RICE UNIVERSITY
Meteorite evidence for deep crustal magma chambers on Mars suggests crustal growth driven by underplating and intrusion
by
Heather Anne Dalton
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE
Master of Science
APPROVED, THESIS COMMITTEE:
Cin-Ty Lee, A^ociate Professor Earth Science
Raj deep Dasgupta, Assistant Professor Earth Science
rian Lenardic/Professor Earth Science ,
Dale Sawyer, Professor Earth Science
HOUSTON, TEXAS JANUARY 2010 UMI Number: 1485997
All rights reserved
INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted.
In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMI Dissertation Publishing
UMI 1485997 Copyright 2010 by ProQuest LLC. All rights reserved. This edition of the work is protected against unauthorized copying under Title 17, United States Code.
ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 ABSTRACT
Meteorite evidence for deep crustal magma chambers on Mars suggests crustal growth
driven by underplating and intrusion
by
Heather Anne Dalton
Meteorite RBT04262 is one of only two Martian meteorites made of composite lithologies. Lithology 1 is composed of coarse-grained olivines enclosed inpoikilitic pigeonites, resembling lherzolitic shergottites. Lithology 2 is finer-grained and composed of olivine phenocrysts set within a groundmass of augite, olivine, plagioclase
(shocked to maskelynite) and accessory phases such as Ca-phosphates, representing an olivine-phyric shergottite. Lithology 1 may be an early-formed cumulate while Lithology
2 may represent a cooled liquid laden with accumulated olivine crystals. Geochemical and textural observations suggest that all components of RBT04262 formed as intrusions.
As the majority of Martian meteorites also have a cumulate origin, it is speculated here that crust formation on Mars may be dominated by magmatic underplating or intrusion rather than by subaerial flows. An intrusive-origin for Martian crust has important implications as intrusions can reset the radiometric ages or magnetic signatures of the
Martian crust without erasing the cratering history. ACKNOWLEDGMENTS
Dr. Cin-Ty Lee, my advisor, and my committee members, Dr. Rajdeep Dasgupta,
Dr. Adrian Lenardic, and Dr. Dale Sawyer were a tremendous help in completing this project. Dr. Anne Peslier very generously gave her time and assistance at the electron microprobe and in reviewing the manuscript. Dr. Alan Brandon provided the thick section studied, and he and Dr. Thomas Lapen gave input on the project. Dr. Tobias
Hoink and Dr. Peter Luffi were great officemates and provided insightful discussions about my project, as well as less professional subjects. Sandra Fleshig, Soookie Sanchez, and Mary Ann Lebar were very helpful in all things administrative. Dr. Chris Barker and
Dr. Volker Gobel gave me my introduction to petrology at Stephen F. Austin State
University and have always encouraged me to excel. Dr. Allen Treiman gave me my start in planetary science when I was an intern at the Lunar and Planetary Institute and continues to offer support and guidance.
I would like to thank my mother, father, brother, and grandmother for always believing in me. My cousin Erin Haid has offered tremendous support and encouragement and has always been there for me when I needed her. Brian Bue and
Morgan Gallagher have been my confidants and shoulders to lean on throughout my time at Rice. I would like to thank Rebekah Hines for her constant encouragement, banter, and laughter. I could not have come as far as I have without Sara Manning, who has been my rock for more than a decade. Emily Bjonnes has been patient, helpful, and kind throughout this process, and I could not have done it without her. Lastly, I would like to thank my cats, Kepler, Tater Tot, and Apollo for always keeping me laughing. TABLE OF CONTENTS
Page
LIST OF TABLES vi
LIST OF FIGURES vii
CHAPTER 1 INTRODUCTION 1
CHAPTER 2 METHODS 2
CHAPTER 3 PETROGRAPHY AND MINERAL
CHEMISTRY 4
Lithology 1 6
Lithology 2 12
Reaction zone between Lithologies 1 and 2 17
Inclusions in megacrysts 17
Shock-related features 18
CHAPTER 4 TRACE ELEMENT GEOCHEMISTRY 19
Silicate minerals 26
Ca-phosphate (merrillite) 29
Oxides 30
CHAPTER 5 CRYSTALLIZATION SEQUENCE 30
CHAPTER 6 TEMPERATURE AND OXYGEN FUGACITY 31
CHAPTER 7 COMPARISON OF RBT04262 WITH OTHER SHERGOTTITES 34
CHAPTER 8 INTERPRETATIONS 36
RBT04262 represents a crystallized magma containing cumulate
clasts 36 V
Petrogenetic model and evidence for a deep magma chamber 38
Implications for Martian crust-formation, rejuvenation, atmospheric
evolution 41
CONCLUSIONS 44
REFERENCES 46 LIST OF TABLES
Table Page
1. Major element composition of Lithology 1 7
2. Major element composition of Lithology 2 and reaction rim between Lithologies 1 and
