GEOSPHERE Petrogenesis of the 91-Mile Peridotite in the Grand Canyon: Ophiolite Or Deep-Arc Fragment? GEOSPHERE, V

Research Paper

GEOSPHERE Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc fragment? GEOSPHERE, v. 17, no. 3 S.J. Seaman1, M.L. Williams1, K.E. Karlstrom2, and P.C. Low1 1Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA https://doi.org/10.1130/GES02302.1 2Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA

10 figures; 5 tables DEDICATION u This manuscript represents one of the last projects that Dr. Sheila J. Seaman was working on before her untimely passing. It is completed in CORRESPONDENCE: [email protected] her honor as a beloved researcher, teacher, and colleague.

CITATION: Seaman, S.J., Williams, M.L., Karlstrom, K.E., and Low, P.C., 2021, Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc ABSTRACT ■■ INTRODUCTION fragment?: Geosphere, v. 17, no. 3, p. 786–803, https:// doi.org/10.1130/GES02302.1. Recognition of fundamental tectonic boundaries has been extremely diffi- The broad Proterozoic orogenic belt of southwestern North America has cult in the (>1000-km- wide) Protero zoic accretionary orogen of southwestern been interpreted in terms of the assembly and accretion of both continental Science Editor: Shanaka de Silva North America, where the main rock types are similar over large areas, and and oceanic terranes, blocks, and/or provinces between ca. 1.8 and 1.0 Ga (Ben-

Received 16 June 2020 where the region has experienced multiple postaccretionary deformation nett and DePaolo, 1987; Karlstrom and Bowring, 1988; Bowring and Karlstrom, Revision received 22 November 2020 events. Discrete ultramafic bodies are present in a number of areas that may 1990; Whitmeyer and Karlstrom, 2007). Geologic mapping, isotopic analyses, Accepted 3 February 2021 mark important boundaries, especially if they can be shown to represent and geochemical investigations have been carried out for several decades, but tectonic fragments of ophiolite complexes. However, most ultramafic bodies suture zones between tectonic blocks (or provinces) are still extremely difficult Published online 21 April 2021 are small and intensely altered, precluding petrogenetic analysis. The 91-Mile to identify. This is probably due, at least in part, to the intensity of multiple peridotite in the Grand Canyon is the largest and best preserved ultramafic syn- and postassembly deformational and metamorphic events, but the lack body known in the southwest United States. It presents a special opportu- of clear suture boundaries has led to questions about the tectonic significance nity for tectonic analysis that may illuminate the significance of ultramafic of the blocks and provinces themselves. Specifically, do the tectonic blocks rocks in other parts of the orogen. The 91-Mile peridotite exhibits spectacular represent microplates that were assembled into Proterozoic continental crust, cumulate layering. Contacts with the surrounding Vishnu Schist are inter- or, alternatively, do they represent tectonically rearranged but not exotic com- preted to be tectonic, except along one margin, where intrusive relations have ponents of a single crustal province? Ultramafic rocks occur as tectonic lenses been interpreted. Assemblages include olivine, clinopyroxene, orthopyrox- (meters to rarely hundreds of meters in diameter) throughout the Proterozoic ene, magnetite, and phlogopite, with very rare plagioclase. Textures suggest orogen, and many occur in or near suspected tectonic boundary regions (Fig. 1). that phlogopite is the result of late intercumulus crystallization. Whole-rock This has led to speculation that these exotic rocks may represent fragments compositions and especially mineral modes and compositions support deri- of ophiolites (back-arc or oceanic crust) that mark sutures, or they may serve vation from an arc-related mafic magma. K-enriched subduction-related fluid as markers of other types of significant tectonic boundaries within the orogen. in the mantle wedge is interpreted to have given rise to a K-rich, hydrous, Field relations and geochemical constraints are required to illuminate the high-pressure partial melt that produced early magnetite, Al-rich diopside, significance of the ultramafic bodies. However, most of the ultramafic occur- and primary phlogopite. The modes of silicate minerals, all with high Mg#, the rences consist of relatively small and isolated blocks, and most are strongly sequence of crystallization, and the lack of early plagioclase are all consistent altered and/or metamorphosed. Primary structures and textures are generally with crystallization at relatively high pressures. Thus, the 91-Mile peridotite not preserved, and geochemical data can be suspect because of alteration. body is not an ophiolite fragment that represents the closure of a former The 91-Mile peridotite, exposed in the Upper Gorge of the Grand Canyon, is ocean basin. It does, however, mark a significant tectonic boundary where a significant anomaly in terms of its size and the degree of preservation. The lower-crustal arc cumulates have been juxtaposed against middle-crustal ultramafic body is ~1 km in diameter, and primary assemblages and textures schists and granitoids. are superbly preserved. This body is of particular interest because it occurs within several kilometers of the proposed Crystal suture zone between the This paper is published under the terms of the isotopically distinct Mojave and Yavapai crustal provinces (Fig. 1; Ilg et al., CC‑BY-NC license. Michael Williams https://orcid.org/0000-0003-0901-3396 1996; Karlstrom and Williams, 2006; Holland et al., 2015).

GEOSPHERE | Volume 17 | Number 3 Seaman et al. | 91-Mile peridotite in the Grand Canyon Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/3/786/5319423/786.pdf 786 by guest on 02 October 2021 Research Paper

The goal of this study was to characterize the 91-Mile peridotite exposure and to use field, petrographic, and compositional information derived from 42o the peridotite to constrain its provenance and emplacement history, and spe- E WYOMING PROVINCE MCC Green Mountain EMG Magmatic Arc cifically to determine whether the peridotite is a fragment of an ophiolite LOC

(i.e., a tectonic sliver from a mid-ocean ridge or back-arc/intra-arc basin) or FMB BGsz BCsz a cumulate that developed in an arc-associated magma chamber. The mini- o CHEYENNE BELT 40 FCSCsz MMsz mally altered, metamorphosed, or deformed nature of the 91-Mile peridotite

provides a special opportunity to interpret field relations, primary textural Hsz features, mineral assemblages, and compositional characteristics, and to evaluate the tectonic setting of formation of these basement rocks. Interpretations o PPZ 38 derived from this well-preserved exposure can provide insight into the petro- MOJAVE PROVINCE Salida-Gunnison

genesis of other ultramafic occurrences within the Grand Canyon and across Magmatic Arc YAVAPAI Dubois Terrane

the orogenic belt and may ultimately help to answer the question of whether PROVINCE Virgin Mtns MOJ-YAV TRANSITION

the ultramafic bodies are associated with, and can be used to identify, major MGG 91-Mile Peridotite o Gold Butte (UGG) NM tectonic boundaries within the orogen. 36 LGG Pecos Terrane

YAV-MAZ TRANSITION ■■ REGIONAL GEOLOGY Zuni Mtns o PO 34

The lithosphere of the Grand Canyon region of southwestern Laurentia was assembled between 1800 Ma and 1400 Ma (Condie, 1992; Whitmeyer and Karl- MAZATZAL PROVINCE strom, 2007) through the accretion of a 1000-km-wide accretionary orogenic complex interpreted to be composed of island-arc terranes, continental fragments, 32o and their syntectonic cover (e.g., Karlstrom and Bowring, 1988; Bowring and Karlstrom, 1990; Whitmeyer and Karlstrom, 2007). This 1000-km-wide orogen has 0 100 been divided into three northeast-southwest– trending Proterozoic crustal prov- km inces, the Mojave, Yavapai, and Mazatzal Provinces (Fig. 1; Karlstrom and Bowring, 30o 1988; Whitmeyer and Karlstrom, 2007). The Mojave and Yavapai Provinces are o o o both made up of 1840–1700 Ma rocks that were deformed and metamorphosed 114 110 106 W during the ca. 1700 Ma Yavapai orogeny (Holland et al., 2015). However, the Figure 1. Map of southwestern North America showing locations of Proterozoic provinces Mojave Province has a distinctive isotopic signature indicating the cryptic pres- and province boundaries and the locations of Proterozoic ultramafic exposures (yellow ence of Archean crustal material (Wooden and DeWitt, 1991; Holland et al., 2018). dots), from Low (2009); adapted from Karlstrom and Bowring (1988), Bowring and Karl- strom (1990), Condie (1992), Aleinikoff et al. (1993), Premo and Fanning (2000), Premo The boundary between the Mojave and the Yavapai Provinces is shown in and Loucks (2000), Bryant et al. (2001), Tyson et al. (2002), Strickland et al. (2003), and Figure 1 as a broad (75-km-wide) northeast-trending “Moj-Yav transition zone” Cavosie and Selverstone (2003). UGG, MGG, LGG—Upper, Middle, Lower Granite Gorges, (Fig. 1), defined mainly on the basis of Pb, Nd, and Hf isotopes (Bennett and Grand Canyon. Dashed black lines—block and terrane boundaries proposed by Condie DePaolo, 1987; Wooden and DeWitt, 1991; Duebendorfer et al., 2006; Holland (1992). BCsz—Big Creek Gneiss shear zone; BGsz—Buckskin Gulch shear zone; EMG— Elkhorn Mountain Gabbro; FMB—Farwell Mountain belt; LOC—Lake Owen Complex; et al., 2015, 2018). The Mojave Province to the west is made up of 1.84–1.68 Ga Hsz—Homestake shear zone; MCC—Mulien Creek Complex; MMsz—Moose Mountain metasedimentary and plutonic rocks that contain detrital and inherited zircon shear zone; NM—Nacimiento Mountains; PO—Payson ophiolite; PPZ—Poncha Pass zone. evidence for derivation from some juvenile crust, but with older crustal involve- FCSCsz—Fish Creek Soda Creek shear zone. ment indicated by the major 1.8 and 2.5 Ga zircon modes. Metasediments and plutons both get systematically younger eastward, and their Hf isotopic composition gets more juvenile west-to-east and old-to-young (Holland et al., 2018). Proterozoic rocks of the transition zone consist of metasedimentary (Vishnu The Yavapai Province, to the southeast, is similar in age but has a more juvenile, Schist) and metavolcanic (Rama and Brama Schist) sequences intruded by volu- arc-related character than the Mojave Province (Karlstrom et al., 2001); the dom- minous mafic to felsic 1730–1680 Ma plutonic rocks (Ilg et al., 1996; Hawkins et inantly juvenile nature of the Yavapai Province has been supported by recent Hf al., 1996). Geochemical signatures from Yavapai-type crust and Mojave-type isotopic studies (Holland et al., 2015, 2018), although older crust (Bickford and Hill, crust, as well as rocks with mixed signatures, can all be found in close prox- 2007) and older detritus (Shufeldt et al., 2010) have been found in some areas. imity to one another, a situation reflective of both tectonic and geochemical

