composition of the mantle lithosphere beneath south …rjstern/pdfs/rayegeosphere11.pdf · fs 3-5),...

710

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 con-tinental 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 harz burgites with coarse, equigranular textures. Some perido tites con-tain veins of lizardite . There are no pyrox-enites or eclogites . The peridotites contain olivine (Fo89-92), orthopyroxene (En89-92), clino-pyroxene (Wo40-45En45-49Fs3-5), and spinel . Spinel Cr# (Cr/(Cr+Al)) distinguishes lherzo-lites (Cr# = 0.14–0.21) and harz burgites (Cr# = 0.25–0.36). Mineral and major ele-ment compositions indicate that the lherzo-lites are residues after <10% melt extraction from primitive upper mantle and the harz-burgites formed by <15% melt extraction. Calculated oxygen fugacities indicate equili-bration 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, simi lar to lightly metasomatized spinel perido tites 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.

Calculated mean seismic velocities Vs = 4.44 km/sec and Vp =7.87 km/sec show no systematic difference between lherzolites and harz burgites, and agree with present geophysical measure ments of upper mantle velocity beneath Texas. The seismic veloci-ties calculated for these samples will provide important constraints for interpretation of EarthScope and other geophysical data sets.

INTRODUCTION

Mantle xenoliths entrained in alkaline mag-mas 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 ele-ments have overprinted Precambrian litho-spheric 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 Bal-cones 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. Character-ization 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 min-eral compositions for these spinel-peridotite xenoliths. We characterize the depletion his-tory, 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; Whit-meyer and Karlstrom, 2007). In the southeast-ern 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 meta-somatism 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 con-tinental collision (Thomas, 2006). The first rift-ing 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

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Geosphere; June 2011; v. 7; no. 3; p. 710–723; doi:10.1130/GES00618.1; 13 figures; 3 tables; 1 supplemental table file.

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

1Department of Geosciences, University of Texas at Dallas, 800 W. Campbell Road, Richardson, Texas 75080, USA2Department of Geological Sciences, University of Texas at El Paso, 500 W. University Avenue, El Paso, Texas 79968-0555, USA3Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan

*[email protected]

Making the Southern Margin of Laurentia themed issue

Composition of the mantle lithosphere beneath south-central Laurentia

Geosphere, June 2011 711

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 Meso protero-zoic crust from the Pennsylvanian Ouachita orogenic lithosphere. The Balcones Igneous Province trend also approximates the southern limit of cratonic North America and its bound-ary with the southern and western limit of atten-uated transitional crust beneath the Texas Gulf

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

mafic crust of Jurassic age (Mickus et al., 2009); and (4) thinned crust of southern Laurentia. The underlying lithospheric mantle may have a simi-larly 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.

SAMPLE DESCRIPTION

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 speci men, and in thin section have scat-tered pheno crysts (90% olivine and 10% clino-pyroxene). The black groundmass contains finely dispersed clinopyroxene, plagioclase, olivine, nepheline, titaniferous magnetite, melilite, zeo-lite, 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-xeno-lith 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 alumi-nous 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, indi-cating recrystallization under equilibrium con-ditions. Olivines are typically fractured, and some grains display kink banding and undulose

BIP

Knippa

Mesoproterozoic craton (1.4 Ga)

Gulf of Mexico Jurassic oceanic lithosphere

APM

Oklahoma Arkansas

Texas

Louisiana

102°W 100° 98° 96° 94° 92°

26°

28°

30°

32°

34°

36°N

Transitio

nal crustGrenville

Front (approx.)

~1.1 Ga deformation

Splitting time

0.5 s 1.0 s 1.5 s

Mexico

S. OK aulacogen

Oua

chita

Orogen

Llano uplift

N

200 km

Figure 1. Location of Knippa mantle xenolith locality in south-central Texas, showing sim-plified 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 stud-ies 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.

Raye et al.

712 Geosphere, June 2011

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, pent-landite, and pyrrhotite (Fig. 3), but most olivines and orthopyroxenes are unaffected away from the reaction boundaries.

Following the International Union of Geo-logical Sciences (IUGS) classification for ultra-mafic rocks (LeBas and Streckeisen, 1991), Knippa xenoliths are lherzolites (modal clino-pyroxene >5%) and harzburgites (<5% clinopy-roxene) 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 frac-tured and have clinopyroxenes that are smaller

(~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 deter-mine 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., orthopy-roxene, 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 com-position 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

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 prepa-ration. 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 pro-vided a representative whole-rock sample of each xenolith. Aliquots of the crushed mate-rial 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 Sup-plemental Table File1 generally represent the

Ol

Sp

CpxOl

Opx

Opx

A B

Triplejunction OpxOpx

Cpx

Opx Ol

Ol

Lizardite

Ol LizarditeVeins

C D

Ol veinsOl

Lizarditeveins Ol

Opx

Sp

E F

OlOl Cpx

1 mm1 mm

1 mm200 µm

1 mm500 µm

Figure 2. Photomicrograph of Knippa peridotites showing (A) elongated spinel (Sp) grain surrounded by orthopyroxene (Opx) and clinopyroxene (Cpx); (B) olivine (Ol) grains show-ing 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.

1Supplemental Table File. Excel file of four tables : Table S1: Representative Microprobe Analysis of Olivine Compositions; Table S2: Representative Microprobe Analysis of Orthopyroxene Composi tions; Table S3: Representative Microprobe Analy sis of Clino-pyroxene Compositions; and Table S4: Representa-tive 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.



Lizardite

Apatite

Pentlandite

Lizardite

50 µm

2

1

2

13

12

3

Lizardite Pentlandite Apatite1 2 Avg(n=5) Std.dev At% 1 2 3 1 2 3

SiO2 37.09 37.95 37.73 0.56 Fe 26.94 26.88 24.31 26.05 1.50 SiO2 0.46 0.46 0.46

TiO2 0.00 0.00 0.00 0.00 Ni 40.40 38.85 40.35 39.87 0.88 TiO2 0.00 0.00 0.08

Al2O3 0.10 0.00 0.14 0.17 S 33.56 34.89 31.78 33.41 1.56 Al2O3 0.02 0.00 0.00

Cr2O3 0.00 0.00 0.06 0.10 Cr2O3 0.00 0.00 0.12FeO 7.28 7.41 7.62 0.49 FeO 0.09 0.06 0.55MnO 0.01 0.00 0.06 0.10 MnO 0.00 0.00 0.04NiO 0.27 0.14 0.24 0.09 MgO 2.66 0.45 0.44MgO 39.66 38.39 37.82 2.17 CaO 53.48 53.66 54.34CaO 0.00 0.00 0.15 0.27 Na2O 0.22 0.23 0.16

Na2O 0.01 0.00 0.01 0.01 K2O 0.00 0.00 0.02

K2O 0.00 0.00 0.00 0.00 P2O5 38.95 39.46 38.18Total 84.42 83.89 83.85 0.59 F 2.67 3.00 2.51

Cl 0.40 0.37 1.26Total 98.95 97.69 98.16

Avg Std.dev

50

60

70

80

9010

20

30

40

50

Ol

Opx Cpx

Dunite

Harzburgite Wehrlite

LherzoliteHzbSerp HzbLhzSerp LhzYoung and Lee

Ol

CpxOpx

Peridotite

Pyroxenite

(2009)

Figure 3. Backscattered-electron image of accessory phases within lizardite veins. Common accessory phases are apatite and pentlandite. Numbers show repre-sentative microprobe analyses listed in the table below.

