
RR2
61
65
26
28
10
6
4
1
K2R3
72
18
9
1
K2G
76
13
10
2
K2R5
83
10
6
1
K2B
68
23
8
1
Kn31
_
_
_
_
Kn34
_
_
_
_
TABLE 1. WHOLE-ROCK COMPOSITION OF KNIPPA PERIDOTITES (continued)
Serpentine lherzolites
Harzburgites
Rock type
Sample
Kn43
Kn44
Kn2
K2D
Kn9
Kn28
Kn29
Kn30
43.57
43.52
42.60
41.50
42.54
44.27
44.59
43.07
SiO2
TiO2
0.04
0.04
0.04
0.03
0.07
0.05
0.01
0.04
2.15
2.45
2.45
1.12
1.27
1.04
1.65
1.10
Al2O3
FeO
7.81
7.83
7.51
7.37
8.59
6.87
7.20
7.47
MnO
0.12
0.12
0.11
0.11
0.13
0.10
0.12
0.11
MgO
42.36
41.21
44.24
44.76
44.79
44.98
42.58
45.47
CaO
1.98
2.56
1.12
0.69
0.64
0.46
1.27
0.72
0.10
0.16
0.07
0.04
0.05
0.04
0.08
0.06
Na2O
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
K2O
0.02
0.04
0.05
0.01
0.03
0.02
0.03
0.02
P2O5
LOI
0.00
0.00
0.00
2.34
0.00
0.00
0.00
0.00
Total
98.16
97.94
98.21
97.98
98.13
97.83
97.53
98.06
VISUAL MODE ESTIMATE
Rock type
Serpentine lherzolites
Harzburgites
Sample
Kn43
Kn44
Kn2
K2D
Kn9
Kn28
Kn29
Kn30
Ol
64
63
73
80
82
80
78
78
Opx
24
23
24
15
15
15
16
18
Cpx
10
12
2
3
2
4
4
3
Sp
2
2
1
2
1
1
2
1
CALCULATED MODE ESTIMATE
Rock type
Serpentine lherzolites
Harzburgites
Sample
Kn43
Kn44
Kn2
K2D
Kn9
Kn28
Kn29
Kn30
Ol
_
_
70
81
_
_
_
_
Opx
_
_
22
15
_
_
_
_
Cpx
_
_
5
3
_
_
_
_
Sp
_
_
4
1
_
_
_
_
Note: Cpx—clinopyroxene; LOI—loss on ignition; OL—olivine; Opx—orthopyroxene; Sp—spinel.
714
Geosphere, June 2011
Kn36
_
_
_
_
(continued)
Serpentine harzburgites
K2C
43.50
0.02
1.87
7.48
0.13
42.68
1.92
0.08
0.01
0.01
2.42
100.12
K2F6
42.76
0.03
1.12
6.69
0.12
46.13
0.75
0.07
0.17
0.02
2.03
99.89
K2R2
43.12
0.04
3.23
7.69
0.12
41.73
2.27
0.14
0.02
0.03
0.00
98.38
K2C
80
16
3
1
Serpentine harzburgites
K2F6
K2R2
80
77
15
15
4
6
1
2
K2C
81
16
2
1
Serpentine harzburgites
K2F6
K2R2
81
79
17
14
3
5
1
1
(continued)
0.33
0.00
0.04
0.05
0.13
0.00
0.02
0.22
0.03
0.01
0.00
0.54
56.09
0.00
2.64
0.33
5.35
0.00
0.01
35.00
0.61
0.04
0.00
100.07
92
0.08
0.01
0.12
0.01
0.06
0.01
0.01
0.24
0.03
0.00
0.00
0.14
55.10
0.01
3.13
0.38
5.68
0.06
0.04
33.79
0.56
0.04
0.00
98.78
91
0.21
0.00
0.03
0.02
0.09
0.00
0.02
0.06
0.01
0.00
0.00
0.40
56.32
0.00
2.66
0.35
5.19
0.00
0.02
34.90
0.62
0.06
0.00
100.15
92
High
St. dev.
Kn2
St. dev.
0.24
0.01
0.13
0.05
0.15
0.00
0.02
0.21
0.03
0.02
0.00
0.39
55.57
0.00
2.93
0.28
5.90
0.00
0.02
34.57
0.51
0.03
0.00
99.83
91
K2F1
St. dev.
Sample
Olivine
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Fo
Rock type
Geosphere, June 2011
56.53
0.03
3.26
0.35
5.68
0.07
0.03
34.33
0.46
0.04
0.00
100.78
91
0.24
0.00
0.16
0.03
0.36
0.00
0.00
0.52
0.12
0.02
0.00
0.40
0.15
0.01
0.07
0.02
0.07
0.03
0.03
0.09
0.02
0.01
0.00
0.16
K2B
Sample
K2G
Orthopyroxene
55.97
SiO2
TiO2
0.00
3.77
Al2O3
Cr2O3
0.12
FeO
7.03
MnO
0.00
NiO
0.00
MgO
32.86
CaO
0.74
0.09
Na2O
0.00
K2O
Total
100.59
Mg#
89
Low
St. dev.
High
St. dev.
K2F2
41.09
0.07
0.00
0.00
9.37
0.00
0.00
49.28
0.05
0.02
0.00
99.89
90
0.18
0.00
0.00
0.01
0.23
0.02
0.04
0.20
0.01
0.01
0.00
0.32
0.30
0.00
0.00
0.00
0.47
0.00
0.00
0.25
0.03
0.00
0.00
0.07
40.75
0.00
0.00
0.00
12.02
0.00
0.00
47.91
0.05
0.00
0.00
100.74
88
41.15
0.00
0.00
0.01
8.62
0.06
0.30
50.45
0.01
0.01
0.00
100.63
91
Lherzolite
K2F2
St. dev.