mixing of rocks from both provinces, and it has not been possible to pinpoint Grenville orogeny between 1200 and 1000 Ma. Figure 1 shows these provinces discrete tectonic boundaries. as well as other suggested boundaries and terminologies involving proposed The Mazatzal Province (Fig. 1) includes 1700–1650 Ma rocks that are inter- terranes (Condie, 1992) and shear zone–bounded subterranes or blocks that preted to have been built on Yavapai-aged crust in a continental arc (Karlstrom differ, at least to some degree, in composition, metamorphic grade, and/or tec- et al., 2016). Deformation took place during the 1650–1600 Ma Mazatzal orogeny tonic history (Karlstrom and Bowring, 1988; Ilg et al., 1996; Dumond et al., 2007). and, in many regions, again at ca. 1450–1400 Ma during the Picuris orogeny The Grand Canyon contains three main basement exposures, the Upper, (Daniel et al., 2013; Mako et al., 2015). The Granite-Rhyolite Province contains Middle, and Lower Granite Gorges (Fig. 1), which each provide 100% exposed ca. 1.5 Ga juvenile crust that was added at this time (Bickford et al., 2015). The transects across parts of the Proterozoic orogen, including the Mojave-Yavapai Texas extension of the Grenville Province occurs well to the south of the Grand transition zone. The Upper Granite Gorge (Fig. 2), exposed from river mile Canyon and includes younger rocks (1400–1100 Ma) that were deformed in the 76 to 120 (where river mile [RM] is defined downriver from Lee’s Ferry, just

117° 25’ W Shinumu pillow 112° 00’ W basalts UPPER GRANITE GORGE Map Area Colorado River BASS Lee’s SHEAR ZONE Ferry

GARNET ANTIFORM CRYSTAL SHEAR ZONE N 91-Mile BRIGHTSHEAR ANGEL ZONE Mile 98 Peridotite ultrama c

Mile 83 EXPLANATION Crystal ultrama c Biotite ± muscovite granite and pegmatite VISHNU 1698 - 1662 Ma pillow basalts SHEAR ZONE Arc plutons: Hornblende - biotite granodiorite, tonalite, diorite, and gabbro: 1750 - 1713 Ma Horn Creek Ultrama c rocks: coarse grained relict pillow basalts cumulate textures (undated) 91-MILE

ANTIFORM Colorado Vishnu Schist: biotite - muscovite - quartz ZOROASTER River schist and pelitic schist ANTIFORM Rama and Brahma Schists: Interlayered felsic (Rama) to ma c (Brahma) metavolcanic rocks: 1750 - 1740 Ma SOCKDOLAGER Elves Chasm gneissic granodiorite: 1840 Ma ANTIFORM 36° 02’ N

Axial trace

Shear zone 25 km

Figure 2. Geologic map of the Upper Granite Gorge of Grand Canyon showing ultramafic exposures at river mile (RM) 83, 91, and 98, distribution of meta-basalts of the Brahma Schist (stars—pillow basalt locations), and relevant metamorphic domains, adapted from Ilg et al. (1996), Dumond et al., (2007), and Holland et al., (2015).

below Glen Canyon Dam), has been subdivided into six shear zone–bounded ■■ ANALYTICAL METHODS lithotectonic blocks (Ilg et al., 1996; Hawkins et al., 1996; Dumond et al., 2007). Most rocks contain at least two deformational fabrics: an early NW-striking, Field mapping, structural analysis, and sample collection took place over

shallowly dipping foliation (S1) that is refolded and variably transposed by a several field seasons by S.J. Seaman and coauthors on park-permitted research

later NE-striking, steeply dipping foliation (S2). Peak metamorphism occurred river trips in Grand Canyon (1995–2012) led by the University of New Mexico between 1705 and 1680 Ma (Hawkins et al., 1996) during the second phase of (UNM). Whole-rock major-element analyses were collected from fused glass deformation. All blocks were metamorphosed at pressures of ~0.6–0.7 GPa; discs in the X-ray fluorescence laboratory at the University of Massachusetts peak temperatures tend to alternate from block to block from 500–600 °C to using a Philips MRS wavelength-dispersive spectrometer under the supervision more than 700 °C (Dumond et al., 2007). of J.M. Rhodes. Whole-rock trace-element analyses were collected by inductively The Crystal shear zone (at RM 98; Fig. 2) was proposed to be the eastern coupled plasma–mass spectrometry (ICP-MS) at Union College, Schenectady, edge of the suture zone between the Mojave and Yavapai Provinces (Ilg et al., New York, using a PerkinElmer/Sciex Elan 6100 DRC under the direction of Kurt 1996; Hawkins et al., 1996), and the Gneiss Canyon shear zone (RM 234–242) Hollocher and by Paul Lamothe at the U.S. Geological Survey (USGS), Denver, was interpreted as the western boundary of the transition zone (Karlstrom et Colorado. Trace-element and rare earth element (REE) analyses of minerals al., 2003). Both shear zones show east-to-west steps to more evolved (more were collected by laser-ablation ICP-MS at Boston University using a VG Plasma Mojave-like) isotopic compositions of plutons, high D2 strain, and lenses of Quad ExCell ICP-MS equipped with a Merchantek LUV213 laser-ablation ICP-MS ultramafic rocks and pillow basalt (and carbonate in the Gneiss Canyon shear system, under the direction of Terry Plank. Microprobe analyses were collected zone). Recent detrital zircon results from the Vishnu Schist document a bimodal in the Electron Microprobe/Scanning Electron Microscopy Facility in the Depart- detrital zircon spectrum with peaks at 1.8 Ga and 2.5 Ga across the entire Grand ment of Geosciences at the University of Massachusetts, using a Cameca SX-50 Canyon transect, with no change across the Crystal or Gneiss Canyon shear electron microprobe, under the direction of Michael Jercinovic. Ferric-ferrous zones, suggesting that any suturing would have predated or been synchro- Fe determinations for minerals were based on stoichiometry (see Low, 2009). nous with 1.75 Ga Vishnu Schist deposition (Shufeldt et al., 2010; Holland et al., 2015). Hf isotopic results from plutons, however, show variable mixing of juvenile and evolved crust within heterogeneous lower-crustal melt-source ■■ CHARACTERISTICS OF THE 91-MILE PERIDOTITE regions in the transition zone but an overall change to juvenile granodiorites east of the Crystal shear zone (Holland et al., 2015). Holland et al. (2015) inter- Field Relations preted the Mojave-Yavapai boundary to be an ~200-km-wide middle-crustal duplex system in which the 1.75 Ga Vishnu Schist was deposited across The 91-Mile peridotite is a NE-trending, pod-shaped ultramafic body located sutured (or suturing) Mojave and Yavapai crust in an accretionary complex. north of the Colorado River, ~1 km up 91-Mile Canyon (Fig. 3). A small out- This distributed boundary has tectonic lenses of plutons that carry the isotopic crop on the Colorado River may be connected (in the subsurface) to the main

signature of their respective crustal isotopic provinces, now imbricated with body along the hinge of a south-plunging F2 fold (Ilg et al., 1996). The main a metasedimentary cover that is compositionally similar across the zone. The body occupies an area of ~0.75 km2. Most of the ultramafic body consists of cover sediments are interpreted to have been derived mainly from the Mojave layered olivine websterite or lherzolite. Major minerals include olivine, diop- Province crust, but were deposited on (i.e., overlapped) both Mojave (evolved) side, orthopyroxene, magnetite, phlogopite, and minor amphibole (pargasite, and Yavapai (juvenile) crust at 1.75 Ga before further shortening and tectonic edenite, and magnesiohastingsite; terminology after Leake et al., 2004). The imbrication by thrusting during the 1.74–1.70 Ga Yavapai orogeny. most striking characteristics of the ultramafic rocks when viewed in the field Three relatively large occurrences of ultramafic rocks are present within and are their coarse grain size (0.5–3 cm) and strong mineralogic layering, partic- on the east side of the Crystal shear zone in the Upper Gorge of the Grand Can- ularly toward the southern end of the body (Fig. 4), with layers persisting for yon (Fig. 2), near RM 83, RM 91, and RM 98, and smaller lenses are found in the tens of meters along strike. Nearest the mouth of the canyon, ~4–5-cm-thick Gneiss Canyon shear zone (RM 245). Of these, the exposure of the 91-Mile peri- layering is defined by variable modal abundances of olivine, pyroxene, and dotite is by far the largest and least altered. The exposure at RM 83 is also large phlogopite. The phlogopite books are several centimeters in diameter and are (0.29 km2), but its primary minerals have been pervasively altered to chlorite, ser- oriented with their basal plane typically at an angle to layering. The layering pentine, and fine-grained clay minerals. Ultramafic exposures at RM 98 consist is interpreted to be cumulate layering, although much of the phlogopite is of two small groupings (each smaller than 0.11 km2) of mafic/ultramafic lenses interpreted to be an intercumulus, postdeposition phase (see below). within Vishnu Schist. Meter-scale folded and boudinaged ultramafic lenses are The layering in the 91-Mile peridotite is northwest-striking and steeply