Figure 4. Modal mineralogy of Knippa peridotites obtained by visual esti-mates. 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—orthopy-roxene; Cpx—clinopyroxene; Serp—serpentine.

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 sum-marized in Table 2. In order to examine the equilibrium among mineral phases, attention was paid to evaluating phase homogeneity.

Minerals in Knippa peridotite xenoliths are homogeneous with little grain-to-grain chem-ical 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 xeno-liths (Table 1) was determined in two separate labs. Nine samples were sent to Activation Lab-oratories 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

Rock type Lherzolites

Sample Kn1 Kn3 Kn4 Kn6 Kn7 Kn12 Kn13 K2F3 RR1 RR2 K2R3 K2G K2R5 K2B Kn31 Kn34 Kn36SiO2 43.70 42.05 42.65 43.15 42.76 42.40 42.59 41.60 43.33 44.12 42.09 45.12 40.84 42.77 44.01 42.76 44.56TiO2 0.08 0.03 0.02 0.03 0.07 0.04 0.07 0.03 0.03 0.05 0.02 0.02 0.04 0.04 0.05 0.05 0.05Al2O3 3.21 3.63 4.06 3.27 3.24 2.39 2.83 1.49 3.83 1.91 0.93 1.99 1.55 2.53 2.34 1.98 1.91FeO 7.48 7.86 7.45 7.61 8.04 7.73 8.21 7.50 7.56 8.04 7.18 5.40 7.60 7.66 7.95 7.93 7.22MnO 0.12 0.12 0.11 0.12 0.12 0.11 0.12 0.12 0.12 0.12 0.11 0.12 0.12 0.12 0.13 0.12 0.11MgO 40.54 42.64 42.29 41.85 41.88 44.04 42.10 45.47 41.13 42.83 45.91 44.82 44.71 43.17 40.83 42.86 42.29CaO 2.66 1.70 1.53 2.17 1.91 1.81 2.12 1.05 2.11 1.34 0.66 1.86 1.32 2.49 2.53 2.04 1.70Na2O 0.15 0.11 0.10 0.13 0.16 0.11 0.13 0.06 0.10 0.11 0.03 0.09 0.09 0.17 0.16 0.11 0.08K2O 0.02 0.01 0.01 0.02 0.04 0.02 0.04 0.01 0.04 0.02 0.01 0.03 0.01 0.03 0.01 0.02 0.01P2O5 0.03 0.02 0.02 0.02 0.02 0.02 0.04 0.01 0.01 0.02 0.02 0.01 0.02 0.03 0.07 0.02 0.01LOI 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.69 0.00 0.00 1.97 0.75 3.58 0.87 0.00 0.00 0.00Total 97.98 98.16 98.24 98.35 98.24 98.67 98.26 100.02 98.26 98.56 98.93 100.22 99.87 99.88 98.07 97.89 97.93VISUAL MODE ESTIMATERock type LherzolitesSample Kn1 Kn3 Kn4 Kn6 Kn7 Kn12 Kn13 K2F3 RR1 RR2 K2R3 K2G K2R5 K2B Kn31 Kn34 Kn36Ol 63 69 60 65 69 76 70 68 65 70 76 75 80 70 66 65 69Opx 25 20 28 23 20 13 20 21 23 22 15 12 12 22 22 25 20Cpx 10 8 9 10 9 9 8 10 10 7 7 10 7 6 10 8 9Sp 2 3 3 2 2 2 2 1 2 1 2 3 1 2 2 2 2CALCULATED MODE ESTIMATERock type LherzolitesSample Kn1 Kn3 Kn4 Kn6 Kn7 Kn12 Kn13 K2F3 RR1 RR2 K2R3 K2G K2R5 K2B Kn31 Kn34 Kn36Ol 61 67 63 64 66 76 68 68 61 65 72 76 83 68 _ _ _Opx 25 21 26 24 23 15 20 17 26 28 18 13 10 23 _ _ _Cpx 12 8 7 10 9 7 9 12 10 6 9 10 6 8 _ _ _Sp 2 4 4 3 2 1 3 3 4 1 1 2 1 1 _ _ _

(continued)

TABLE 1. WHOLE-ROCK COMPOSITION OF KNIPPA PERIDOTITES (continued)

Rock type Serpentine lherzolites Harzburgites Serpentine harzburgites

Sample Kn43 Kn44 Kn2 K2D Kn9 Kn28 Kn29 Kn30 K2C K2F6 K2R2SiO2 43.57 43.52 42.60 41.50 42.54 44.27 44.59 43.07 43.50 42.76 43.12TiO2 0.04 0.04 0.04 0.03 0.07 0.05 0.01 0.04 0.02 0.03 0.04Al2O3 2.15 2.45 2.45 1.12 1.27 1.04 1.65 1.10 1.87 1.12 3.23FeO 7.81 7.83 7.51 7.37 8.59 6.87 7.20 7.47 7.48 6.69 7.69MnO 0.12 0.12 0.11 0.11 0.13 0.10 0.12 0.11 0.13 0.12 0.12MgO 42.36 41.21 44.24 44.76 44.79 44.98 42.58 45.47 42.68 46.13 41.73CaO 1.98 2.56 1.12 0.69 0.64 0.46 1.27 0.72 1.92 0.75 2.27Na2O 0.10 0.16 0.07 0.04 0.05 0.04 0.08 0.06 0.08 0.07 0.14K2O 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.17 0.02P2O5 0.02 0.04 0.05 0.01 0.03 0.02 0.03 0.02 0.01 0.02 0.03LOI 0.00 0.00 0.00 2.34 0.00 0.00 0.00 0.00 2.42 2.03 0.00Total 98.16 97.94 98.21 97.98 98.13 97.83 97.53 98.06 100.12 99.89 98.38VISUAL MODE ESTIMATERock type Serpentine lherzolites Harzburgites Serpentine harzburgitesSample Kn43 Kn44 Kn2 K2D Kn9 Kn28 Kn29 Kn30 K2C K2F6 K2R2Ol 64 63 73 80 82 80 78 78 80 80 77Opx 24 23 24 15 15 15 16 18 16 15 15Cpx 10 12 2 3 2 4 4 3 3 4 6Sp 2 2 1 2 1 1 2 1 1 1 2CALCULATED MODE ESTIMATERock type Serpentine lherzolites Harzburgites Serpentine harzburgitesSample Kn43 Kn44 Kn2 K2D Kn9 Kn28 Kn29 Kn30 K2C K2F6 K2R2Ol _ _ 70 81 _ _ _ _ 81 79 81Opx _ _ 22 15 _ _ _ _ 16 17 14Cpx _ _ 5 3 _ _ _ _ 2 3 5Sp _ _ 4 1 _ _ _ _ 1 1 1

Note: Cpx—clinopyroxene; LOI—loss on ignition; OL—olivine; Opx—orthopyroxene; Sp—spinel.