K2G
55.86
0.15
3.72
0.42
5.78
0.00
0.04
33.17
0.71
0.09
0.00
99.92
91
Low
0.40
0.04
0.14
0.03
0.18
0.00
0.03
0.16
0.04
0.02
0.00
0.35
High
St. dev.
K2C
40.69
0.00
0.00
0.00
9.29
0.00
0.00
49.67
0.06
0.00
0.00
99.72
91
0.16
0.00
0.00
0.00
0.12
0.00
0.04
0.21
0.03
0.00
0.00
0.33
40.78
0.00
0.00
0.00
9.17
0.00
0.32
50.60
0.04
0.00
0.00
100.93
91
Serpentine lherzolite
0.28
0.05
0.00
0.01
0.20
0.00
0.00
0.37
0.01
0.01
0.00
0.81
K2D
St. dev.
55.89
0.11
2.95
0.34
5.89
0.13
0.04
34.32
0.48
0.03
0.00
100.19
91
Low
0.42
0.02
0.28
0.03
0.08
0.03
0.05
0.27
0.04
0.01
0.00
0.50
40.96
0.15
0.00
0.00
0.00
0.00
0.00
0.00
8.52
0.17
0.00
0.00
0.29
0.05
51.09
0.35
0.03
0.01
0.01
0.00
0.00
0.00
100.90
0.39
91
Serpentine harzburgite
Low
High
K2R2
St. dev.
K2F5
St. dev.
0.16
0.01
0.00
0.01
0.25
0.02
0.05
0.19
0.00
0.00
0.00
0.22
40.13
0.00
0.00
0.01
9.07
0.11
0.33
50.00
0.00
0.00
0.00
99.68
91
0.14
0.00
0.00
0.00
0.12
0.00
0.07
0.24
0.01
0.01
0.00
0.14
41.17
0.00
0.00
0.00
8.34
0.00
0.21
51.42
0.03
0.02
0.00
101.22
92
Harzburgite
St. dev.
0.23
0.00
0.00
0.01
0.35
0.00
0.00
0.27
0.06
0.00
0.00
0.42
High
St. dev.
K2F5
St. dev.
K2R2
Low
High
Kn2
St. dev.
Low
High
K2F1
St. dev.
Low
Low
Lherzolite
Major phases in Knippa peridotites are compositionally homogeneous and do not show
significant grain-to-grain chemical variations.
Olivine compositional range is Fo89 to Fo92 with
one exception of Fo88 (Table 2; Fig. 5). The Fo
content of Knippa olivines falls in the range
expected for Proterozoic (Fo92 to Fo93) and especially Phanerozoic (Fo91to Fo92) lithospheric
mantle (Gaul et al., 2000). Harzburgite olivines
are slightly more magnesian (Fo90–92) than lherzolite olivines (Fo88–91).
Orthopyroxenes are enstatite with a composition of Wo1–2 En88–91 Fs8–11 (Table 2; Fig. 5).
Orthopyroxene Mg# ( = 100*Mg/[Mg + Fe2+])
correlates with coexisting olivine Mg# (Fig. 6A).
Lherzolite orthopyroxenes have higher Al2O3
(3%–4%) and lower Cr2O3 (0.1%–0.4%) content than harzburgite orthopyroxenes (2%–3%
Al2O3; 0.2%–0.4% Cr2O3) (Fig. 6B).
Clinopyroxenes are apple-green diopsides,
with a composition of Wo40–45En45–49Fs3–5 (Table 2;
Fig. 5) with Mg# between 90 and 93. Al2O3 and
Cr2O3 in lherzolite clinopyroxene range from
2% to 6% and 0.5% to 1.1%, respectively, while
those in harzburgite clinopyroxene range from
3% to 5% and 0.67% to 1.08%.
Spinel compositions are summarized in
Table 2. Based on Cr# ( = Cr/Cr + Al), there are
two distinct spinel compositions (Fig. 5). Harzburgite spinels have higher Cr# (mean = 0.29)
than lherzolites (mean = 0.17).
Serpentine is lizardite, and thin serpentine veins contain small grains of pentlandite
([Fe,Ni]9S8), apatite (Ca5[PO4]3), and trace
amounts of pyrrhotite (Fe1 –xS). Pentlandite contains 29.88–40.39 atomic% Ni, 24.31–35.18
atomic% Fe, and 31.78–34.89 atomic% S. The
contents of CaO and P2O5 in apatite are 33–34
and 38–39 wt%, respectively, and are fluorapatite
(Ca5PO4)3F) with 2.5–3 wt% fluorine (Fig. 3).
Rock type
Mineral Chemistry
TABLE 2. REPRESENTATIVE MICROPROBE ANALYSIS OF MINERAL COMPOSITIONS
Serpentine lherzolite
Harzburgite
RESULTS
High
K2B
St. dev.
X-ray fluorescence (XRF) spectrometry at Institute for Research on Earth Evolution (IFREE),
Japan Agency for Marine-Earth Science and
Technology (JAMSTEC), using high-dilution
(10:1) fused glass beads. Prior to the fusion, the
whole-rock powders were weighed and ignited
in ceramic crucibles for ≥4 h at 900°C. The
ignited powders were then weighed together with
lithium tetraborate and fused in a platinum-gold
alloy crucible at 1180 °C to produce glass beads,
which were analyzed using ultramafic reference
rock samples at external standards. The XRF
technique was designed to make high accuracy
analyses of major elements in peridotites such as
Si, Mg, Fe, Al, and Ca.