also found at RM 245. The lack of continuous outcrop and the advanced alteration southwest-dipping, similar to the early (S0/S1) foliation in adjacent Vishnu of these rocks make them much less interpretable than those of the large and Schist metasediments (Fig. 3). Contacts between the ultramafic body and the mostly unaltered 91-Mile body, which was the focus of this study. surrounding Vishnu Schist are sharp and locally truncate the peridotite layering

and Vishnu layering, suggesting tectonic emplacement. However, some contact and metamorphism. Modal composition varies little as a function of position areas have deformed and serpentinized inclusions of ultramafic rocks in the within the unit, with the exception that phlogopite crystals become coarser neighboring schist, and the side canyon contact at the downstream margin and pyroxene crystals become finer and less abundant northeastward (inter- (Fig. 3) has a ragged, possibly intrusive contact with the schist. Tectonic folia- preted to be upward based on decreasing Mg# in olivine and clinopyroxene; tions and lineations are not well developed within the interior of the ultramafic see below) across the sequence. rock body, but there is local folding and fracturing near the contacts. Clinopyroxene (diopside) and phlogopite are the dominant minerals seen Most of the minerals in the 91-Mile peridotite are interpreted to be pri- in hand specimens of the 91-Mile peridotite. Diopside crystals range from mary igneous minerals, including olivine, diopside, orthopyroxene, amphibole 1 mm to 5 mm in diameter. They are blocky and black and occur in layers a (pargasite, edenite, and magnesiohastingsite), and phlogopite (Fig. 4). Ser- few millimeters to over 1 cm in thickness. Large (to >3-cm-diameter) phlogo- pentine occurs locally and is interpreted to be the product of fluid infiltration pite crystals impart a bronzy sheen to weathered surfaces of the peridotite

S0 S1 75 less strongly layered 43 69

45 79

63 55 30 S S 0 1 68 dunite inclusions Trinity Pluton 65 45 35 S2 81 Ninety-one Mile Creek

75 75 S2 50 orientation of S2 87 cumulate layers 50 River large (to 5 cm) cpx crystals 30 78 60 75 Vichnu80 Schist 76 Horn Vishnu F2 Pluton 30-60 Schist 82

Ultrama c bodies N strike and dip of cumulate 45 layering in ultrama c body Amphibolites and basalts strike and dip of 87 foliation (undierentiated) 20 trend and plunge Granodioritic plutons strike and dip of directions of an 30 45 S 2 foliation anticlinal fold 83 Vishnu Schist strike and dip of bedding and 0 km 1 20 trend and plunge of lineation

Figure 3. 91-Mile peridotite outcrop distribution, cumulate layering, and deformational fabrics. Inset shows detail of structure in and around the peridotite body; cpx—clinopyroxene. Ninetyone Mile creek is located at 112.1°W, 36.1°N.

almost entirely olivine (or serpentine pseudomorphs after olivine) with minor diopside, enstatite, Cr-spinel, and pargasite.

Geochemistry

Major-Element Concentrations

Cumulate-textured peridotite and pyroxenite samples of the 91-Mile per-

idotite range in SiO2 from ~45 to 51 wt% and in MgO from ~20 to 28 wt%.

They are low in Al2O3 (~1.0–6.5 wt%) and high in K2O (~1.5–2.0 wt%), and they

have CaO concentrations from 4.5 to 7.0 wt% (Table 1). CaO, K2O, and Al2O3 are all inversely correlated with MgO. Mg# (molar Mg/[Mg + Fe]) is 0.80–0.83 in all except two less-Mg-rich peridotite samples and over 0.84 in the dunite Figure 4. Outcrop view of cumulate layers near the exposed base of the 91-Mile peridotite. See text for discussion. inclusions. Cr and Ni concentrations are 1600–2100 ppm and 800–1150 ppm, respectively, for the peridotites and ~1700–2200 ppm (Cr) and 1200–1700 ppm (Ni) for the dunite inclusions (Table 2). and commonly host olivine inclusions. Orthopyroxene also occurs as large Major-element compositions define a general trend consistent with pro- cumulate crystals, but it is less abundant than diopside, and the two are indis- gressive accumulation of minerals crystallizing from a primitive basalt; the

tinguishable in hand specimen. most Mg-rich cumulates are the most depleted in K2O, Na2O, and Al2O3, consistent with early crystallization of Mg-rich olivine and Mg-rich diopside. With increasing removal of Mg-rich olivine and diopside from the basaltic parent, Petrographic Characteristics those phases became less magnesian and more Fe-rich. The dunite enclaves

are the most primitive material, with SiO2 ~41 wt% and MgO ~40 wt%. Thin section analysis showed that most samples of 91-Mile peridotite are dominated by coarse diopside, olivine, and phlogopite (Fig. 5A). Diopside crystals host concentric sprays of hundreds of tiny (micron-sized) magne- Trace-Element and Rare Earth Element Concentrations tite crystals that define concentric growth zones in the diopside (Fig. 5B). The euhedral crystal shapes further support the interpretation of preserved Samples of the 91-Mile peridotite are characterized by significant enrich- igneous textures. Olivine occurs as inclusions in the large diopside crystals ment in the most compatible elements and extreme depletion in incompatible and as later-forming crystals. Generally, the olivine inclusions interrupt the elements. Cr concentrations range from 1320 to 2240 ppm, and Ni abundances concentric zones of magnetite, so that magnetite is absent or scarce within range from 660 to 1160 ppm. Rb concentrations range from 50 to 100 ppm, and ~1 mm of olivine crystals (Fig. 5C). Phlogopite crystals are orange in thin sec- Ba abundances range from 390 to 800 ppm. Zr concentrations range from 40 tion under plane light. In rare instances, phlogopite appears to have replaced to 50 ppm, and Nb abundances range from 2 to 7 ppm (Table 2). The rocks are orthopyroxene, but generally phlogopite occurs as a large, interstitial phase enriched in the large ion lithophile elements (LILEs), particularly K, Rb, Ba, and surrounding olivine, diopside, and orthopyroxene (Fig. 5A). Orthopyroxene Pb, and they are depleted in the high field strength elements (HFSEs) relative occurs both as large cumulate crystals and as reaction rims around olivine to mid-ocean-ridge basalt (MORB) (Fig. 7A). (Fig. 5D; Low, 2009). Based on inclusion relationships, the crystallization order Rare earth element (REE) patterns are negatively sloped and slightly con- of major minerals was olivine, diopside, orthopyroxene, with minor primary cave upward, with the light rare earth elements (LREEs) enriched relative to amphibole, followed by phlogopite. Trace (to 2%) Na-rich plagioclase occurs the heavy rare earth elements (HREEs). Overall REE abundances range from in some samples as an interstitial phase. ~40 times chondritic composition for the LREEs to ~6 times chondritic com- Cumulate layers near the southern end of the main body contain swarms position for the HREEs (Fig. 7B). Dunite enclaves have a similarly sloping REE of spheroidal, olivine-rich (i.e., dunite) enclaves 2–10 cm in length (Fig. 6). The pattern (Ce/Yb = 5.7–6.8) but with overall abundances that range from ~7 times long axes are contained within the cumulate layering but are not particularly chondritic concentrations for the LREEs to ~2 times for the HREEs. The dunite lineated. Irregular, somewhat flattened boundaries between the enclaves and inclusions are also enriched in LILEs relative to HFSEs (Pb/Ce = 11–34), with the surrounding cumulate suggest that the enclaves may have been crystal overall abundances in both LILEs and HFSEs much lower than those in the mushes when they were incorporated into the cumulates. The enclaves are host cumulate samples.

Ol Phl Di Di

Figure 5. Photomicrographs showing silicate minerals from 91-Mile peridotite samples. (A) Cumulate diopside (Di), olivine (Ol), and orthopyroxene (Opx) crystals with interstitial 0.1mm 0.2mm phlogopite (Phl). (B) Euhedral diopside crystals with abundant magnetite inclusions that define growth faces. (C) Diopside crystal with olivine inclusions showing reduced abundance of magnetite near olivine inclusions. (D) Orthopyroxene reaction rim surrounding olivine and separating olivine from amphibole (Amp). Ol Ol

Opx Amp 0.2mm Opx 100 µm

Figure 6. High Mg# dunite enclave–bearing layer in the 91-Mile peridotite near the mouth of 91-Mile Canyon, Upper Granite Gorge, Grand Canyon. Pen is 14 cm in length.