X-ray fluorescence (XRF) spectrometry at Insti-tute 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.

RESULTS

Mineral Chemistry

Major phases in Knippa peridotites are com-positionally 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 espe-cially Phanerozoic (Fo91to Fo92) lithospheric mantle (Gaul et al., 2000). Harzburgite olivines are slightly more magnesian (Fo90–92) than lher-zolite olivines (Fo88–91).

Orthopyroxenes are enstatite with a compo-sition 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%) con-tent 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). Harz-burgite spinels have higher Cr# (mean = 0.29) than lherzolites (mean = 0.17).

Serpentine is lizardite, and thin serpen-tine veins contain small grains of pentlandite ([Fe,Ni]9S8), apatite (Ca5[PO4]3), and trace amounts of pyrrhotite (Fe1 –xS). Pentlandite con-tains 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).

TAB

LE 2

. RE

PR

ES

EN

TATI

VE

MIC

RO

PR

OB

E A

NA

LYS

IS O

F M

INE

RA

L C

OM

PO

SIT

ION

SR

ock

type

Lher

zolit

eS

erpe

ntin

e lh

erzo

lite

Har

zbur

gite

Ser

pent

ine

harz

burg

ite

Low

Hig

hLo

wH

igh

Low

Hig

hLo

wH

igh

Sam

ple

K2G

St.

dev.

K2B

St.

dev.

K2F

2S

t. de

v.K

2F1

St.

dev.

K2D

St.

dev.

Kn2

St.

dev.

K2R

2S

t. de

v.K

2F5

St.

dev.

Oliv

ine

SiO

240

.75

0.30

41.1

50.

1841

.09

0.28

40.7

80.

1640

.69

0.23

41.1

70.

1440

.13

0.16

40.9

60.

15Ti

O2

0.00

0.00

0.00

0.00

0.07

0.05

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.00

Al 2O

30.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00C

r 2O

30.

000.

000.

010.

010.

000.

010.

000.

000.

000.

010.

000.

000.

010.

010.

000.

00Fe

O12

.02

0.47

8.62

0.23

9.37

0.20

9.17

0.12

9.29

0.35

8.34

0.12

9.07

0.25

8.52

0.17

MnO

0.00

0.00

0.06

0.02

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.11

0.02

0.00

0.00

NiO

0.00

0.00

0.30

0.04

0.00

0.00

0.32

0.04

0.00

0.00

0.21

0.07

0.33

0.05

0.29

0.05

MgO

47.9

10.

2550

.45

0.20

49.2

80.

3750

.60

0.21

49.6

70.

2751

.42

0.24

50.0

00.

1951

.09

0.35

CaO

0.05

0.03

0.01

0.01

0.05

0.01

0.04

0.03

0.06

0.06

0.03

0.01

0.00

0.00

0.03

0.01

Na 2

O0.

000.

000.

010.

010.

020.

010.

000.

000.

000.

000.

020.

010.

000.

000.

010.

00K

2O0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

000.

00To

tal

100.

740.

0710

0.63

0.32

99.8

90.

8110

0.93

0.33

99.7

20.

4210

1.22

0.14

99.6

80.

2210

0.90

0.39

Fo88

9190

9191

9291

91R

ock

type

Lher

zolit

eS

erpe

ntin

e lh

erzo

lite

Har

zbur

gite

Ser

pent

ine

harz

burg

iteLo

wH

igh

Low

Hig

hLo

wH

igh

Low

Hig

hS

ampl

eK

2GS

t. de

v.K

2BS

t. de

v.K

2F2

St.

dev.

K2F

1S

t. de

v.K

2CS

t. de

v.K

n2S

t. de

v.K

2R2

St.

dev.

K2F

5S

t. de

v.O

rthop

yrox

ene

SiO

255

.97

0.24

56.5

30.

1555

.86

0.40

55.5

70.

2455

.89

0.42

56.3

20.

2155

.10

0.08

56.0

90.

33Ti

O2

0.00

0.00

0.03

0.01

0.15

0.04

0.00

0.01

0.11

0.02

0.00

0.00

0.01

0.01

0.00

0.00

Al 2O

33.

770.

163.

260.

073.

720.

142.

930.

132.

950.

282.

660.

033.

130.

122.

640.

04C

r 2O

30.

120.

030.

350.

020.

420.

030.

280.

050.

340.

030.

350.

020.

380.

010.

330.

05Fe

O7.

030.

365.

680.

075.

780.

185.

900.

155.

890.

085.

190.

095.

680.

065.

350.

13M

nO0.

000.

000.

070.

030.

000.

000.

000.

000.

130.

030.

000.

000.

060.

010.

000.

00N

iO0.

000.

000.

030.

030.

040.

030.

020.

020.

040.

050.

020.

020.

040.

010.

010.

02M

gO32

.86

0.52

34.3

30.

0933

.17

0.16

34.5

70.

2134

.32

0.27

34.9

00.

0633

.79

0.24

35.0

00.

22C

aO0.

740.

120.

460.

020.

710.

040.

510.

030.

480.

040.

620.

010.

560.

030.

610.

03N

a 2O

0.09

0.02

0.04

0.01

0.09

0.02

0.03

0.02

0.03

0.01

0.06

0.00

0.04

0.00

0.04

0.01

K2O

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Tota

l10

0.59

0.40

100.

780.

1699

.92

0.35

99.8

30.

3910

0.19

0.50

100.

150.

4098

.78

0.14

100.

070.

54M

g#89

9191

9191

9291

92(c

ontin

ued

)

Raye et al.


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 whole-rock contents of Al2O3 and CaO in Knippa peridotite xenoliths anticorrelate with MgO content (Fig. 7). For comparison, the compo-sition of primitive (undepleted) upper mantle (Hart and Zindler, 1986; Jagoutz et al., 1979; McDonough and Sun, 1995), forearc perido-tite (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 perido tites have high whole-rock MgO and low CaO and Al2O3 compared to fertile model–primitive mantle compositions. Negative cor-relations of Al and Ca oxides with MgO are consistent with Knippa peridotites represent-ing the residue of previous melt depletion. Both lherzolites and harzburgites follow melt-depletion trends.

Seismic Velocity

Seismic velocities were calculated at stan-dard 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 sys-tematic 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 cal-culated 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 differ-ences exist between the seismic velocities of lherzolites and harzburgites calculated by this method.

DISCUSSION

The following section discusses the implica-tions 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.