Serpentine harzburgite
Composition of the mantle lithosphere beneath south-central Laurentia
715
716
0.00
0.04
0.38
0.32
0.59
0.00
0.04
0.11
0.00
0.00
0.00
0.24
0.06
0.16
36.06
30.88
15.17
0.00
0.22
16.92
0.01
0.01
0.00
99.48
0.36
0
0.00
0.07
0.11
0.24
0.00
0.00
0.24
0.00
0.00
0.00
0.04
0
0.10
43.50
23.90
12.93
0.17
0.28
17.99
0.00
0.00
0.01
98.88
0.27
0.01
0.02
0.39
0.30
0.17
0.00
0.04
0.11
0.00
0.00
0.01
0.41
High
St. dev.
0.01
0.04
0.22
0.21
0.21
0.00
0.00
0.08
0.01
0.00
0.00
0.15
0.08
0.24
44.01
21.52
15.86
0.00
0.00
17.70
0.01
0.00
0.00
99.43
0.25
Kn2
St. dev.
0.01
0.03
0.54
0.49
0.61
0.00
0.00
0.11
0.01
0.00
0.01
0.35
0.06
0.27
49.30
18.96
11.42
0.00
0.00
19.06
0.01
0.01
0.02
99.10
0.21
K2F2
St. dev.
K2F4
0
0.00
0.38
0.39
0.27
0.03
0.03
0.33
0.01
0.01
0.01
0.49
0
0.07
49.66
20.78
9.49
0.15
0.23
19.17
0.03
0.01
0.01
99.61
0.22
High
St. dev.
K2F1
52.07
0.67
5.72
1.14
2.73
0.00
0.00
15.92
20.18
1.16
0.01
99.59
91
Whole-rock Geochemistry
Whole-rock major-element composition of
Knippa xenoliths (Table 1) agree with those
calculated by Young and Lee (2009) from
mineral modes and compositions. The wholerock contents of Al2O3 and CaO in Knippa
peridotite xenoliths anticorrelate with MgO
content (Fig. 7). For comparison, the composition of primitive (undepleted) upper mantle
(Hart and Zindler, 1986; Jagoutz et al., 1979;
McDonough and Sun, 1995), forearc peridotite (Parkinson and Pearce, 1998; Parkinson
and Arculus, 1999; Pearce et al., 2000) and
backarc basin (BAB) peridotite (Michibayashi
et al., 2009) are also shown. The Knippa
peridotites have high whole-rock MgO and
low CaO and Al2O3 compared to fertile model–
primitive mantle compositions. Negative correlations of Al and Ca oxides with MgO are
consistent with Knippa peridotites representing the residue of previous melt depletion.
Both lherzolites and harzburgites follow meltdepletion trends.
Seismic Velocity
Seismic velocities were calculated at standard temperature and pressure (STP) (25 °C
and 1 atm) using the method described by Lee
(2003) (Table 3). Vp (Primary wave velocities)
for Knippa xenoliths range from 8.24 to 8.31
km/sec and Vs (Secondary wave velocities)
range from 4.77 to 4.84 km/sec. There is no systematic difference in seismic velocities between
Knippa lherzolites and harzburgites.
The velocities calculated by the method of
Lee (2003) correspond to temperatures that
are lower than those in the upper mantle. For
this reason, seismic velocities were also calculated using the method of Hacker and Abers
(2004) and mineral equilibration temperatures
between 900 and 1060 °C. Table 3 shows that
Vp range from 7.80 to 7.97 km/s and Vs range
from 4.34 to 4.52 km/s. No systematic differences exist between the seismic velocities of
lherzolites and harzburgites calculated by this
method.
0
0.00
0.43
0.07
0.64
0.02
0.03
0.07
0.01
0.00
0.01
0.13
0
0.17
64.43
5.08
10.34
0.09
0.30
20.83
0.02
0.00
0.01
101.27
0.05
Sample
Spinel
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Cr#
Low
St. dev.
DISCUSSION
Kn12
0.43
0.23
0.36
0.13
0.09
0.01
0.02
0.32
0.43
0.09
0.01
0.24
0.20
0.01
0.17
0.02
0.05
0.00
0.01
0.15
0.13
0.02
0.00
0.39
50.50
0.66
5.91
0.34
2.86
0.09
0.02
15.59
21.87
0.89
0.02
99.23
90
52.44
0.39
2.97
0.80
2.26
0.03
0.02
16.55
23.09
0.43
0.01
99.44
93
Lherzolite
K2F2
St. dev.
Kn12
0.09
0.06
49.75
17.30
11.96
0.00
0.32
19.69
0.00
0.00
0.00
99.18
0.19
Low
0.01
0.01
0.36
0.51
0.30
0.00
0.02
0.22
0.00
0.00
0.00
0.54
0.28
0.01
0.29
0.12
0.09
0.00
0.02
0.23
0.19
0.06
0.00
0.28
52.49
0.09
3.65
0.68
2.40
0.00
0.01
16.76
22.34
0.86
0.00
99.27
93
Serpentine lherzolite
0.51
0.25
0.29
0.23
0.26
0.00
0.00
0.43
0.91
0.52
0.01
0.43
St. dev.
High
St. dev.
Low
K2D
0.03
0.17
40.02
28.34
12.76
0.00
0.19
18.41
0.00
0.01
0.01
99.99
0.32
53.14
0.17
0.00
0.00
2.94
0.11
0.67
0.08
2.20
0.07
0.00
0.00
0.01
0.02
17.44
0.08
22.43
0.13
0.71
0.02
0.00
0.00
99.55
0.26
93
Serpentine harzburgite
Low
High
K2R2
St. dev.
K2F6
St. dev.