2 9
3. Trace element composition of Lithology 1 20
4. Trace element composition of Lithology 2 22
5. Calculated temperatures and oxygen fugacities 33 LIST OF FIGURES
Figure Page
1. Backscattered electron image of RBT04262 thick section 5
2. Al Ka, Ca Ka, Fe Ka, and Mg Ka X-ray maps of RBT04262 thick section 6
3. AlKa, CaKa, Fe Ka, and Mg Ka X-ray maps of Lithology 1 8
4. AlKa, CaKa, Fe Ka, and Mg Ka X-ray maps of Lithology 2 10
5. Ternary diagram of pyroxene compositions present in RBT04262 11
6. Plot of FeO and MgO content of RBT04262 12
7. Backscattered electron image of a portion of Lithology 2 13
8. Ternary diagram of feldspar compositions present in Lithology 2 15
9. Profiles of C^Oa, T1O2, and FeO across an ulvospinel crystal in Lithology 2 16
10. Q2O3 and Ti02 contents of ulvospinels within minerals in Lithologies 1 and 2 16
11. B SE image of melt inclusions in olivines in Lithology 1 and Lithology 2 18
12. Representative REE patterns of phases in RBT04262 19
13. Chemical compositions of silicate phases inRBT04262 27
14. Plot of REE compositions of olivine divided by augite and pigeonite divided by
augite in Lithology 2, both cores and rims, and olivine divided by pigeonite in
Lithologies 1 and 2 28
15. Comparison of REE concentrations in RBT04262 with other shergottites relative to
CI chondrite reference 35
16. Cartoon of crystallization sequence of RBT04262, with locations of Lithologies 1 and
2 labeled 41 1
1. Introduction
One of the key questions in planetary geology is how planetary crusts form and evolve. Are planetary crusts formed from above by effusive outpourings of lavas or are they formed from below by underplating or intrusions? On Earth, both extremes of crust formation are represented: oceanic crust at mid-ocean ridges is largely formed by the extrusion of basaltic magmas while continental crust is formed by a combination of extrusive volcanism and the emplacement of plutons and sills above subduction zones or collisional belts. Of interest here are the crust formation mechanisms on Mars.
Extrusive- versus intrusive-dominated scenarios have profoundly different implications for Mars. For example, if the Martian crust is formed primarily from below, one might expect inefficient degassing of volatiles from the planet. In contrast, if the Martian crust is built primarily by extrusive volcanism, efficient degassing would be expected. On
Earth (Allegre et al., 1996), the 40Ar/36Ar signature of the Martian atmosphere has been used to put constraints on the extent of degassing from the Martian mantle (Bogard et al.,
2001; Mathew et al, 2003). Because it is usually assumed that degassing is controlled by magmatism, degassing should accompany crust formation. However, if magmas are emplaced at depth where magmas do not degas, the relationship between degassing and crust formation is no longer so clear. Finally, intrusive versus extrusive crustal build-up may have different implications for thermal resetting of internal radiogenic isochrons and the magnetic properties of the Martian surface. In particular, magma emplacement mechanisms may be important in understanding the apparent incompatibility between young (<500 My) radiometric ages of many Martian meteorites and the dominance of very old (Gy) cratered surfaces on Mars. 2
Martian meteorites may shed light on crust formation mechanisms because many of these meteorites were probably excavated from the Martian subsurface, potentially providing windows into different levels of the Martian crust. Here, we investigate in detail the petrogenesis of Martian meteorite RBT04262, a composite meteorite composed of two distinct lithologies separated by a reaction zone. The meteorite was found by the
2004-2005 Antarctic Search for Meteorites (ANSMET) team in the Roberts Massif region. It weighs 204.6 grams and has dimensions of 6.5 x 5.5 x 3.5 cm. RBT04262 has been paired with RBT04261, 4.0 x 3.5 x 2.5 cm in size and weighing 78.8 grams. Both were partially covered by a fusion crust at the time of their discoveries (Satterwhite and
Righter, 2007). A Lu-Hf isochron was constructed from four minerals in RBT04262, giving an age of 225 ±21 Ma (Lapen et al., 2008). RBT04262 is unique in that it is one of only two composite type SNC meteorites, the other being EETA79001 (McSween and
Jarosewich, 1983). We will show that RBT04262 is composed of an ultramafic pyroxene-olivine cumulate "xenolith" embedded within a picritic host basalt.
Geochemical studies of these two lithologies indicate that they may be cogenetic and that the cumulate lithology crystallized in the presence of plagioclase, but was likely separated from the plagioclase in a deep crustal magma chamber.