TAB

LE 2

. RE

PR

ES

EN

TATI

VE

MIC

RO

PR

OB

E A

NA

LYS

IS O

F M

INE

RA

L C

OM

PO

SIT

ION

S (c

ontin

ued

)

Roc

k ty

peLh

erzo

lite

Ser

pent

ine

lher

zolit

eH

arzb

urgi

teS

erpe

ntin

e ha

rzbu

rgite

Low

Hig

hLo

wH

igh

Low

Hig

hLo

wH

igh

Sam

ple

Kn1

2S

t. de

v.K

n13

St.

dev.

K2F

2S

t. de

v.K

2F1

St.

dev.

K2C

St.

dev.

Kn2

St.

dev.

K2R

2S

t. de

v.K

2F5

St.

dev.

Clin

opyr

oxen

eS

iO2

50.5

00.

2052

.44

0.43

52.0

70.

5152

.49

0.28

51.9

90.

3053

.25

0.11

52.1

70.

0453

.14

0.17

TiO

20.

660.

010.

390.

230.

670.

250.

090.

010.

490.

050.

160.

020.

090.

010.

000.

00A

l 2O3

5.91

0.17

2.97

0.36

5.72

0.29

3.65

0.29

4.57

0.29

3.37

0.09

3.60

0.07

2.94

0.11

Cr 2

O3

0.34

0.02

0.80

0.13

1.14

0.23

0.68

0.12

0.94

0.13

0.88

0.07

0.85

0.06

0.67

0.08

FeO

2.86

0.05

2.26

0.09

2.73

0.26

2.40

0.09

2.83

0.06

2.29

0.10

2.50

0.06

2.20

0.07

MnO

0.09

0.00

0.03

0.01

0.00

0.00

0.00

0.00

0.09

0.02

0.00

0.00

0.06

0.01

0.00

0.00

NiO

0.02

0.01

0.02

0.02

0.00

0.00

0.01

0.02

0.02

0.02

0.00

0.01

0.05

0.02

0.01

0.02

MgO

15.5

90.

1516

.55

0.32

15.9

20.

4316

.76

0.23

15.7

60.

1916

.95

0.08

16.8

10.

1417

.44

0.08

CaO

21.8

70.

1323

.09

0.43

20.1

80.

9122

.34

0.19

21.7

20.

1221

.33

0.11

21.9

90.

1122

.43

0.13

Na 2

O0.

890.

020.

430.

091.

160.

520.

860.

061.

180.

051.

100.

030.

820.

020.

710.

02K

2O0.

020.

000.

010.

010.

010.

010.

000.

000.

010.

010.

000.

000.

000.

000.

000.

00To

tal

99.2

30.

3999

.44

0.24

99.5

90.

4399

.27

0.28

99.6

00.

2699

.62

0.11

98.9

40.

1799

.55

0.26

Mg#

9093

9193

9193

9293

Roc

k ty

peLh

erzo

lite

Ser

pent

ine

lher

zolit

eH

arzb

urgi

teS

erpe

ntin

e ha

rzbu

rgite

Low

Hig

hLo

wH

igh

Low

Hig

hLo

wH

igh

Sam

ple

Kn1

2S

t. de

v.K

2F4

St.

dev.

K2F

1S

t. de

v.K

2F2

St.

dev.

K2D

St.

dev.

Kn2

St.

dev.

K2R

2S

t. de

v.K

2F6

St.

dev.

Spi

nel

SiO

20

00

00.

090.

010.

060.

010.

080.

010.

030.

010

00.

060.

00Ti

O2

0.17

0.00

0.07

0.00

0.06

0.01

0.27

0.03

0.24

0.04

0.17

0.02

0.10

0.00

0.16

0.04

Al 2O

364

.43

0.43

49.6

60.

3849

.75

0.36

49.3

00.

5444

.01

0.22

40.0

20.

3943

.50

0.07

36.0

60.

38C

r 2O

35.

080.

0720

.78

0.39

17.3

00.

5118

.96

0.49

21.5

20.

2128

.34

0.30

23.9

00.

1130

.88

0.32

FeO

10.3

40.

649.

490.

2711

.96

0.30

11.4

20.

6115

.86

0.21

12.7

60.

1712

.93

0.24

15.1

70.

59M

nO0.

090.

020.

150.

030.

000.

000.

000.

000.

000.

000.

000.

000.

170.

000.

000.

00N

iO0.

300.

030.

230.

030.

320.

020.

000.

000.

000.

000.

190.

040.

280.

000.

220.

04M

gO20

.83

0.07

19.1

70.

3319

.69

0.22

19.0

60.

1117

.70

0.08

18.4

10.

1117

.99

0.24

16.9

20.

11C

aO0.

020.

010.

030.

010.

000.

000.

010.

010.

010.

010.

000.

000.

000.

000.

010.

00N

a 2O

0.00

0.00

0.01

0.01

0.00

0.00

0.01

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.01

0.00

K2O

0.01

0.01

0.01

0.01

0.00

0.00

0.02

0.01

0.00

0.00

0.01

0.01

0.01

0.00

0.00

0.00

Tota

l10

1.27

0.13

99.6

10.

4999

.18

0.54

99.1

00.

3599

.43

0.15

99.9

90.

4198

.88

0.04

99.4

80.

24C

r#0.

050.

220.

190.

210.

250.

320.

270.

36



Temperature

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 con-stituent phases is also evident from absence of chemical zoning and grain-to-grain chemical variations as well as by the recognition of typi-cal equilibrium textures in thin section.

Several methods have been proposed for esti-mat ing the equilibrium temperatures of spinel-peridotite mineral assemblages. Four of the most commonly used geothermometers (with abbreviations in parentheses) are: (1) the two-pyroxene 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 co exist-ing 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 perido-tites, facilitating comparison with other studies .

Knippa xenoliths have equilibrium tempera-tures 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 geo thermometers give similar results that closely correlate, with r2 = 0.917 (Fig. 8), but with somewhat higher minimum temperature of 890 °C. These temperatures denote thinner and warmer lithosphere than typical cratonic litho-sphere 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 igne-ous activity, in which case the temperature esti-mates 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 Bal-cones Igneous Province event and the thermal inertia of the lithosphere.

Oxygen Fugacity

The oxidation state of the upper mantle, which can be calculated from the equilib-rium reaction involving coexisting olivine, orthopyroxene, and spinel, has been exten-sively applied in spinel peridotites (Ballhaus et al., 1991; Mattioli and Wood, 1988; Nell and

HzbSerp HzbLhzSerp Lhz

En

50

60

70

80

90

100

Di

90 80 70100 60

40 0102030Cr# in spinel

Fo in olivine92 91 90 89 88 87

Figure 5. Composition of primary minerals in Knippa perido-tites. 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 composi-tion 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.

84

86

88

90

92

94

Mg#

Ol

86 88 90 92 940

1

2

3

4

Mg# opx

Al 2O

3 in

Opx

HzbSerp HzbLhzSerp LhzYoung and Lee (2009)Forearc peridotiteBAB peridotiteA

B

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 com-parison 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 deple-tion. Hzb—harzburgite; Lhz—lherzolite; Serp—serpentine.

Raye et al.


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 xeno-liths were calculated using electron micro-probe 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 determina-tions of Fe3+.