TABLE 1. MAJOR-ELEMENT ANALYSES wt OF 91-MILE PERIDOTITE SAMPLES Sample: S5-91.1-3 S5-91.1-5 S5-91.1-8 S96-91-2 S96-91- S99-91-1 S99-91-2 S99-91-3 S99-91-5 S99-91-6

SiO2 50.8 5.13 7.08 0.8 5.75 5.38 5.7 .82 9.09 5.97

TiO2 0.88 0.33 0.1 0.10 0.09 0.7 0.35 0.39 0.0 0.36

Al2O3 6.8 .75 5.00 1.31 0.88 .89 .51 5.13 6.05 5.07

Fe2O3 11.08 11.91 11.67 1.28 13.28 12.7 12.97 12.92 10.60 12.56 MnO 0.30 0.19 0.19 0.18 0.18 0.20 0.22 0.20 0.17 0.20 MgO 19.61 28.6 25.38 38.08 36.20 26.31 26.87 28.6 2.10 28.2 CaO 6.77 5.15 6.22 2.0 2.08 6.96 6.83 .5 6.35 .55

Na2O 0.65 1.32 0. 0.09 0.17 0.09 0.79 0.63 0.56 0.58

K2O 2.07 1.1 1.7 0.18 0.06 1.5 1.27 2.00 2.05 2.09

P2O6 0.16 0.20 0.21 0.0 0.02 0.17 0.17 0.21 0.22 0.2 Total 98.8 99.03 98.3 97.50 98.71 98.76 99.72 99.21 99.59 99.85

TABLE 2. TRACE-ELEMENT AND RARE EARTH ELEMENT ANALYSES ppm OF 91-MILE PERIDOTITES Sample S6-83-1 S99-83-2 S99-83- S5-91.1-3 S5-91.1-5 S5-91.1-8 S99-91-1 S99-91-2 S99-91-2i S99-91-3 S99-91-5 S99-91-6 S99-91-7 S02-91-P1 S6-98.1-1 S99-98-1 S99-98-2 S99-98-3 La 9.10 18.00 8.00 7.55 9.60 10.30 12.00 8.00 2.13 9.00 16.00 9.00 13.00 16.20 31.70 6.00 12.00 5.00 Ce 21.10 36.00 19.00 2.20 21.0 21.10 13.00 15.00 5.2 18.00 26.00 21.00 32.00 3.30 75.20 17.00 33.00 15.00 Pr 2.78 na na .13 2.86 2.90 na na 0.7 na na na na 0.5 10.20 na na na Nd 13.50 na na 21.15 12.33 12.0 na na 3.31 na na na na 2.20 5.0 na na na Sm 3.27 na na 5.90 2.52 2.77 na na 0.73 na na na na 0.51 9.5 na na na Eu 0.98 na na 1.50 0.6 0.71 na na 0.20 na na na na 0.13 2.65 na na na Gd 3.15 na na 5.97 2.02 2.30 na na 0.70 na na na na 0.9 8.21 na na na Tb 0.9 na na 1.0 0.25 0.30 na na 0.10 na na na na 0.09 1.12 na na na Dy 2.85 na na 5.87 1.65 2.10 na na 0.62 na na na na 0.53 5.85 na na na Ho 0.57 na na 1.17 0.3 0.1 na na 0.12 na na na na 0.10 1.07 na na na Er 1.59 na na 3.06 0.92 1.10 na na 0.31 na na na na 0.30 2.72 na na na Tm 0.22 na na 0. 0.13 0.16 na na 0.05 na na na na 0.05 0.35 na na na Yb 1.31 na na 2.70 0.87 1.00 na na 0.31 na na na na 0.28 2.12 na na na Lu 0.21 na na 0.31 0.11 0.12 na na 0.05 na na na na bdl 0.32 na na na Sr 208.90 83.00 176.00 35.30 353.0 238.80 319.00 385.00 83.79 282.00 171.00 256.00 107.00 32.70 221.60 550.00 1085.00 600.00 Rb 77.90 32.30 75.00 97.50 50.20 na 52.80 3.20 5.01 63.20 77.00 73.00 5.20 2.0 1.10 89.60 76.70 62.60 Ba 793.30 9.00 88.00 386.00 78.00 581.30 80.00 635.00 50.7 721.00 12.00 712.00 77.00 27.0 67.0 251.00 212.00 337.00 Th 1.02 3.00 2.00 1. 2.20 1.50 2.00 1.00 0.51 1.00 2.00 1.00 2.00 0.26 3.53 1.00 2.00 1.00 Nb 3.53 .60 2.70 7.10 3.0 3.10 2.00 1.90 0.6 2.30 2.70 2.30 3.80 na 7.20 3.50 3.90 3.00 Pb 3.07 6.00 7.00 7.1 6.71 3.30 5.00 6.00 1.7 .00 2.00 .00 5.00 3.32 6.67 6.00 15.00 5.00 r 35.80 50.00 8.00 5.70 51.70 39.00 37.00 0.00 9.19 1.00 9.00 2.00 69.00 na 13.30 65.00 110.00 62.00 Hf 1.12 na na 1.7 1.9 1.20 na na 0.31 na na na na na 3.68 na na na Ti na 610.00 560.00 na na na 530.00 20.00 na 20.00 20.00 370.00 1790.00 300.00 na 690.00 960.00 720.00 Y 1.50 17.0 1.00 37.90 8.80 na 10.00 10.00 2.99 9.20 12.70 9.0 23.10 2.00 25.80 16.0 21.10 16.0 n na 128.00 90.00 na na na 102.00 10.00 na 99.00 87.00 101.00 130.00 51.00 na 92.00 69.00 98.00 Ni 75.60 617.00 731.00 660.70 1052.00 761.90 1037.00 1057.00 1203.00 1158.00 958.00 1120.00 73.00 1670.00 196.10 35.00 239.00 318.00 Cr 110.00 166.00 1999.00 1318.00 1993.00 2103.00 166.00 159.00 2239.00 1872.00 2131.00 165.00 12.00 1670.00 802.50 1105.00 663.00 1062.00 128.70 157.00 158.00 153.00 88.60 101.90 109.00 95.00 31.90 90.00 103.00 86.00 758.00 0.20 182.10 211.00 17.00 212.00 Cs 5.20 na na 11.80 1.15 21.0 na na 0.13 na na na na 0.17 3.70 na na na 0.58 bdl bdl 1.01 0.88 0.50 bdl bdl 0.20 bdl bdl bdl 2.00 0.12 1.3 bdl bdl bdl Ga na 13.00 9.00 na na na 6.00 6.00 na 6.00 7.00 6.00 20.00 1.10 na 12.00 13.00 12.00 Lab nion nion nion nion nion nion nion nion nion nion nion nion nion SGS nion nion nion nion Rock type Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Peridotite Dunite Peridotite Peridotite Peridotite Peridotite Notes: nanot analyed; bdlbelow detection limit. Lab: nionnion College, Schenectady, New York; SGS.S. Geological Survey, Denver, Colorado.

Mineral Compositions Olivine. All rocks of the 91-Mile peridotite contain either olivine or serpentine pseudomorphs after olivine. Modal olivine (including pseudomorphs) ranges from 6% to 80%. Olivine Mg# ranges from 0.77 to 0.92 and averages 0.83 throughout the peridotite exposure (Table 3). NiO concentrations range from 0.1 to 0.3 wt%. CaO concentrations are less than 0.001 wt%. Olivine has relatively flat REE patterns with a very slight enrichment in HREEs relative to LREEs (La/Lu < 0.78) and an overall abundance of 0.1 and 1.0 relative to chondrites (Fig. 8) (Sun and McDonough, 1989). Diopside. Clinopyroxene (Cpx) is abundant throughout the body with a modal abundance of greater than 20% in some samples. Based on mineral inclusion relationships, it is interpreted to have crystallized after magnetite and olivine and contemporaneously with orthopyroxene. Mg# in diopside ranges from 0.84 to 0.94 (Table 3). Commonly, diopside exhibits alternating concentric bands with abundant magnetite inclusions (Fig. 5B). Diopside in magnetite-rich bands has slightly elevated Al content compared with diopside in magnetite-poor bands. REE plots show concave-downward patterns with overall abundance between 4 and 60 times chondrites (Table 4; Fig. 8). Diopside is enriched in LREEs relative to HREEs (La/Lu > 1.2) with a negative Eu anomaly. Diopside from the 91-Mile peridotite generally shows a slight LREE depletion relative the middle rare earth elements (MREEs) (Fig. 8). Downes (2001) used the La/Nd ratio to represent the LREE/MREE slope of the REE pattern and the Sm/Yb ratio to represent the MREE/HREE slope of the REE pattern, such that Rock/Chondrite a REE pattern can be plotted as a single point on a plot of La/Nd versus Sm/ Yb. By these criteria, the diopside in the 91-Mile peridotite would be classified as LREE-depleted and MREE-enriched diopside. Phlogopite. Phlogopite is a major component of all samples, with modal Figure 7. Whole-rock geochemical characteristics of 91-Mile peridotite abundances ranging from 3% to 25% and generally increasing as modal oliv- (lherzolites, wehrlites) and dunite enclaves of the 91-Mile peridotite. ine decreases. Phlogopite occurs in three textural settings: (1) as inclusions (A) Trace elements normalized to primitive mantle. (B) Rare earth in olivine, both with and without amphibole, (2) as large inclusion-rich crys- elements normalized to chondrite composition. Normalization con- tals, and (3) as secondary crystals that have partially replaced orthopyroxene. centrations for both plots are from McDonough and Sun (1995). Three instances of phlogopite included in olivine were observed in thin section. Admittedly, these rare apparent inclusions may actually be later phlogopite filling embayments projecting from the third dimension. However, if they are possibly casting doubt on the inclusion interpretation. REEs in phlogopite truly inclusions, they suggest that at least some phlogopite was present early have low abundance and typically show relative enrichment of HREEs (La/Lu in the crystallization history. < 0.7) with a large positive Eu anomaly (Table 4; Fig. 8). Large, primary phlogopite crystals are tabular. They contain inclusions of Orthopyroxene. Cumulate orthopyroxene crystals are euhedral to subeuhe- olivine, pyroxene, and ubiquitous elongated inclusions of magnetite parallel dral and are generally over twice the diameter of coexisting olivine crystals, to 001 planes. The phlogopite crystals are interpreted to be magmatic crys- with a range in diameter from 1 to 5 mm and an average of ~4 mm. They com- tals that formed in the late stages of the crystallization history (after olivine, monly contain inclusions of olivine and magnetite, and they locally occur as orthopyroxene, and diopside but before matrix amphibole) from an evolved, coronas surrounding olivine crystals. The Mg# of orthopyroxene ranges from interstitial, hydrous melt. Phlogopite pseudomorphs after orthopyroxene occur 0.80 to 0.85 (bronzite) with a very limited range within individual samples, i.e.,