0.04
0.01
0.07
0.06
0.06
0.01
0.02
0.14
0.11
0.02
0.00
0.17
52.17
0.09
3.60
0.85
2.50
0.06
0.05
16.81
21.99
0.82
0.00
98.94
92
0.11
0.02
0.09
0.07
0.10
0.00
0.01
0.08
0.11
0.03
0.00
0.11
0.30
0.05
0.29
0.13
0.06
0.02
0.02
0.19
0.12
0.05
0.01
0.26
51.99
0.49
4.57
0.94
2.83
0.09
0.02
15.76
21.72
1.18
0.01
99.60
91
53.25
0.16
3.37
0.88
2.29
0.00
0.00
16.95
21.33
1.10
0.00
99.62
93
Harzburgite
High
St. dev.
K2F5
St. dev.
St. dev.
K2C
High
Kn2
St. dev.
Low
High
K2F1
St. dev.
Low
High
Kn13
St. dev.
Sample
Clinopyroxene
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
NiO
MgO
CaO
Na2O
K2O
Total
Mg#
Rock type
Rock type
Low
Lherzolite
TABLE 2. REPRESENTATIVE MICROPROBE ANALYSIS OF MINERAL COMPOSITIONS (continued)
Serpentine lherzolite
Harzburgite
K2R2
Low
Serpentine harzburgite
Raye et al.
Geosphere, June 2011
The following section discusses the implications of the compositional data reported above.
We focus on the equilibration temperatures of
Knippa xenoliths, the oxidation state of the
mantle beneath Knippa, and estimates of partial
melting and depletion of the peridotites. Finally,
we compare the calculated seismic velocities
to geophysical seismic-velocity models for this
part of southern Laurentia.
Composition of the mantle lithosphere beneath south-central Laurentia
Knippa xenoliths have equilibrium temperatures of 824–1058 °C for T/BKN, except for
two samples with temperatures of 760 and
1150 °C (Table 3). The T/BKN and T/Wells
geothermometers give similar results that
closely correlate, with r 2 = 0.917 (Fig. 8), but
with somewhat higher minimum temperature of
890 °C. These temperatures denote thinner and
warmer lithosphere than typical cratonic lithosphere but cooler than Phanerozoic lithospheric
mantle as discussed by Young and Lee (2009).
No significant temperature differences exist
between Knippa lherzolites and harzburgites.
These temperature estimates pertain to the
Late Cretaceous time of xenolith entrainment.
The thermal structure of the upper mantle may
have been disturbed by Balcones Province igneous activity, in which case the temperature estimates may be higher than conditions that exist
today. Alternatively, the lithospheric thermal
structure may not have changed significantly
since that time, due to the brevity of the Balcones Igneous Province event and the thermal
inertia of the lithosphere.
Di 50
Hzb
Serp Hzb
Lhz
Serp Lhz
60
70
80
90
100
En
100
92
40
90
80
91
30
70
60
90
89
Fo in olivine
20
Cr# in spinel
88
10
87
0
Figure 5. Composition of primary minerals in Knippa peridotites. Olivine compositional range is generally restricted to
(Fo89–92). Orthopyroxenes are enstatite (En) with a composition of
Wo1–2En88–91Fs8–11. Clinopyroxenes are diopside (Di) with a composition of Wo40–45En45–49Fs3–5. Spinel in harzburgites (Hzb) have higher
Cr# (100*Cr/[Cr + Al]) ranging from 25 to 36 than those in lherzolites
(Lhz) ranging from 15 to 21. Serp—serpentine.
The oxidation state of the upper mantle,
which can be calculated from the equilibrium reaction involving coexisting olivine,
orthopyroxene, and spinel, has been extensively applied in spinel peridotites (Ballhaus
et al., 1991; Mattioli and Wood, 1988; Nell and
94
Figure 6. (A) Mg# ( = 100*Mg/
[Mg + Fe2+]) of orthopyroxene
(Opx) and Mg# of olivine (Ol)
showing a positive correlation
indicating equilibrium between
the mineral phases. Forearc
peridotite data used for comparison are from Parkinson
and Pearce (1998), Parkinson
and Arculus (1999), and Pearce
et al. (2000), and backarc basin
peridotites (BAB) are from
Arai and Ishimaru (2008) and
Ohara et al. (2002). (B) Al2O3
in orthopyroxene decreases
and Mg# increases with melt
depletion. Negative correlation
between these two parameters
probably reflects melt depletion. Hzb—harzburgite; Lhz—
lherzolite; Serp—serpentine.
92
Mg# Ol
Mantle temperatures can be estimated
using thermometric techniques on equilibrated
samples. Equilibrium between mineral phases
is essential for such estimates. Attainment
of equilibrium among mineral phases of the
Knippa xenoliths and absence of disturbance
during serpentinization and after entrainment in
basanite melt are inferred from the correlation
between olivine and orthopyroxene Mg# with
a nearly constant slope (Fig. 6A). In addition,
as described in the sections on petrography and
mineral chemistry, homogeneity among constituent phases is also evident from absence of
chemical zoning and grain-to-grain chemical
variations as well as by the recognition of typical equilibrium textures in thin section.
Several methods have been proposed for estimating the equilibrium temperatures of spinelperidotite mineral assemblages. Four of the
most commonly used geothermometers (with
abbreviations in parentheses) are: (1) the twopyroxene thermometer of Brey and Kohler
(1990) (T/BKN), (2) the Ca-in-orthopyroxene
thermometer of Brey et al. (1990) (T/BK_Ca),
(3) the thermometer of Witt-Eickschen and Seck
(1991) based on Cr-Al partitioning between
orthopyroxene and spinel coexisting with
olivine (T/WS), and (4) the thermometer of
Wells (1977) based on iron solubility in coexisting pyroxenes (T/Wells). Knippa xenolith tem-
peratures were calculated assuming a pressure
of 2.0 GPa. This pressure was chosen because
it falls within the spinel-peridotite stability
field and is commonly used for spinel peridotites, facilitating comparison with other studies.