Calculated oxygen-fugacity values (Table 3) are given relative to the fayalite-magnetite-quartz (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-

ity for unserpentinized harzburgite (0.00 ± 0.64) is indistinguishable from that of serpentinized harz burgite (–0.62 ± 0.49). Similarly, serpen-tinized lherzolite (–1.6 ± 1.45) has an oxygen fugacity that is indistinguishable from that of unserpentinized lherzolite (–0.94 ± 0.70).

Comparing Knippa peridotite oxygen-fugacity 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

0

1

2

3

4

5

0

1

2

3

4

5Hzb

Serp Hzb

Lhz

Serp Lhz

Al2O3 CaOPUM

PUM

Young and Lee (2009)

MgO (wt%) MgO (wt%)

Batch 1.5 GPa

FractionalFractional

2.0 GPa2.5 GPa

35 40 45 50 35 40 45 50A B

BAB peridotite

Forearc peridotite

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).

TABLE 3. TEMPERATURE, OXYGEN FUGACITY, AND SEISMIC VELOCITY OF SELECTED SAMPLES CALCULATED AT P = 20 KBAR

Sampleno.

T/Wells(°C)

T/BKN(°C) !logfO2

Vp (STP)(km/s)

Vs (STP)(km/s)

Vp (T)(km/s)

Vs (T)(km/s)

Kn1 934 889 – 8.25 4.81 7.82 4.42Kn2 969 950 – 8.30 4.84 7.88 4.46Kn3 991 987 !0.46 8.30 4.83 7.83 4.42Kn4 949 937 !0.46 8.29 4.83 7.85 4.44Kn6 910 858 !2.60 8.27 4.82 7.88 4.45Kn7 960 949 ! 8.29 4.82 7.82 4.41Kn8 939 939 !1.13 ! ! 7.85 4.43Kn9 968 975 !0.69 8.30 4.83 7.88 4.44Kn10 856 760 !0.99 ! ! 7.92 4.48Kn12 920 870 ! 8.27 4.80 7.87 4.42Kn13 890 825 !0.57 8.27 4.81 7.91 4.46RR1 941 909 !2.03 8.28 4.82 7.84 4.43RR2 947 926 !1.50 8.25 4.82 7.80 4.41K2B 910 855 !0.44 8.27 4.82 7.88 4.46K2C 897 860 0.19 8.28 4.82 7.94 4.48K2D 894 846 0.22 8.30 4.82 7.93 4.47K2E 917 840 !0.47 ! ! 7.86 4.44K2F1 914 840 !0.58 ! ! 7.87 4.45K2F2 1023 1058 !2.63 ! ! 7.72 4.35K2F3 1068 1151 !0.64 8.31 4.83 7.72 4.34K2F4 927 860 – ! ! 7.88 4.45K2F5 948 885 !0.92 ! ! 7.92 4.48K2F6 876 781 !0.05 8.30 4.83 7.99 4.52K3G 926 876 !0.24 8.24 4.77 7.84 4.39K2H 926 917 0.57 ! ! 7.88 4.45K2R2 946 891 !0.88 8.29 4.82 7.92 4.46K2R3 966 967 !2.48 8.28 4.82 7.82 4.40K2R4 860 770 0.62 ! ! 7.97 4.50K2R5 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.



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) dis-tinguishes 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, start-ing 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 reac-tion 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 serpen-tinized and unserpentinized peridotites, again confirming that the serpentinization is probably a late-stage, low-temperature phenomenon.

Spinel Cr# correlates with Fo content of co exist ing olivines and Cr contents of coexisting orthopyroxene and clinopyroxene. These com-positional variations are interpreted to have been caused by extraction of a basaltic partial melt and have been observed in xenolith suites worldwide

(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 experimen-tal studies of Jaques and Green (1980) show that Knippa lherzolites were produced by <10% melting, whereas harzburgites were produced by

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 relation-ship 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 compo-sition and reflect moderate extraction of partial melts. As melt depletion increases, whole-rock compositions show decreasing Al2O3 and CaO and increasing MgO. With progressive melt-ing, 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,

0.1 0.3 0.5 0.7 0.9–3

–2

–1

0

1

2

)Q

MF(

2Of

gol

Cr # in spinel

HzbSerp HzbLhz

Stronglymetasomatized

Lightlymetasomatized

Prim

itive

peridotite

Serp Lhz

BAB peridotiteForearcYoung and Lee (2009)

Abyssalspinelperidotite

Figure 9. Oxygen fugacity of Knippa peridotites. Oxy-gen fugacities fall near the fayalite-magnetite-quartz buf-fer ("log fO2FMQ = –2.6–0.61). Knippa lherzolites and harz-burgites plot in the field of lightly metasomatized spinel peridotites (Ballhaus, 1993). In contrast, forearc perido-tites have distinctly higher Cr# and oxygen fugacity. Forearc perido tite data are from Par-kinson and Pearce (1998), Parkinson and Arculus (1999), and Pearce et al. (2000), and backarc basin (BAB) peridotite data are from Arai and Ishi-maru (2008). The less oxidized forearc samples are from, for example, Pali Aike. See text for further discussion.

800 900 1000 1100800

900

1000

1100

T B

KN

(°C

)

y = 0.5291x + 459.97R2 = 0.91


Mean Wells = 936 °CMean BKN = 916 °C

T/Wells 1977(°C)

/


Figure 8. Equilibration temper-ature 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 r2 = 0.91. There are no signifi-cant temperature differences be tween lherzolites (Lhz) and harz burgites (Hzb) or between serpentinized versus fresh perido-tites, suggesting that these lithologies are mixed in the 0litho sphere beneath Texas. Serp—serpentine.

Raye et al.


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 Fig-ure 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 reac-tion 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 incompat-ible 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 metasoma-tized continental lithosphere (Young and Lee, 2009) or backarc peridotites (Arai and Ishimaru, 2008). However, neither melt depletion nor oxi-dation state is specific to tectonic setting. Iso-topic and trace-element data (in progress) may help constrain these models.

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 (Cor-pus 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 inter-preted 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

0.0

0.2

0.4

0.6

0.8

1.0

TiO2 wt% in spinel

0.8 1.00.2 0.4 0.6

Cr#

in s

pin

el

FMM

5%

10%

15%

20%

Partialmelting


MORB Mantle

Melt-rockreaction

Young and Lee (2009)FMM = Fertile

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.

1

+

1

40%Serp HzbLhz

++ +

+30%

Forearc peridotite+BAB peridotite

pine

l

++ +++

++

+++

++++

+0.5 20% FMM = Fertile

MORB Mantle

Cr#

in s

++++ +++

++10%

++

0

ContinentalPeridotite

FMM095 90 85 80

Mg# of olivine


Hzb

Serp. Lhz

crystallization

FractionalPartial

melting

Figure 11. Knippa peridotite compositions plotted on the olivine-spinel mantle array (OSMA) of Arai (1994). OSMA is a mantle-peridotite restite trend. Peridotites from different tectonic settings plot in differ-ent 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) indi-cate 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 back-arc 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.