as optically continuous, inclusion-poor crystals up to 10 mm across. little or no zoning (Table 3). Both CaO and Al2O3 show a negative correlation Phlogopite has Mg# from 0.81 to 0.87, with less than 5% variation among with Mg#. MnO is poorly correlated with Mg#. The orthopyroxene is generally samples (Table 3). The one analysis made to date of phlogopite included enriched in HREEs relative to LREEs (La/Lu < 0.07) and ranges from 0.05 to 5 in olivine was essentially the same as other analyses from matrix crystals, times chondritic composition (Table 4; Fig. 8).

TABLE 3. MINERAL ANALYSES FROM THE 91-MILE PERIDOTITE Clinopyroene Olivine Orthopyroene Sample S96- S99- S02-91- S96- S96- S96- S96- S99- S5- S5-91-8 S99- S99- S02- S99- S99- S99- S99- 91- 91-2 P1l 91-7 91- 91- 91- 91-6B 91.1-1 91-3M 91-2 91-c 91-6M 91-6B 91-1 91-2

SiO2 5.79 5.08 5.80 5.22 53.91 0.22 0.80 39.70 39.38 39.77 39.27 39.10 56.8 55.92 57.10 5.75 55.6

Al2O3 0.1 2.06 1.63 5.71 6.53 0.00 0.00 0.02 0.01 0.01 0.0 0.00 0.21 0.23 0.25 2.0 0.7 FeO 1.6 3.73 2.91 .75 .59 13.79 13.71 16.86 16.60 16.27 17.15 20.57 10.8 9.33 10.96 12.71 11.63 MgO 17.78 15.12 16.8 20.22 21.0 6.12 6.16 3.88 3.02 3.87 3.6 39.57 31.95 31.83 31.22 28.73 30.8 MnO 0.07 0.16 0.1 0.06 0.07 0.22 0.23 0.32 0.31 0.31 0.31 0.37 0.39 0.1 0.2 0.25 0.30 CaO 2.51 22.52 22.52 12.30 11.50 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.15 0.20 0.21 0.5 0.23

Na2O 0.25 1.10 0.79 1.68 1.75 na na na na na na na 0.00 0.0 0.02 0.03 0.00

TiO2 0.09 0.16 0.10 0. 0.59 na na 0.16 0.1 0.16 0.28 na 0.0 0.06 0.08 0.03 0.03

Cr2O3 0.3 0.1 0.5 0.6 0.83 0.00 0.01 0.00 0.00 0.05 0.00 0.03 0.05 0.01 0.02 0.0 0.00

Fe2O3 0.66 0.92 0.6 0.00 0.02 na na na na na na na 0.05 2.01 0.00 0.00 1.56

K2O 0.02 na na 0.281 na na na na na na na na na na na na na Total 100.5 100.25 100.7 100.11 100.82 100.35 100.90 100.96 99.5 100. 100.52 99.63 100.15 100.05 100.30 99.03 100.33 Si 1.981 1.972 1.979 1.926 1.897 1.000 1.007 0.997 1.002 1.000 0.993 1.011 1.993 1.968 2.002 1.963 1.969 Al3 0.018 0.089 0.069 0.239 0.271 – – 0.001 0.000 0.000 0.001 0.000 0.009 0.010 0.011 0.086 0.019 Fe2 0.050 0.11 0.088 0.11 0.135 0.287 0.283 0.35 0.353 0.32 0.363 0.5 0.307 0.275 0.322 0.381 0.3 Mg2 0.959 0.822 0.907 1.071 1.10 1.709 1.698 1.62 1.632 1.65 1.638 1.525 1.670 1.670 1.632 1.535 1.608 Mn2 0.002 0.005 0.00 0.002 0.002 0.005 0.005 0.007 0.007 0.007 0.007 0.008 0.012 0.012 0.013 0.008 0.009 Ca2 0.950 0.880 0.871 0.68 0.3 0.000 bdl 0.000 – 0.000 – 0.000 0.006 0.008 0.008 0.017 0.009 Na2 0.011 0.078 0.056 0.115 0.119 – – – – – – – – 0.003 0.002 0.002 – Ti 0.002 0.00 0.003 0.012 0.016 – – 0.003 0.003 0.003 0.006 – 0.001 0.002 0.002 0.001 0.001 Cr3 0.010 0.012 0.015 0.013 0.023 – 0.000 – – 0.001 – 0.001 0.002 0.000 0.000 0.001 – Fe3 0.018 0.025 0.012 – 0.001 0.001 0.053 – – 0.02 K2 0.001 – – 0.013 – – – – – – Total .000 .001 .00 .000 .000 3.000 2.993 3.003 2.998 2.999 3.007 2.989 .000 .000 3.991 3.99 .000 Mg 0.935 0.858 0.902 0.892 0.891 0.856 0.857 0.823 0.822 0.828 0.819 0.77 continued

TABLE 3. MINERAL ANALYSES FROM THE 91-MILE PERIDOTITE continued Spinel group minerals Amphiboles Phlogopite interstitial Sample S99- S99- S99- S99- S99- S99- S99- S02-91- S02-91- S02-91- S99- S99- S02- S02- S5- 91-2 91-2 91-2 91-2 91-2 91-2 91-7? P1l P1l P1l 91-2 91-2 91-P1 91-P1 91.1-8

SiO2 0.09 0.09 0.12 0.101 0.18 .8 3.5 7.03 .53 6.63 1.01 0.02 1.1 0.55 1.05

Al2O3 3.21 12.86 2.1 9.23 12.02 12.01 11.57 9.77 9.26 9.61 15.18 1.79 1.53 1.75 15.02 FeO 8.3 22. 25.28 26.97 27.83 .36 8.97 0.68 2.29 2.58 3.95 .59 5.60 5.2 5.26 MgO 0.85 2.5 8.18 3.38 3.18 16.92 10.79 19.09 17.71 18.2 25.10 2.51 23.8 23.62 2.57 MnO 0.3 0.50 0.38 0.6 0.5 0.09 0.33 0.15 0.09 0.10 0.03 0.02 0.03 0.10 0.02 CaO 0.03 0.12 0.06 0.23 0.09 11.77 11.1 11.95 12.05 12.11 0.01 0.0 0.03 0.03 0.00

Na2O 0.09 0.10 na na na 3.51 1.7 3.05 3.00 3.11 0.68 0.62 0.87 0.88 0.53

TiO2 2.16 0.16 0.02 2.56 1.5 0.2 0.88 0.51 1. 0.67 0.62 0.56 1.3 1.57 1.00

Cr2O3 12.68 38.33 18.99 2.93 31.77 0.27 0.07 0.27 0.88 0.36 0.50 0.9 0.1 0.8 0.19

Fe2O3 72.02 20.87 3.08 31.67 21.75 3.25 8.9 5.51 6.76 .3 na na na na na

K2O 0.17 0.87 na na na 0.61 0.00 0.00 0.00 0.00 9.81 10.07 10.92 10.62 10.80 Total 99.97 98.80 98.52 99.53 98.81 97.87 97.95 98.00 98.00 98.00 96.90 9.70 98. 98.02 98. Si 0.003 0.003 0.00 0.00 0.006 6.21 6.398 6.61 6.382 6.605 2.85 2.839 2.861 2.830 2.83 Al3 0.13 0.532 1.81 0.382 0.9 2.028 2.003 1.619 1.56 1.60 1.25 1.237 1.191 1.21 1.226 Fe2 0.26 0.659 0.627 0.792 0.812 0.522 1.102 0.080 0.275 0.305 0.230 0.272 0.326 0.317 0.30 Mg2 0.08 0.128 0.362 0.177 0.165 3.613 2.363 .003 3.78 3.890 2.60 2.593 2.35 2.58 2.537 Mn2 0.011 0.015 0.010 0.01 0.013 0.011 0.01 0.017 0.011 0.011 0.002 0.001 0.002 0.006 0.001 Ca2 0.001 0.00 0.002 0.009 0.003 1.806 1.796 1.801 1.850 1.838 0.001 0.003 0.003 0.002 – Na2 0.003 0.003 – – – 0.97 0.20 0.831 0.833 0.85 0.092 0.085 0.118 0.119 0.072 Ti 0.061 0.00 0.001 0.068 0.00 0.026 0.097 0.05 0.155 0.071 0.033 0.030 0.075 0.082 0.052 Cr3 0.379 1.065 0.5 0.692 0.876 0.030 0.008 0.029 0.099 0.00 0.028 0.027 0.022 0.026 0.010 Fe3 2.09 0.552 0.069 0.837 0.571 0.350 0.988 0.583 0.729 0.72 K2 0.005 0.022 – – – 0.112 0.000 0.000 0.000 0.000 0.870 0.911 0.969 0.96 0.95 Total 2.967 2.989 2.999 2.973 2.983 15.892 15.215 15.632 15.682 15.692 7.958 7.997 8.000 8.000 8.000 Mg 0.86 0.82 0.807 0.813 0.82 Note: alues for maor elements are given in wt. Element values are cations per formula unit. nanot analyed.