90
Hzb
Serp Hzb
Lhz
Serp Lhz
Young and Lee (2009)
Forearc peridotite
BAB peridotite
88
86
84 A
4
Al2O3 in Opx
Temperature
Oxygen Fugacity
3
2
1
0
B
86
88
90
92
94
Mg# opx
Geosphere, June 2011
717
Raye et al.
5
Al2O3
5
Hzb
Serp Hzb
Lhz
Serp Lhz
PUM
4
4
Young and Lee (2009)
3
CaO
Forearc peridotite
Batch
Fractional
Fractional
PUM
1.5 GPa
2.0 GPa
2.5 GPa
3
BAB peridotite
2
2
1
1
0
A
35
40
45
50
0
B
35
40
MgO (wt%)
MgO (wt%)
45
50
Figure 7. Major-element compositions of Knippa peridotites. (A) Al2O3 plotted as a function of MgO.
(B) CaO plotted as a function of MgO. Experimental batch and fractional melt extraction curves at
1.5 GPa (dashed and long line) and 2.5 GPa (short line) of Asimow (1999) are plotted, starting from
average Primitive Upper Mantle (PUM) of Jagoutz et al. (1979), McDonough and Sun (1995), and Hart
and Zindler (1986). Knippa harzburgites experienced more melt extraction than the lherzolites. Forearc
peridotite data are from Parkinson and Pearce (1998), Parkinson and Arculus (1999), and Pearce et al.
(2000), and backarc basin (BAB) peridotites are from Michibayashi et al. (2009).
Wood, 1991; O’Neill and Wall, 1987; Wood,
1990; Wood and Virgo, 1989). The equilibrium
reaction is as follows:
6 Fe2SiO4 + O2 = 3Fe2Si2O6 + 2Fe3O4.
Olivine
Orthopyroxene Spinel
The oxygen-fugacity values for Knippa xenoliths were calculated using electron microprobe analyses, which yield total iron content
of spinels. The distribution of Fe2+ and Fe3+ in
spinel was inferred from stoichiometry. The
mantle fugacity values obtained by this method
(Ballhaus, 1993; Mattioli et al., 1989) differ
little from those obtained by direct determinations of Fe3+.
Calculated oxygen-fugacity values (Table 3)
are given relative to the fayalite-magnetitequartz (FMQ) buffer at 2.0 GPa. The oxygen
fugacity relative to FMQ buffer is little affected
by changes in estimated pressure within the
range of the spinel-lherzolite stability field
(Wood, 1990). Changing the pressure to 1.5
GPa causes a shift of oxygen fugacity of <0.001
log units. Oxygen-fugacity values of Knippa
xenoliths fall in a relatively narrow range from
below the FMQ buffer (∆log fO2FMQ = –2.6 to
0.61; Fig. 9). The harzburgites show slightly
higher oxygen fugacity than the lherzolites.
The ∆log fO2FMQ values for lherzolite are 0 to
–2.6; whereas for harzburgite, the values are –1
to +0.61 (Fig. 9). Mean oxygen-fugacity value
and standard deviation (relative to FMQ) for
unserpentinized harzburgite is 0.00 ± 0.64 and
for lherzolite is –0.94 ± 0.70. Serpentinization
did not affect oxygen fugacity: oxygen fugac-
718
ity for unserpentinized harzburgite (0.00 ± 0.64)
is indistinguishable from that of serpentinized
harzburgite (–0.62 ± 0.49). Similarly, serpentinized lherzolite (–1.6 ± 1.45) has an oxygen
fugacity that is indistinguishable from that of
unserpentinized lherzolite (–0.94 ± 0.70).
Comparing Knippa peridotite oxygenfugacity estimates with the oxygen-fugacity
range of global Proterozoic and Phanerozoic
subcontinental lithospheric spinel peridotites
(Ballhaus at al., 1991; Ballhaus, 1993), the
Knippa peridotites plot in the field of lightly
metasomatized peridotites (Fig. 9). Knippa
TABLE 3. TEMPERATURE, OXYGEN FUGACITY, AND SEISMIC VELOCITY
OF SELECTED SAMPLES CALCULATED AT P = 20 KBAR
Sample
T/Wells
T/BKN
Vp (STP)
Vs (STP)
Vp (T)
Vs (T)
no.