Figure 12. Melt depletion of Knippa peridotites, based on Cr# of spinel. Chromium being compatible in spinel, increases in the residue with increas-ing melt extraction. Hellebrand et al. (2001) calculated fractional melt percentage as a function of spinel Cr#, yielding the rela-tionship F = 10*ln (Cr#) +24, where F = melt fraction. This relationship is valid for spinel Cr#s between 0.1 and 0.6. Knippa lherzolites (Lhz) are formed by <10% melt extrac-tion 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.

0.1

0.3

0.5

0.7

0.9

F (% melt fraction)

Cr#

in s

pine

l


Forearc peridotite

23%

16%

5%

11%

BAB peridotite


0 10 20 30



igneous rocks (Fig. 13C). Gravity and refraction data suggest that the lower crustal layer dips to the northwest and that thick Paleozoic sedi-mentary 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 presum-ably by thrusting and other deformation.

Our seismic velocities calculated at appro-priate 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, mineral-chemical, 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

was less than 10% for lherzolite and 10%–15% for harzburgite. This melt-depleted mantle sub-sequently 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 late-stage, low-temperature phenomenon. The com-bination 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 consis-tent 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 mod-els generated by Earthscope’s new data set.

ACKNOWLEDGMENTS

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 infor-mation on the Balcones Igneous Province. This work is supported by Texas Norman Hackerman Advanced Research Program grant 003661-0003-2006 to Eliza-beth Y. Anthony and Robert J. Stern.

REFERENCES CITED

Anthony, E.Y., 2005, Source regions of granites and their links to tectonic environment: Examples from the western United States: Lithos, v. 80, p. 61–74, doi: 10.1016/j.lithos.2004.04.058.

Arai, S., 1992, Chemistry of chromian spinel in volcanic rocks as a potential guide to magma chemistry: Lon-don, Mineralogical Magazine, v. 56, p. 173–184.

Arai, S., 1994, Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation: Chemical Geology, v. 113, p. 191–204, doi: 10.1016/0009-2541(94)90066-3.

Arai, S., and Ishimaru, S., 2008, Insights into petrologi-cal characteristics of the lithosphere of mantle wedge beneath arcs through peridotite xenoliths: A review: Journal of Petrology, v. 49, p. 665–695, doi: 10.1093/petrology/egm069.

Asimow, P.D., 1999, A model that reconciles major- and trace-element data from abyssal peridotites: Earth and Planetary Science Letters, v. 169, p. 303–319, doi: 10.1016/S0012-821X(99)00084-9.

Ballhaus, C., 1993, Redox states of lithospheric and astheno-spheric upper mantle: Contributions to Mineralogy and Petrology, v. 114, p. 331–348, doi: 10.1007/BF01046536.

Ballhaus, C., Berry, R.G., and Green, D.H., 1991, High-pressure experimental calibration of the olivine-ortho-pyroxene-spinel oxygen geobarometer: Implications for the oxygen state of the upper mantle: Contributions to Mineralogy and Petrology, v. 107, p. 27–40, doi: 10.1007/BF00311183.

Barker, D.S., Mitchell, R.H., and McKay, D., 1987, Late Cretaceous nephelinite to phonolite magmatism in the Balcones province, Texas, in Morris, E.M., and Pasteris , J.D., eds., Mantle Metasomatism and Alkaline Magmatism: Geological Society of America Special Paper, v. 215, p. 293–304.

Barnes, M.A., Anthony, E.Y., Williams, I., and Asquith, G.B., 2002, Architecture of a 1.38–1.34 Ga granite-rhyolite complex as revealed by geochronology and isotopic and elemental geochemistry of subsurface samples from west Texas, USA: Precambrian Research, v. 119, p. 9–43, doi: 10.1016/S0301-9268(02)00116-X.

Bickford, M.E., Soegaard, K., Nielsen, K.C., and McLelland , J.M., 2000, Geology and geochronology of Grenville-

Llano uplift0

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20

30

40

100 200 300 400 500

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10

20

30

40

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th (k

m)

T1 T2 T3 C D H

B

C

N

Oua

chita

Syst

em

HOUSAM

COR

LAR

EDN

GulfCoast

UpperCrust

LowerCrust

D

HC

MOHO

A

Knippa

Distance (km)D

epth

(km

)

Uppermantle

54

1

2

3

Uppercrust

LowerCrust

Sedi-ments

Sediments

Upper mantle

Transitional lithosphere

MOHO

Llanouplift

Knippa

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 struc-ture 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.

Raye et al.


age rocks in the Van Horn and Franklin Mountains area, west Texas: Implications for the tectonic evolu-tion of Laurentia during the Grenville: Geological Society of America Bulletin, v. 112, p. 1134–1148, doi: 10.1130/0016-7606(2000)112<1134:GAGOGR>2.0.CO;2.

Bird, D.E., Bruke, K., Hall, S.A., and Casey, J.F., 2005, Gulf of Mexico tectonic history: Hotspot traces crustal boundaries and early salt distribution: American Asso-ciation of Petroleum Geologists Bulletin, v. 89, p. 311–328, doi: 10.1306/10280404026.

Bonatti, E., and Michael, P.J., 1989, Mantle peridotites from continental rifts to ocean basins to subduction zones: Earth and Planetary Science Letters, v. 91, p. 297–311, doi: 10.1016/0012-821X(89)90005-8.

Brey, G.P., and Kohler, T., 1990, Geothermobarometry in four-phase lherzolites: II. New thermobarometers, and practical assessment of existing thermobarometers: Journal of Petrology, v. 31, p. 1353–1378.

Brey, G.P., Köhler, T., and Nickel, K.G., 1990, Geothermo-barometry in four-phase lherzolites: I. Experimental results from 10 to 60 kb: Journal of Petrology, v. 31, p. 1313–1352.

Cram, I.H., Jr., 1961, A crustal refraction survey in South Texas: Geophysics, v. 26, p. 560–573.

Cram, I.H., Jr., 1962, Crustal structure of Texas coastal plain region: Am. Assoc. Petroleum Geologists Bull., v. 46, p. 1721–1727.

Dickinson, W.R., Gehrels, G.E., and Stern, R.J., 2010, Late Triassic Texas uplift preceding Jurassic opening of the Gulf of Mexico: Evidence from U-Pb ages of detrital zircons: Geosphere, v. 6, no. 5, p. 641–662, doi: 10.1130/GES00532.1.

Dorman, J., Worzel, J.L., Leyden, R., Crook, T.N., and Hatziemmanuel , M., 1972, Crustal section from seis-mic regraction measurements near Victoria, Texas: Geophysics, v. 37, p. 325–336.

Edwards, S.J., and Malpas, J., 1996, Melt-peridotite inter-actions in shallow mantle at the East Pacific Rise: Evidence from ODP site 895 (Hess Deep): London, Mineralogical Magazine, v. 60, no. 1, p. 191–206.