100.00 100.000

Mile 91 Olivine Mile 91 Phlogopite

10.00 10.000

1.00 1.000

0.10 0.100

0.01 0.010 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100.0 Mile 91 - Diopside 100.0000 Mile 91 Opx

10.0000

10.0 1.0000

0.1000

0.0100 1.0 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Figure 8. Rare earth element concentrations of olivine, phlogopite, diopside, and orthopyroxene (Opx) in the 91-Mile peridotite, normalized to chondritic values of McDonough and Sun (1995). Diopside crystals from the 91-Mile peridotite are relatively depleted in both light rare earth elements (LREEs) and heavy rare earth elements (HREEs), resulting in an N-shaped pattern somewhat similar to patterns of diopside crystals from Finero and Vulture (Italy), both in a subduction setting.

TABLE . TRACE-ELEMENT AND RARE EARTH ELEMENT MINERAL ANALYSES FROM THE 91-MILE PERIDOTITE Sample S96-91-13ib S99-99-1 S99-91-1 S99-91-2 Phase olivine op olivine op cp phlog olivine cp phlog olivine cp phlog La 0.02 0.03 0.2 0.0 5.1 0.2 0.2 3.2 0.1 bdl 7.7 0.1 Ce 0.05 0.09 0.2 0.2 15.9 0.6 0.6 12.0 0.1 0.0 25.0 0.0 Pr 0.0 0.01 0.1 0.0 3.2 0.1 0.1 2. 0.0 bdl .8 bdl Nd 0.05 0.07 0.1 0.1 18.0 0.5 0.5 12. 0.2 0.2 23. 0.0 Sm 0.06 0.0 0.2 0.1 5. 0.2 0.1 3.9 0.1 bdl 7.5 0.1 Eu 0.02 0.02 0.0 0.0 1. 0.1 0.1 1.0 2.0 na 1.7 1.6 Gd 0.02 0.0 0.0 0.2 5.7 0.2 0.1 .0 bdl bdl 6.7 bdl Tb 0.01 0.01 bdl 0.0 0.8 0.0 0.0 0.6 bdl bdl 1.0 bdl Dy 0.02 0.03 0.0 0.3 .8 0.3 0.2 3.7 0.0 0.0 5.8 bdl Ho 0.01 0.01 0.0 0.1 0.9 0.1 0.0 0.8 0.0 0.0 1.0 bdl Er 0.02 0.01 0.1 0.3 2.6 0.2 0.1 2.6 0.0 bdl 2.7 bdl Tm bdl 0.01 0.0 0.1 0.3 0.0 0.0 0. bdl bdl 0. bdl Yb 0.05 0.0 0.2 0.7 2.0 0.5 0.1 2.0 0.0 0.0 2.2 bdl Lu bdl 0.0100 0.0 0.1 0.3 0.1 0.0 0.3 bdl 0.0 0.3 0.0 Sr 1.23 1.88 7.3 1.5 335.5 9.1 6.7 150.3 96.5 0.6 23. 81.7 Rb na na na na na na 0.6 3.3 316.7 bdl 1.2 331.1 Ba 0.09 0.07 1.9 0.3 7. 8.1 7.2 3.3 5386.5 0. 1.9 600.8 Th 0.02 0.03 bdl 0.0 0.8 0.5 0.1 0.8 0.0 0.0 1.1 bdl Nb na na 0.06 0.01 0.01 0.03 0.0 0.1 6.32 bdl 0.06 5.76 Pb na na na na na na 2.1 2.1 17.1 0.1 3.6 16.9 r 1.2 0.2 1.0 3.3 23.9 1.9 1.3 31.2 6.6 bdl 26.7 7.2 Hf na na 0.0 0.2 1.5 0.1 0.0 1. 0.3 0.0 1.0 0.2 Y 0.0 0.1 0.1 2.5 22.8 1.5 0.9 18.1 0. 0.1 28.3 0.3 n na na na na na na 292.7 58.0 87.1 253.8 16.1 82.9 Ni na na 1680.6 360. na na 666.1 131.8 35.3 807.7 96.8 33.1 Cr na na 2.9 138. na na 1801.1 1219.1 115.0 53.0 2222.7 1691.7 na na na na na na 36. 83.0 39.8 1.5 11.6 280.8 Cs 0.0 0.0 na na na na 0.0 0.1 2.3 0.1 0.1 3.5 0.0 0.0 0.0 0.0 0. 0.1 0.1 0.2 0.0 0.0 0.5 0.0 Ta na na 0.0 0.0 0.0 0.0 bdl 0.1 0.7 na 0.0 0.5 Be 95. 102.1 na na na na na na na na na na Co na na 163. 56. na na 220.2 1.6 70.1 215.2 33.7 66.1 Li na na na na na na 10. 11.6 15.5 15.1 13.7 12.3 Sc na na na na na na 11. 52. 12.3 9.3 86.7 10.1 Cu na na na na na na 132.8 7.7 32.6 1.7 85.8 19.6 Lab MN MN MN MN MN MN B B B B B B La/Nd 0.7 0.1 1.36 0.17 0.28 0.3 0.36 0.26 0.33 bdl 0.33 1.75 Sm/Yb 1.20 1.00 1.06 0.12 2.72 0.1 0.6 1.96 9.00 bdl 3.50 bdl Notes: alues for elements are given in ppm; oporthopyroene; cpclinopyroene; phlogphlogopite; nanot analyed; bdlbelow detection limit. Labs: MNMemorial niversity, Newfoundland; BBoston niversity, Massachusetts.

Amphibole. Amphibole is generally a minor component in most samples, last) in the crystallization sequence. Plagioclase crystals are also generally but it is abundant (up to 44 modal percent) in several samples. Three textural found in contact with interstitial amphibole and are always separated from varieties were observed: (1) interstitial amphibole that surrounds olivine and olivine by a layer of amphibole and bronzite and from coronitic bronzite by a pyroxene; (2) amphibole that is symplectically intergrown with orthopyroxene layer of amphibole. It was not observed in contact with diopside. Plagioclase in coronas around olivine or occurs as inclusions in olivine; and (3) amphi- crystals are unzoned, with compositions ranging from albite to oligoclase

bole that has partially replaced diopside (Fig. 5D). The three textural types are (An5-An14 Or0.3-Or1 Ab85-Ab93 with an average of An9Or0.6Ab90). somewhat compositionally distinct. Both interstitial amphibole and symplectic/ Magnetite (Mt)-chromite (Cr)-spinel (Sp). Oxide minerals are magne- inclusion amphibole are pargasite, magnesiohastingsite, or edenite. Amphi- tite-dominated solid solutions with varying amounts of chromite end member

bole that replaces diopside is magnesiohornblende. It has high Mg# (0.88–0.92), (Fig. 9; Table 3). Magnetite compositions range from Mt79Sp6Ct15 to Mt25Sp25Ct50. although slightly lower than those of diopside grains in the same samples. Magnetite solid solutions define a trend generally parallel to the Fe-Ti trend of Mg# values of interstitial amphiboles are notably lower (0.80–0.85) (Table 3). Barnes and Roeder (2001), which has been attributed to evolution of spinel com- A third compositional type, tschermakitic amphibole, with Mg# 0.54–0.56, was positions during fractional crystallization of olivine and pyroxene, commonly observed in only one sample. accentuated by reaction with intercumulus magma (cf. Barnes and Roeder, 2001). Plagioclase. Plagioclase occurs in only the most evolved cumulate samples. Spinel-chromite phases were identified in two contexts. First, a dunite All observed plagioclase is interstitial. It appears to have formed late (possibly enclave from near the base of the exposure hosts spinel that plots midway

Al Hercynite

spinel-chr

In olivine omi

te solid sol

Cr-Al-Trend

Figure 9. Spinel mineral compositions from 91-Mile peridotite, using magnetite-spinel trends defined by u tion Barnes and Roeder (2001). See text for discussion; serp—serpentine; cpx—clinopyroxene.