(°C)
(°C)
∆logfO2
(km/s)
(km/s)
(km/s)
(km/s)
Kn1
934
889
–
8.25
4.81
7.82
4.42
Kn2
969
950
–
8.30
4.84
7.88
4.46
Kn3
991
987
−0.46
8.30
4.83
7.83
4.42
Kn4
949
937
−0.46
8.29
4.83
7.85
4.44
Kn6
910
858
−2.60
8.27
4.82
7.88
4.45
Kn7
960
949
−
8.29
4.82
7.82
4.41
Kn8
939
939
−1.13
−
−
7.85
4.43
Kn9
968
975
−0.69
8.30
4.83
7.88
4.44
Kn10
856
760
−0.99
−
−
7.92
4.48
Kn12
920
870
−
8.27
4.80
7.87
4.42
Kn13
890
825
−0.57
8.27
4.81
7.91
4.46
RR1
941
909
−2.03
8.28
4.82
7.84
4.43
RR2
947
926
−1.50
8.25
4.82
7.80
4.41
K2B
910
855
−0.44
8.27
4.82
7.88
4.46
K2C
897
860
0.19
8.28
4.82
7.94
4.48
K2D
894
846
0.22
8.30
4.82
7.93
4.47
K2E
917
840
−0.47
−
−
7.86
4.44
K2F1
914
840
−0.58
−
−
7.87
4.45
K2F2
1023
1058
−2.63
−
−
7.72
4.35
K2F3
1068
1151
−0.64
8.31
4.83
7.72
4.34
K2F4
927
860
–
−
−
7.88
4.45
K2F5
948
885
−0.92
−
−
7.92
4.48
K2F6
876
781
−0.05
8.30
4.83
7.99
4.52
K3G
926
876
−0.24
8.24
4.77
7.84
4.39
K2H
926
917
0.57
−
−
7.88
4.45
K2R2
946
891
−0.88
8.29
4.82
7.92
4.46
K2R3
966
967
−2.48
8.28
4.82
7.82
4.40
K2R4
860
770
0.62
−
−
7.97
4.50
K2R5
984
960
−0.74
8.30
4.82
7.88
4.42
Note: STP—standard temperature and pressure; T/BKN—two-pyroxene thermometer of Brey and Kohler
(1990); T/Wells—thermometer of Wells (1977); T(936 °C)—Mean Knippa temperature; Vp—primary wave
velocity; Vs—secondary wave velocity.
Geosphere, June 2011
10%–15% melting, which is consistent with the
estimate based on Cr# versus TiO2 plot. As noted
above, spinel Cr# is a much more sensitive index
of melt extraction than olivine Mg# (Hellebrand
et al., 2001). Hellebrand et al. (2001) calculated
fractional melt percentage as a function of spinel
Cr#, yielding the relationship: F = 10*ln (Cr#)
+24, where F = melt percentage. This relationship is thought to be valid for spinel Cr# between
10 and 60. Based on Hellebrand et al.’s (2001)
model, Knippa lherzolites reflect 5%–9%, and
harzburgites reflect 11%–14% melt extraction
from a primitive mantle source (Fig. 12). These
melt-depletion signatures are less than current
data sets for forearc peridotite (Parkinson and
Pearce, 1998; Parkinson and Arculus, 1999;
Pearce et al., 2000) but quite consistent with
backarc basin peridotite (Arai and Ishimaru,
2008; Ohara et al., 2002).
The whole-rock compositions are consistent
with the results obtained from mineral composition and reflect moderate extraction of partial
melts. As melt depletion increases, whole-rock
compositions show decreasing Al2O3 and CaO
and increasing MgO. With progressive melting, Al2O3 in orthopyroxene decreases (Fig.
6B), olivine Fo content increases, modal olivine
increases, and modal clinopyroxene decreases.
The whole-rock contents of Al2O3 and CaO,
which reflect variations in melt extraction from
primitive (undepleted) upper mantle (PUM;
Hart and Zindler, 1986; Jagoutz et al., 1979;
McDonough and Sun, 1995), are plotted as a
function of MgO (Fig. 7). Experimental batch
and fractional melting curves of Asimow (1999)
are plotted starting from PUM compositions,
1100 y = 0.5291x + 459.97
R2 = 0.91
Mean Wells = 936 °C
Mean BKN = 916 °C
1000
900
Hzb
Serp Hzb
Lhz
Serp Lhz
Young and Lee (2009)
800
peridotites overlap the abyssal and backarc
basin peridotite field (Arai and Ishimaru, 2008)
and have lower oxygen fugacities compared to
forearc peridotites.
Partial Melting and Depletion of the
Knippa Lithospheric Mantle
The Cr# versus TiO2 in spinel (Fig. 10) distinguishes between spinels that experienced
melt-rock interaction and those defining a partial
melting trend (Arai, 1992; Zhou et al., 1996).
Figure 10 shows a partial melting trend, starting from fertile mid-ocean ridge basalt (MORB)
mantle with 0.18% TiO2, and is superimposed on
published experimental results of Johnson et al.
(1990). Details of the method are explained in
Pearce et al. (2000). Ti, being an incompatible
element, should rapidly decrease as the degree
of melting increases and Cr# increases. The
trend for Knippa peridotites toward higher Ti,
especially for harzburgites, therefore implies
melt-mantle interaction through reaction or melt
impregnation by a Ti-rich melt (Edwards and
Malpas, 1996; Kelemen et al., 1995). This reaction would have followed depletion of the host
harzburgite as origin of the Ti-rich trend point to
the most Cr#-rich melting trend of the peridotite.
No significant difference is seen between serpentinized and unserpentinized peridotites, again
confirming that the serpentinization is probably
a late-stage, low-temperature phenomenon.
Spinel Cr# correlates with Fo content of
coexisting olivines and Cr contents of coexisting
orthopyroxene and clinopyroxene. These compositional variations are interpreted to have been
caused by extraction of a basaltic partial melt and
have been observed in xenolith suites worldwide
900
1000
1100
T/Wells 1977(°C)
(Bonatti and Michael, 1989; Frey et al., 1985;
Frey and Prinz, 1978; Hauri and Hart, 1994).
Knippa xenoliths plot within the olivine-spinel
mantle array (OSMA; Fig. 11), interpreted as a
mantle-peridotite restite trend (Arai, 1994). With
greater extents of partial melting, olivine Fo
increases slightly, but the Cr# of spinel increases
greatly, defining the olivine-spinel mantle array.
Peridotites from different tectonic settings
occupy distinct parts of the array. Melting curves
from Pearce et al. (2000) based on experimental studies of Jaques and Green (1980) show
that Knippa lherzolites were produced by <10%
melting, whereas harzburgites were produced by
Figure 9. Oxygen fugacity
of Knippa peridotites. Oxygen fugacities fall near the
fayalite-magnetite-quartz buffer (∆log fO2FMQ = –2.6–0.61).