Frey, F.A., and Prinz, M., 1978, Ultramafic inclusions from San Carlos, Arizona: Petrology and geochemical data bearing on their petrogenesis: Earth and Planetary Science Letters, v. 38, p. 129–176, doi: 10.1016/ 0012-821X(78)90130-9.

Frey, F.A., Suen, C.J., and Stockman, H., 1985, The Ronda high temperature peridotite: Geochemistry and petro-genesis: Geochimica et Cosmochimica Acta, v. 49, p. 2469–2491, doi: 10.1016/0016-7037(85)90247-9.

Galloway, W.E., Bedout, D.G., and Fisher, W.L., Jr., Cabrera-Castro, R., Lugo-Rivera, J. E., and Scott, T.M., 1991, Cenozoic, in Salvador, A., ed., The Gulf of Mexico Basin: Geological Society of America, Geol-ogy of North America, v. J, p. 245–324.

Gao, S.S., Liu, K.H., Stern, R.J., Keller, R.G., Hogan, J.P., Pulliam, J., and Anthony, E.Y., 2008, Characteristics of mantle fabric beneath the south-central United States: Constraints from shear-wave splitting measurements: Geosphere, v. 4, p. 411–417.

Gaul, O.F., Griffin, W.L., O’Reilly, S.Y., and Pearson, N.J., 2000, Mapping olivine composition in the lithospheric mantle: Earth and Planetary Science Letters, v. 182, p. 223–235, doi: 10.1016/S0012-821X(00)00243-0.

Griffin, W.R., Foland, K.A., Stern, R.J., and Leybourne, M.I., 2010, Geochronology of bimodal alkaline vol-canism in the Balcones Igneous Province, Texas: Impli-cations for Cretaceous intraplate magmatism in the northern Gulf of Mexico Magmatic Zone: The Journal of Geology, v. 118, p. 1–21, doi: 10.1086/648532.

Hacker, B.R., and Abers, G.A., 2004, Subduction Factory 3: An Excel worksheet and macro for calculating the densities, seismic wave speeds, and H2O contents of minerals and rocks at pressure and temperature: Geochemistry, Geophysics, Geosystems, v. 5, p. 1–7. doi:10.1029/2003GC000614.

Hales, A.L., Helsley, C.E., and Nation, J.B., 1970, Crustal structure study of the Gulf Coast of Texas: Am. Assoc. Petroleum Geologists Bull., v. 54, p. 2040–2057.

Hart, S.R., and Zindler, A., 1986, In search of a bulk Earth composition: Chemical Geology, v. 57, p. 247–267, doi: 10.1016/0009-2541(86)90053-7.

Hauri, E.H., and Hart, S.R., 1994, Constraints on melt migration from mantle plumes: A trace element study of peridotite xenoliths from Savai’i, Western Samoa: Journal of Geophysical Research, v. 99, p. 24,301–24,321, doi: 10.1029/94JB01553.

Hellebrand, E., Snow, J.E., Dick, H.J.B., and Hoffmann, A.W., 2001, Coupled major and trace elements as indicators of the extent of melting in mid-ocean-ridge peridotites: Nature, v. 410, p. 677–681, doi: 10.1038/35070546.

Herrmann, W., and Berry, R.F., 2002, MINSQ—A least-squares spreadsheet method for calculating mineral proportions from whole-rock major-element analyses: Geochemistry: Exploration, Environment, Analysis, v. 2, no. 4, p. 361–368, doi: 10.1144/1467-787302-010.

Jagoutz, E., Palme, H., Baddenhausen, H., Blum, K., Cendales , M., Dreibus, G., Spettel, B., Lorenz, V., and Wanke, H., 1979, The abundances of major, minor, and trace elements in the Earth’s mantle as derived from primitive ultramafic nodules: Houston, Proceed-ings of the 10th Lunar and Planetary Science Confer-ence, Pergamon, p. 2031–2050.

Jaques, A.L., and Green, D.H., 1980, Anhydrous melt-ing of peridotite at 0–15 kb pressure and the genesis of tholeiitic basalts: Contributions to Mineralogy and Petrology, v. 73, p. 287–310, doi: 10.1007/BF00381447.

Johnson, K.T.M., Dick, H.J.B., and Shimizu, N., 1990, Melting in the oceanic upper mantle: An ion micro-probe study of diopsides in abyssal peridotites: Journal of Geophysical Research, v. 95, p. 2661–2678, doi: 10.1029/JB095iB03p02661.

Kelemen, P.B., Shimizu, N., and Salters, V.J.M., 1995, Extraction of mid-ocean ridge basalt from the upwell-ing mantle by focused flow of melt in dunite channels: Nature, v. 375, p. 747–753, doi: 10.1038/375747a0.

Keller, G.R., and Shurbet, D.H., 1975, Crustal struc-ture of the Texas Gulf Coastal Plain: Geological Society of America Bulletin, v. 86, p. 807–810, doi: 10.1130/0016-7606(1975)86<807:CSOTTG> 2.0.CO;2.

LeBas, M.J., and Streckeisen, A.L., 1991, The IUGS sys-tematics of igneous rocks: Journal of the Geological Society of London, v. 148, p. 825–833, doi: 10.1144/gsjgs.148.5.0825.

Lee, C.-T.A., 2003, Compositional variation of density and seismic velocities in natural peridotites at STP condi-tions: Implications for seismic imaging of composi-tional heterogeneities in the upper mantle: Journal of Geophysical Research, v. 108, p. 1–20.

Mattioli, G., and Wood, B., 1988, Magnetite activities across the MgAl2O4-Fe3O4 spinel join, with application to thermobarometric estimates of upper mantle oxygen fugacity: Contributions to Mineralogy and Petrology, v. 98, p. 148–162, doi: 10.1007/BF00402108.

Mattioli, G., Baker, M., Rutter, M., and Stolper, E., 1989, Upper mantle oxygen fugacity and its relationship to metasomatism: The Journal of Geology, v. 97, p. 521–536, doi: 10.1086/629332.

McDonough, W., and Sun, S., 1995, The composition of the Earth: Chemical Geology, v. 120, p. 223–253, doi: 10.1016/0009-2541(94)00140-4.

Michibayashi, K., Ohara, Y., Stern, R.J., Fryer, P., Kimura, J.-I., Tasaka, M., Harigane, Y., and Ishi, T., 2009, Peridotites from a ductile shear zone within back-arc lithospheric mantle, southern Mariana Trench: Results of a Shinkai 6500 dive: Geochemistry Geo physics Geosystems, v. 10, Q05X06, 17 p., doi: 10.1029/ 2008GC002197.

Mickus, K., Stern, R., Keller, G.R., and Anthony, E., 2009, Potential field evidence for a volcanic rifted margin along the Texas Gulf Coast: Geology, v. 37, p. 387–390, doi: 10.1130/G25465A.1.

Mickus, K., and Keller, G.R., 1992, Lithospheric structure of the south-central U.S: Geology, v. 20, p. 335–338, doi: 10.1130/0091-7613(1992)020<0335:LSOTSC> 2.3.CO;2.