Fe-Ti-Trend

In serp

In cpx In dunite

In olivine lherzolite Fe Magnetite Chromite Cr

between the chromite and spinel end members, close to the Cr-Al trend of the Rb and Ba enrichment is consistent with interaction with slab-derived LILE- Barnes and Roeder (2001). Second, magnetite inclusions in olivine have rich fluids, supporting the interpretation of subduction-related magmatism. exsolved spinel with a composition between the chromite and spinel end Several processes are known to cause enrichment in LREEs in peridotites

members. Spinel compositions are approximately Mt0.11Sp0.60Ct0.28. Thus, spinel and specifically in clinopyroxene. These include: (1) interaction with more fel- group minerals that crystallized earlier (inclusions in olivine) have a higher sic silicate magmas (Nielson and Noller, 1987; Downes, 2001), (2) interaction concentration of spinel end member (Cr-Al trend?) than those that crystallized with hydrous subduction-related fluids (Hartmann and Hans Wedepohl, 1993; later and follow the Fe-Ti trend. Zanetti et al., 1999; Downes, 2001), and (3) interaction with mantle-derived carbonate magmas (Yaxley et al., 1991; Ionov et al., 1993, 1997; Downes, 2001). Passage of silicate magmas or fluids through the mantle by flow along grain ■■ DISCUSSION: TECTONIC SETTING OF THE 91-MILE PERIDOTITE boundaries can result in hornblende- or pyroxene-rich veins, or hornblende and/or clinopyroxene pseudomorphs of primary mantle minerals (Navon and The Proterozoic orogen of southwestern North America preserves a record Stolper, 1987; Kelemen et al., 1990; Takazawa et al., 1992). Partial melting of of the accretion and evolution of much of the midsection of the North Amer- the altered rocks could then give rise to LREE-enriched basaltic melts, similar ican continent. Ultramafic rocks have been suggested to mark the location of to the 91-Mile parent, wherein major-element concentrations are not strongly major boundaries or even sutures between orogenic blocks or provinces. The affected, but enrichment in LREE, Sr, Zr, Hf, and possibly Nb occurs (Downes, 91-Mile peridotite is by far the largest and least altered of the known ultramafic 2001). The pervasive Nb depletion of the 91-Mile parent is also consistent with bodies in the Southwest and as such provides an opportunity to assess the an origin in a subduction setting. provenance and tectonic significance of at least this one body. REE patterns of clinopyroxene crystals have been used as indicators of Perhaps the most fundamental observation/conclusion is that the 91-Mile tectonic settings. Clinopyroxene from ultramafic massifs is generally LREE peridotite is a cumulate derived by layered accumulation of crystals from an igneous parent. Primary igneous cumulate phases include olivine, orthopyroxene, clinopyroxene, magnetite/spinel, and minor late-stage plagioclase. TABLE 5. CALCLATED PARENT MELT COMPOSITIONS FROM Phlogopite crystallized mainly as an intercumulus phase. Serpentine and amphi- CLINOPYROENE CP ANALYSES bole occur primarily as replacements of olivine and pyroxene, respectively. Cp analyses KD Parent composition cp-1 cp-2 cp-3 cp/melt Parent-1 Parent-2 Parent-3 Parent Melt La 3.2 7.67 6.12 0.0536 60 13 11 Ce 11.98 2.98 23.01 0.0858 10 291 268 Pr 2.38 .78 3.78 0.1 17 3 27 Compositional characteristics of the melt from which the cumulates crys- Nd 12.38 23.3 19.99 0.1873 66 125 107 tallized are key factors in evaluating the tectonic setting in which the peridotite Sm 3.86 7.53 5.9 0.291 13 26 20 originated. Bulk-rock trace-element variation and REE patterns are shown in Eu 0.95 1.73 1.52 0.32 3 5 5 Figure 7 relative to mantle peridotites and chondrites (from McDonough and Gd .0 6.71 7.38 0. 10 17 18 Sun, 1995). The 91-Mile ultramafic rocks are depleted in Nb and Zr and enriched Tb 0.6 0.97 1 0.2 1 2 2 Dy 3.68 5.75 5.88 0.2 8 13 13 in Ba, similar to subduction-fluxed, phlogopite-bearing xenoliths from North Er 2.59 2.68 3.01 0.387 7 7 8 Hesse, Germany, and from Vulture, Italy (Downes, 2001). Yb 1.97 2.15 2.9 0.3 5 5 6 Because the bulk of the exposure consists of cumulates, characteristics of Sr 150.31 23. 337 0.1283 1172 1827 2627 REE patterns depend on relative proportions of cumulate minerals, and as a Ba 3.28 1.88 11.39 0.002 21,60 70 5695 result, the individual mineral compositions may be more useful petrogenetic Th 0.78 1.06 1.17 0.012 65 88 98 indicators than the bulk rocks. Trace-element and REE concentrations in diop- Nb 0.1 0.06 0.1 0.0077 53 8 18 r 31.19 26.65 3.5 0.123 253 216 353 side crystals from the body provide an opportunity to calculate, at least to a first Hf 1.35 1.03 2.01 0.256 5 8 order, the trace-element and REE concentrations of the melt from which the Y 18.08 28.26 36.19 0.67 39 61 77 diopside crystallized. Experimentally determined clinopyroxene/melt partition Ta 0.05 0.02 0.0 0.019 3 1 2 coefficients of Hart and Dunn (1993) and Sobolev et al. (1996) were used for K 0.362 0.26 na 0.0072 50.28 59 na the calculation (Table 5). The calculated melt composition (Fig. 10) is similar to Ti 0.238 0.1 na 0.38 0.62 1.07 na bulk-rock cumulate analyses, with LREE enrichment and HREE depletion (Fig Notes: alues for elements are given in ppm. Distribution coefficients KD are from 10A) and strong depletion in Nb, Zr, and Ti and enrichment in Ba (Fig. 10B). The Hart and Dunn 1993, with the eception of the distribution coefficient for Ba, which is from Sobolev et al. 1996. nanot analyed. negative Nb and Zr anomalies are typical of subduction-related basalts, and

depleted, similar to diopside from the 91-Mile body. N-shaped REE patterns of clinopyroxene, like those of the 91-Mile body, are relatively rare; Downes 1000 (2001) suggested that the pattern is consistent with subsolidus redistribution of REEs with coexisting garnet. Garnet is common in the host Vishnu Schist,

but it is not present within the 91-Mile peridotite. e Other trace-element concentrations can be used to identify interaction with i t 100 nd r

subduction-related fluids. Hydrous fluids commonly transport LILEs, so the o h C enrichment in Rb and Ba of the 91-Mile peridotite parent basalt is consistent / l e with its origin in a subduction setting. HFSEs, such as Nb and Zr, are insolu- p 10

ble in hydrous fluids, and they are typically depleted in basalts generated in Sa m subduction settings, which is also consistent with the modeled parent basaltic Clinopyroxene magma for the 91-Mile peridotite. Xenoliths from Vulture (central Italy) are samples of crust overlying a subduction zone. They are phlogopite-rich fea- 1 tures, most likely as a result of interaction with K-bearing subduction zone La Ce Pr Nd Sm Eu Gd Tb Dy Er Yb hydrous fluid (Downes, 2001). We suggest that K-enriched subduction-related fluid in the mantle wedge gave rise to a K-rich, hydrous, high-pressure partial melt that produced early magnetite, Al-rich diopside, and the phlogopite of 10000.0 the 91-Mile peridotite. 1000.0

1le 00.0 Pressure of Crystallization and Abundance of Water in the Parent Melt t a n M 10.0 We attempted to constrain the pressure of crystallization of the 91-Mile e i ti v peridotite. Both the order of crystallization of minerals and the composition 1.0

of the minerals provide information about pressure and temperature condi- /P r i m l e

tions of formation. At low pressure, the order of crystallization of minerals in p 0.1 Clinopyroxene

a primitive basalt is expected to be olivine, plagioclase, clinopyroxene, and Sa m 0.0 orthopyroxene, because the crystallization of olivine depletes the liquid in Mg Ba Th Nb K La Ce Pr Sr Nd Zr Sm Eu Ti Dy Y Yb before clinopyroxene crystallizes, leaving the Mg# of clinopyroxene relatively low (<82) (Elthon et al., 1982; Bağci et al., 2006). In contrast, clinopyroxene Figure 10. (A) Rare earth element and (B) trace-element concentra- crystals with higher Mg# suggest higher-pressure (>0.7 GPa) crystallization. tions of diopside crystals and of the calculated parent melt from Presnall et al. (1978) showed that the diopside field expands with increasing which they crystallized. See Low (2009) for further explanation. The calculated melt is strongly enriched in light rare earth elements, pressure in the Di-Fo-An system, until at >0.7 GPa, plagioclase is no longer similar to both subduction-related basalts and ocean-island basalts. stable. As pressure increases, the amount of olivine that crystallizes prior to The strongly negative Nb and Zr anomalies of the parent melt are crystallization of diopside decreases, and the Mg# of olivine in equilibrium with typical of subduction-related basalts, and the Rb and Ba enrichment diopside increases. Rock types that are typical of low-pressure crystallization is consistent with interaction with slab-derived large ion lithophile element–rich fluids. Blue solid lines—clinopyroxene composition; of basalt include dunites, troctolites, and olivine gabbros, because plagioclase black solid lines—calculated parent composition; dashed line—ref- crystallizes early, along with olivine, at low pressures. High-pressure crys- erence arc basalt composition (A—Cascades average; B—Cascades tallization of basalt produces dunite, wehrlite, clinopyroxenite, websterite, and Costa Rica averages), from Turner and Langmuir (2015). and lherzolite, all with high Mg# (Elthon et al., 1982; Elthon, 1992; Parlak et al., 1996, 2000) and all characteristic of the 91-Mile peridotite. At pressure >1.0 GPa, clinopyroxene crystallizes before plagioclase (Presnall et al., 1978). Thus, the modes of silicate minerals, the sequence of crystallization, and the inclusions in olivine have a higher concentration of spinel, more consis- lack of early plagioclase all support crystallization of the 91-Mile peridotite at tent with the Cr-Al trend (Fig. 9; Irvine, 1967). Barnes and Roeder (2001) relatively high pressures, probably on the order of 1.0 GPa. suggested that the Cr-Al trend in high-pressure plutonic settings is prob- Magnetite-chromite solid solution phases in the 91-Mile peridotite follow ably the result of equilibration between Al-bearing pyroxene and Mg- and the Fe-Ti trend of Barnes and Roeder (2001), while (early?) spinel group Al-rich spinel minerals, consistent with the apparent evolution from Al-richer