Knippa lherzolites and harzburgites plot in the field of
lightly metasomatized spinel
peridotites (Ballhaus, 1993).
In contrast, forearc peridotites have distinctly higher Cr#
and oxygen fugacity. Forearc
peridotite data are from Parkinson and Pearce (1998),
Parkinson and Arculus (1999),
and Pearce et al. (2000), and
backarc basin (BAB) peridotite
data are from Arai and Ishimaru (2008). The less oxidized
forearc samples are from, for
example, Pali Aike. See text for
further discussion.
Geosphere, June 2011
2
Strongly
metasomatized
1
0
–1
–2
–3
Primitive
800
log f O2 (FMQ)
Figure 8. Equilibration temperature for Knippa peridotites.
Temperature was calculated at
2 GPa using the two-pyroxene
thermometer (T/BKN) of Brey
and Kohler (1990) and the
thermometer of Wells (1977)
(T/Wells). Knippa xenoliths
have equilibrium temperatures
of 824–1058 °C (T/BKN). The
two approaches agree with r 2 =
0.91. There are no significant temperature differences
between lherzolites (Lhz) and
harzburgites (Hzb) or between
serpentinized versus fresh peridotites, suggesting that these
lithologies are mixed in the
0litho sphere beneath Texas.
Serp—serpentine.
T/ BKN (°C)
Composition of the mantle lithosphere beneath south-central Laurentia
0.1
Lightly
metasomatized
0.3
Abyssal
spinel
peridotite
0.5
Cr # in spinel
Hzb
Serp Hzb
Lhz
Serp Lhz
Young and Lee (2009)
Forearc peridotite
BAB peridotite
0.7
0.9
719
Raye et al.
1.0
Hzb
Serp Hzb
Lhz
Serp Lhz
Young and Lee (2009)
FMM = Fertile
Partial
melting
0.6
%
Cr# in spinel
Cr# in spinel
0.8
1
MORB Mantle
20%
0.4
0.5
15%
al
Parti g
in
melt
5%
Melt-rock
reaction
FMM
0.2
0.4
0.6
0.8
95
1.0
Figure 10. Chromium is compatible in spinel and increases with melt
extraction, while titanium is incompatible and decreases with
melt extraction. Increase of Ti content in the Cr# versus TiO2 wt%
of spinel is thus indicative of an early melt-extraction event followed
by subsequent melt-rock reaction. The harzburgites appear to have
undergone more melt-rock reaction than the lherzolites.
Seismic-Velocity Structure beneath Texas
Calculated velocities were compared with
geophysical measurements of seismic velocity
beneath the region. Rayleigh wave-dispersion
experiments carried out by Keller and Shurbet
(1975) using different stations in Texas (Corpus Christi, Edinburg, Laredo, San Marcos, and
Houston, Fig. 13A) show that crustal structure
is generally similar along all profiles extending
from the Llano uplift southeastward to the Gulf
of Mexico. A generalized crustal structure model
proposed by Keller and Shurbet (1975) is shown
in Fig. 13B. Based on Rayleigh wave-dispersion
data, the upper layers (Vp ≤5.2 km/s) are interpreted as Mesozoic and Cenozoic sedimentary
rocks, the upper crustal layer (Vp >5.2 km/s)
is interpreted to consist primarily of Paleozoic
metamorphic rocks, and the lower crustal layer
(Vp ≤6.9 km/s) is interpreted to comprise mafic
85
90
Mg# of olivine
80
Figure 11. Knippa peridotite compositions
plotted on the olivine-spinel mantle array
(OSMA) of Arai (1994). OSMA is a mantleperidotite restite trend. Peridotites from
different tectonic settings plot in different parts of this trend. Knippa peridotites
plot within the continental field. Melting
curve (dashed line with % melting) from
Pearce et al. (2000) based on experimental
studies of Jaques and Green (1980) indicate that the lherzolites were produced by
<10% melting, whereas the harzburgites
were produced by 10%–15% melting. All
forearc peridotite data are from Parkinson
and Pearce (1998), Parkinson and Arculus
(1999), and Pearce et al. (2000), and backarc basin (BAB) peridotite data are from
Arai and Ishimaru (2008) and Ohara et al.
(2002). FMM—fertile MORB mantle;
MORB—mid-ocean ridge basalt; Serp—
serpentine.
0.9
Figure 12. Melt depletion of
Knippa peridotites, based on
Cr# of spinel. Chromium being
0.7
compatible in spinel, increases
23%
in the residue with increasing melt extraction. Hellebrand
0.5
et al. (2001) calculated fractional
Hzb
11%
melt percentage as a function
Serp Hzb
of spinel Cr#, yielding the relaLhz
0.3
5%
Serp Lhz
tionship F = 10*ln (Cr#) +24,
Young and Lee (2009)
16%
where F = melt fraction. This
Forearc peridotite
BAB peridotite
relationship is valid for spinel
0.1
30
20
0
10
Cr#s between 0.1 and 0.6.
Knippa lherzolites (Lhz) are
F (% melt fraction)
formed by <10% melt extraction and harzburgites (Hzb) by ~15% melt extraction. All forearc peridotite data are from
Parkinson and Pearce (1998), Parkinson and Arculus (1999), and Pearce et al. (2000), and
backarc basin (BAB) peridotite data are from Arai and Ishimaru (2008) and Ohara et al.
(2002). Serp—serpentine.