Mosher, S., 1993, Western extensions of Grenville age rocks: Texas, in Reed, J.C., Jr., Bickford, M.E., Houston, R.S., Link, P.K., Rankin, D.W., Sims, P.K., and Van Schmus, W.R., eds., Precambrian: Conterminous U.S: Boulder, Colorado, Geological Society of America, The Geol-ogy of North America, v. C-2, p. 365–378.

Mosher, S., 1998, Tectonic evolution of the Southern Lau-rentian Grenville orogenic belt: Geological Soci-ety of America Bulletin, v. 110, p. 1357–1375, doi: 10.1130/0016-7606(1998)110<1357:TEOTSL> 2.3.CO;2.

Nell, J., and Wood, B., 1991, High-temperature electri-cal measurements and thermodynamic properties of Fe3O4-FeCr2O4-MgCr2O4-MgAl2O4-FeAl2O4 spinels: The American Mineralogist, v. 74, p. 339–351.

Ohara, H., Stern, R.J., Ishi, T., Yurimoto, H., and Yamazaki, T., 2002, Peridotites from the Mariana Trough: First look at the mantle beneath an active back-arc basin: Contributions to Mineralogy and Petrology, v. 143, p. 1–18, doi: 10.1007/s00410-001-0329-2.

O’Neill, H., and Wall, V., 1987, The olivine-spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the upper mantle: Journal of Petrol-ogy, v. 28, p. 1169–1192.

Parkinson, I.J., and Arculus, R.J., 1999, The redox-state of subduction zones: Insights from arc-peridotites: Chemical Geology, v. 160, p. 409–423, doi: 10.1016/S0009-2541(99)00110-2.

Parkinson, I.J., and Pearce, J.A., 1998, Peridotites from the Izu-Bonin-Mariana forearc (ODP Leg 125): Evi-dence for mantle melting and melt-mantle inter action in a supra-subduction zone setting: Journal of Petrol-ogy, v. 39, p. 1577–1618, doi: 10.1093/petrology/ 39.9.1577.

Pearce, J.A., Barker, P.F., Edwards, S.J., Parkinson, I.J., and Leat, P.T., 2000, Geochemistry and tectonic sig-nificance of peridotites from the South Sandwich arc-basin system, South Atlantic: Contributions to Miner-alogy and Petrology, v. 139, p. 36–53, doi: 10.1007/s004100050572.

Reese, J.F., Mosher, S., Connelly, J., and Roback, R., 2000, Mesoproterozoic chronostratigraphy of the southeastern Llano uplift, central Texas: Geologi-cal Society of America Bulletin, v. 112, p. 278–291, doi: 10.1130/0016-7606(2000)112<278:MCOTSL> 2.0.CO;2.

Roback, R.C., 1996, Characterization and tectonic evolu-tion of a Mesoproterozoic island arc in the south-ern Grenville orogen, Llano uplift, central Texas: Tectonophysics, v. 265, p. 29–52, doi: 10.1016/S0040-1951(96)00145-X.

Rowley, D.B., and Pindell, J.L., 1989, End Paleozoic–Early Mesozoic western Pangean reconstruction and its implications for the distribution of Precambrian and Paleozoic rocks around Meso-America: Precambrian Research, v. 42, p. 411–444, doi: 10.1016/0301-9268 (89)90022-3.

Satsukawa, T., Michibayashi, K., Raye, U., Anthony, E.Y., Pulliam, J., and Stern, R.J., 2010, Uppermost mantle anisotropy beneath the southern Laurentian margin: Evidence from Knippa peridotite xenoliths, Texas: Geophysical Research Letters, v. 37, p. L20312, doi: 10.1029/2010GL044538.

Sawyer, D.S., Buffler, R.T., and Pilger, R.H.P., Jr., 1991, The crust under the Gulf of Mexico Basin, in Salvador, A., ed., The Gulf of Mexico Basin: Geological Soci-ety of America, The Geology of North America, v. J, p. 53–72.

Spencer, A.B., 1969, Alkalic igneous rocks of the Balcones Province, Texas: Journal of Petrology, v. 10, p. 272–306.

Stern, R.J., Anthony, E.Y., Ren, M., Norton, I., Kimura, J.I., Miyazaki, T., Hanyu, T., Chang, Q., and Hira-hara, Y., 2011, Southern Louisiana Salt Dome xeno-liths: First glimpse of Jurassic (~160 Ma) Gulf of Mexico crust: Geology, doi: 10.1130/G31635.1 (in press).

Thomas, W.A., 2006, Tectonic inheritance at a continen-tal margin: GSA Today, v. 16, p. 4–11, doi: 10.1130/ 1052-5173(2006)016[4:TIAACM]2.0.CO;2.

Thomas, W.A., and Astini, R.A., 1996, The Argentine Pre-cordillera: A traveler from the Ouachita embayment of North American Laurentia: Science, v. 273, p. 752–757, doi: 10.1126/science.273.5276.752.

Wells, P.R.A., 1977, Pyroxene thermometry in simple and complex systems: Contributions to Mineral-ogy and Petrology, v. 62, p. 129–139, doi: 10.1007/BF00372872.



Whitmeyer, S.J., and Karlstrom, K.E., 2007, Tectonic model for the Proterozoic growth of North America: Geo-sphere, v. 3, p. 220–259, doi: 10.1130/GES00055.1.

Wilshire, H.G., McGuire, A.V., Noller, J.S., and Turrin, B.D., 1990, Petrology of lower crustal and upper man-tle xenoliths from the Cima volcanic field, California: Journal of Petrology, v. 32, p. 170–200.

Witt-Eickschen, G., and Seck, H.A., 1991, Solubility of Ca and Al in orthopyroxene from spinel peridotite: An improved version of an empirical geothermometer: Contributions to Mineralogy and Petrology, v. 106, p. 431–439, doi: 10.1007/BF00321986.

Wood, B.J., 1990, An experimental test of the spinel perido tite oxygen barometer: Journal of Geophysi-cal Research, v. 95, p. 15,845–15,851, doi: 10.1029/JB095iB10p15845.

Wood, B.J., and Virgo, D., 1989, Upper mantle oxidation state: Ferric iron contents of lherzolite spinels by 57Fe Mossbauer spectroscopy and resultant oxygen fugacities: Geochimica et Cosmochimica Acta, v. 53, p. 1277–1291, doi: 10.1016/0016-7037(89)90062-8.

Young, H.P., and Lee, C.-T.A., 2009, Fluid-metasomatized mantle beneath the Ouachita belt of southern Lauren-tia: Fate of lithospheric mantle in a continental oro-

genic belt: Lithosphere, v. 1, p. 370–381, doi: 10.1130/L72.1.

Zhou, M.-F., Robinson, P.T., Malpas, J., and Li, Z., 1996, Podiform chromitites in the Loubusa ophiolite (South-ern Tibet): Implications for melt-rock interaction and chromite segregation in the upper mantle: Journal of Petrology, v. 37, p. 3–21, doi: 10.1093/petrology/ 37.1.3.

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