to Al-poorer spinel phases in the 91-Mile peridotite. Further, they showed Ultramafic Rocks and Crustal Boundaries in the Grand Canyon that solid solutions on the Cr-Al trend are indicative of magmas derived from melting of primitive mantle and that they are typical of mantle and The 91-Mile peridotite is surrounded by a series of metamorphosed turbid- lower-crustal rocks. The progression of spinel group compositions in the ites and pelitic sedimentary rocks (the Vishnu Schist) and volcanic rocks (the 91-Mile peridotite is consistent with crystallization in a high-pressure setting Rama and Brahma Schists) that were deposited ca. 1750 Ma in an arc setting and from a melt of mantle origin. This is also compatible with Hf data showing then intruded by 1750–1710 Ma granodioritic plutons before being tectonized that plutons east of the Crystal shear zone are largely juvenile in character during the ca. 1700 Ma Yavapai orogeny. The combination of basalt/dacite vol- (Holland et al., 2915). canic rocks, turbidite host rocks, and calc-alkaline granodiorite to gabbro/diorite Diopside Mg# values in the 91-Mile peridotite range from 0.87 to 0.93, which, plutons is typical of lithologies of modern subduction settings, consistent with like the olivine, are in the range of those of other high-pressure peridotites. the convergent margin interpretation of the Yavapai orogeny. Plutons east of

Further, the Al2O3 concentrations of diopside, relative to Mg#, are consistent the Crystal shear zone have Hf isotope compositions indicative of their deriva- with the trend defined by Medaris (1972) as the high-pressure crystallization tion from ca. 1.75 Ga juvenile mantle. Peak metamorphic pressures of the rocks trend. However, it is important to note that the high Mg# values of olivine and intruded by the plutons are on the order of 0.6–0.7 GPa (Dumond et al., 2007), diopside in the 91-Mile peridotite could, at least in part, also be a consequence but these were achieved after arc pluton emplacement during crustal thicken- of crystallization from a hydrous magma; high magmatic water concentra- ing. Even the granodiorite bodies have vertical tabular shapes and tectonic tions lead to high Fe3+/Fe2+ and thus higher Mg# in the crystallizing minerals contacts more similar to tectonic slices than intruded plutons. The geometry of (Berndt et al., 2005; Botcharnikov et al., 2005; Feig et al., 2006; Gahlan et al., the relatively small and lenticular 91-Mile peridotite suggests tectonic dismem- 2012). The abundant magnetite inclusions in diopside strongly suggest that bering of a larger arc pluton, and the possible intrusive relationships suggest the melt was highly oxidized. late synmagmatic emplacement and juxtaposition against supracrustal rocks. In summary, several lines of evidence, including the lack of plagioclase, Compositional characteristics of the tectonically emplaced 91-Mile peridotite the order of crystallization of minerals, the high Mg# of olivine and diop- are also consistent with an origin in a subduction setting, although we would side, and the evolution of spinel group mineral compositions, all suggest suggest at a deeper level (~30 km) than that at which the sedimentary and that the 91-Mile peridotite crystallized at significant pressures. The presence volcanic rocks accumulated or underwent prograde metamorphism. of late phlogopite as well as the abundant magnetite inclusions in diopside One of the goals of this research was to evaluate if the ultramafic rocks crystals and the high Mg# of olivine and clinopyroxene all suggest that the that are sporadically present throughout the Proterozoic orogen could be parent magma was relatively hydrous. Without the influence of water, one oceanic (i.e., MORB) ophiolite fragments and thus indicators of major accre- might confidently suggest pressures on the order of 1 GPa, but the pressure tionary structures separating disparate terranes. Because the 91-Mile peridotite could be somewhat lower in this hydrous magma. We cautiously conclude was most likely formed in the lower level of a magmatic arc, we reject this that the combination of moderately high-pressure crystallization, a hydrous ophiolite interpretation and the hypothesis that the ultramafic rocks can be LILE-enriched parent melt, and trace-element evidence for interactions with used to identify boundaries between exotic tectonic (continental) fragments. slab-derived fluids are all consistent with crystallization in deeper levels of However, there is no evidence that the host rocks to the 91-Mile peridotite an arc-related magma system. were ever buried to pressures greater than 0.6–0.7 GPa (Dumond et al., 2007), One additional setting to be considered for the 91-Mile peridotite is a back- and probably significantly less, during arc plutonism. If the 91-Mile peridotite arc (i.e., suprasubduction ophiolite) setting. Although this setting may not be crystallized at higher pressure, at least some degree of tectonic juxtaposition entirely excluded, we strongly prefer the arc itself because of the very hydrous is indicated. Some juxtaposition is also suggested by the fact that no other nature of the magmas, the pressure of crystallization, and the associated arc-re- pluton with cumulate layering has been seen in the well-studied and 100% lated plutonic and volcanic rocks. Further, the main tectonic conclusions (see exposed Upper, Middle, or Lower Granite Gorges of the Grand Canyon. The below) reflect the fact that the 91-Mile peridotite almost certainly did not form parallelism of the 91-Mile peridotite cumulate layering with early S1 foliation in a MORB ophiolite setting. and D1 structures suggests that the juxtaposition may have been associated The apparent intrusive contact along one margin of the 91-Mile peridotite with early accretion-related (D1) thrusts in an accretionary complex, as pro- is still somewhat of a puzzle. The planar magmatic layering and the otherwise posed by Holland et al. (2015). Thus, the 91-Mile peridotite may indeed mark the sharp and discordant contacts suggest that the body was emplaced as a rela- location of a significant structure within the Proterozoic orogen, a conclusion tively intact rigid body. The possible intrusive textures occur along only one consistent with its location near the inferred Mojave-Yavapai boundary region. contact and have been somewhat obscured by later deformation. If the contact Several other, generally small and highly altered, ultramafic bodies occur does indeed preserve an intrusive relationship, we would suggest that the in or near shear zones or interpreted tectonic boundaries (Fig. 1). Based on body may have been emplaced as a nearly solid mush, but that some magma the results from the 91-Mile peridotite and also on the fact that no arc-related (crystals and liquid) was expelled along this particular contact. plutons with cumulates have been recognized at the current level of exposure,

we suggest that the association of ultramafic blocks and shear zones or sus- Daniel, C.G., Pfeifer, L.S., Jones, J.V., and McFarlane, C.M., 2013, Detrital zircon evidence for non-Lau- rentian provenance, Mesoproterozoic (ca. 1490–1450 Ma) deposition and orogenesis in a pected tectonic boundaries is more than coincidence. The ultramafic bodies reconstructed orogenic belt, northern New Mexico, USA: Defining the Picuris orogeny: Geolog- may signal the presence of structures capable of juxtaposing deeper levels ical Society of America Bulletin, v. 125, no. 9–10, p. 1423–1441, https://doi .org /10 .1130 /B30804 .1 . of the arc crust with the current midcrustal exposure. Further, the presence Downes, H., 2001, Formation and modification of the shallow sub-continental lithospheric mantle: A review of geochemical evidence from ultramafic xenolith suites and tectonically emplaced of the ultramafic bodies may indicate the general regions to explore for other ultramafic massifs of western and central Europe: Journal of Petrology, v. 42, no. 1, p. 233–250, exotic rocks from deeper in the orogen. https://doi.org/10.1093/petrology/42.1.233. Duebendorfer, E.M., Chamberlain, K.R., and Fry, B., 2006, Mojave-Yavapai boundary zone, southwestern United States: A rifting model for the formation of an isotopically mixed crustal boundary zone: Geology, v. 34, no. 8, p. 681–684, https://doi.org/10.1130/G22581.1. ACKNOWLEDGMENTS Dumond, G., Mahan, K.H., Williams, M.L., and Karlstrom, K.E., 2007, Crustal segmentation, compos- Sheila Seaman passed away on 27 July 2019. Her work on igneous rocks of the Grand Canyon, and ite looping pressure-temperature paths, and magma-enhanced metamorphic field gradients: specifically on the 91-Mile peridotite, benefited from interaction with and participation of many Upper Granite Gorge, Grand Canyon, USA: Geological Society of America Bulletin, v. 119, students and colleagues from the University of Massachusetts and the University of Colorado. p. 202–220, https://doi.org/10.1130/B25903.1. This research was facilitated by grant EAR-0003477 from the NSF Tectonics Program (to Seaman Elthon, D., 1992, Chemical trends in abyssal peridotites: Refertilization of depleted suboceanic and Karlstrom) and Grand Canyon National Park Research and Collecting permits. Steve Turner mantle: Journal of Geophysical Research–Solid Earth, v. 97, no. B6, p. 9015–9025, https://doi and two anonymous reviewers made positive, helpful comments that significantly strengthened .org/10.1029/92JB00723. the manuscript. 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