Cr# in spinel
which again indicate that lherzolites reflect less
melt depletion than do harzburgites. Note that
the harzburgite samples with TiO2-rich spinel
that follow the melt-rock reaction trend in Figure 10 plot around the highest MgO-end of the
whole-rock trend (Fig. 7), implying that melt
depletion formed the harzburgite first, and melt
impregnation occurred later. Effect of melt reaction to the depleted harzburgites shown by Cr#
versus TiO2 in Figure 10 is not identifiable in
other plots, suggesting negligible effect on the
estimates of melt extraction based on major
and compatible elements. However, the effect
should not be underestimated for incompatible elements such as Ti or rare-earth elements
(REEs). According to the tectonic model shown
in figure 7 of Mosher (1998), the mantle beneath
Knippa would have underlain a forearc during
late Mesoproterozoic time. However, Knippa
peridotites have major-element and mineral
compositions that are different from forearc
peridotites and are consistent with metasomatized continental lithosphere (Young and Lee,
2009) or backarc peridotites (Arai and Ishimaru,
2008). However, neither melt depletion nor oxidation state is specific to tectonic setting. Isotopic and trace-element data (in progress) may
help constrain these models.
Continental
Peridotite
FMM
0
TiO2 wt% in spinel
720
cry Frac
sta tio
lli nal
za
tio
n
10%
10%
0.2
0.0
40
+
++ +
+ +
30%
+ ++
++
+
++++
+ +
+
++
20%
+++
+
+
+
+
+
Hzb
Serp Hzb
Lhz
Serp. Lhz
Young and
Lee (2009)
+ Forearc peridotite
BAB peridotite
FMM = Fertile
MORB Mantle
Geosphere, June 2011
Composition of the mantle lithosphere beneath south-central Laurentia
N
D HOU
5
Depth (km)
Knippa
SAM
4
C
H
3
LAR
Tran
Upper
crust
20
Sediments
sitio
nal
litho
sph
Lower
Crust
30
ere
MOHO
40
Upper mantle
t
A
Knippa
Llano uplift
10
ac
hit
a
Ou
Llano
uplift
Syst
em
0
as
1 COR Co
f
l
Gu
B
200
100
400
300
Distance (km)
500
2EDN
T1
T2
T3
C
D
H
0
Sediments
was less than 10% for lherzolite and 10%–15%
for harzburgite. This melt-depleted mantle subsequently underwent melt-mantle interaction
through reaction or melt impregnation by a
Ti-rich melt. No significant difference between
serpentinized and unserpentinized peridotites
confirmed that the serpentinization was a latestage, low-temperature phenomenon. The combination of these characteristics suggests that
this mantle was either a slight metasomatized
continental lithosphere and/or backarc basin
rather than a forearc setting.
Calculated seismic-velocity data are consistent with geophysical profiles from the Llano
uplift southeastward to the coastline of the Gulf
of Mexico. Our velocity calculations will be
useful to constrain improved geophysical models generated by Earthscope’s new data set.
ACKNOWLEDGMENTS
Depth (km)
10
Upper
Crust
We thank Carol Frost, Tim Lawton, Cin-ty Lee,
and Melanie Barnes for their reviews. W.R. Griffin has
been a great help with providing samples and information on the Balcones Igneous Province. This work
is supported by Texas Norman Hackerman Advanced
Research Program grant 003661-0003-2006 to Elizabeth Y. Anthony and Robert J. Stern.
20
Lower
Crust
30
40
Upper
mantle
7.8
7.8
MOHO
8.2
8.41
8.0
C
Figure 13. (A) Index map showing Knippa (star), seismograph stations: LAR—Laredo,
COR—Corpus Christi, EDN—Edinburg, SAM—San Marcos, HOU—Houston (Keller and
Shurbet, 1975), C—Cram (1961, 1962), D—Dorman et al. (1972), H—Hales et al. (1970)
refraction line and gravity profiles 1, 2, 3, and 4. T1 (stations 2, 4, and 5), T2 (stations 1, 2,
and 3), and T3 (stations 1, 3, and 4) are tripartite locations. (B) Generalized crustal structure model proposed by Keller and Shurbet (1975). (C) Crustal structure as interpreted
from seismic velocities. Our calculated upper mantle seismic velocity agrees well with those
velocities obtained from geophysical experiments.
igneous rocks (Fig. 13C). Gravity and refraction
data suggest that the lower crustal layer dips
to the northwest and that thick Paleozoic sedimentary and metasedimentary rocks are present
seaward of the buried Ouachita belt. Seismic
velocities for the crust do not indicate a crustal
granitic layer seaward of the buried Ouachita
belt, but they do indicate that the lower crust is
composed of oceanic crust thickened presumably by thrusting and other deformation.
Our seismic velocities calculated at appropriate temperatures for Knippa peridotites
(7.8–8.3 km/sec) shown in Figure 13C are
consistent with the geophysical measurements
(7.8–8.4 km/sec) shown in Figure 4C (Keller and
Shurbet, 1975). This velocity structure obtained
from gravity data and calculated seismic data
will be refined as data from the Earthscope trans-
portable array investigations of mantle beneath
Texas are interpreted. The velocities reported in
this study will serve as important constraints on
models generated by the Earthscope experiment.
CONCLUSIONS
Peridotite xenoliths in Cretaceous basanites
from the Knippa quarry of south-central Texas
are among a few mantle samples from the
southern margin of Laurentia and thus provide
constraints on the nature of this lithospheric
mantle. Based on our petrographic, mineralchemical, and whole-rock chemical studies, we
estimate that equilibration temperatures at the
time of entrainment were 824–1058 °C and that
∆log fO2FMQ values are 0 to –2.6 for lherzolite
and –1 to +0.61 for harzburgite. Melt depletion
Geosphere, June 2011
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olivine-spinel compositional relationships: Review and
interpretation: Chemical Geology, v. 113, p. 191–204,
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