2. Methods
Major and minor element analysis and chemical mapping of this meteorite were performed using the Cameca SX100 electron microprobe at NASA Johnson Space
Center. Analyses were done on an ~2xl cm polished section (200 urn thick) of the meteorite mounted in epoxy. The microprobe was run with an accelerating voltage of 15 3 kV and a current of 20 nA for the silicate and opaque minerals, and 15 kV and 10 nA for the phosphates. A l^m-diameter electron beam was utilized for olivine, pyroxenes, and opaques, and a 10 [xm-diameter beam was used for glasses (mostly maskelynite), and merrillite. Elemental concentrations were quantified with WDS detectors and calibrated against mineral standards (diopside was used for Si, Ca, and Mg; rhodonite for Mn; albite for Na and Al; orthoclase for K; fayalite for Fe; rutile for Ti; and chromite for Cr in silicates and opaques. In Ca phosphates, apatite was used for P, Ca, and F; tugtupite for
CI; albite for Si and Al; oligoclase for Na; and kaersutite for Mg and Fe.).
Trace element and some major/minor element (Mg, Ca, Fe, Ni, P, and Zn) analyses were performed with the ThermoFinnigan Element 2 laser-ablation inductively- coupled plasma mass spectrometer at Rice University, operating in both low mass resolution (m/Am=300) and medium mass resolution (m/Am=3000) modes, the former for trace elements and the latter for majors and traces below mass 63. The ICP-MS ran for
40 to 50 cycles with 10 cycles dedicated to measuring the composition of the gas blanks before firing the laser. The gas blanks generally remained below 50 cps for rare earth elements. The laser ablation system utilizes a New Wave Nd-Yag 213 ran laser, operated at a power output of 19mJ/cm2 and a pulse rate of 10 Hz. The spot size of the laser varied depending on the material being analyzed: 55 ^m for olivine and pyroxene, 20 to
30 pun for merrillite, and 30 (xm for opaque minerals. Laser transmission was about
27000 cps/ppm on La for a USGS Hawaiian basaltic glass standard BHV02g. Internal normalization (Longerich et al., 1996) using 25Mg for olivine, pyroxene, and opaques and
43Ca for maskelynite and merrillite controlled matrix effects and drift in ablation yield.
External calibration utilized standard glasses BCR2g, BHV02g, and BIRlg with 4 reference concentrations taken from Gao et al. (2002). For each analysis, the limit of detection was determined as three times the standard deviation of the background divided by the sensitivity of the instrument monitored using 25Mg and 43Ca.
3. Petrography and mineral chemistry
RBT0426 represents a composite of two lithologies, which are texturally (Fig. 1) and chemically distinct, as can be seen in the electron probe microanalysis (EMPA) maps, which we will discuss below (Figs. 2-4). Lithology 1 occurs as two distinct pieces of coarse-grained (1-5 mm) crystals surrounded by a second lithology made up of a fine grained (<1 mm) matrix. In terms of texture, Lithology 1 and Lithology 2 also appear to be out of equilibrium, as exemplified by a thin (0.5 mm) transition between the two lithologies. This "reaction zone" consists of high Ca-pyroxene on the edges of the low
Ca-pyroxenes in Lithology 1 (Fig. 2). Major-element chemistries of all minerals analyzed are given in Tables 1-2. 5
Figure 1. Backscattered electron image of RBT04262 thick section. Lithology 1 exists in the bottom right corner of the section, with a small piece in the upper center. Lithology 2 composes the remainder of the section. Between Lithologies 1 and 2 is a reaction rim of high-Ca pyroxene. 6
Al 1 Ca ,„?,,.
* * '•** * j i . • -. * j
; i ' " ' *"** 4»**
!
. . A.
! Fe Mg , ^".«-;ti
•» # •» fe * 1 v <..--- ^a, 1 1 ••' "• -V" '' % •%#* •
* -5 ^ * *-,t "Xte.*"--»- -,
i
5mm Figure 2. Al Kcc, Ca Ka, Fe Ka, and Mg Ka X-ray maps of RBT04262 thick section. Colored scale bars are electron microprobe counts: red indicates high amounts of the element, while blue indicates low amounts of the element. Low-Ca pyroxenes are black in Al, blue in Ca, light blue in Fe, and orange to yellow in Mg. High-Ca pyroxenes are dark blue in Al, yellow in Ca, dark blue in Fe, and green in Mg. Olivines are black in Al and Ca, green to yellow in Fe, and red to orange in Mg. Maskelynite is green to orange in Al, green in Ca, and black in Fe and Mg. Opaques are light blue in Al, black in Ca, yellow to orange in Fe, and blue to green in Mg. Merrilites are black in Al and Fe, red in Ca, and dark blue in Mg.
3.1 Lithology 1
Lithology 1 (Fig. 3) is dominated by large (1-5 mm) low-Ca pyroxenes
(orthopyroxenes and pigeonites). Olivines occur as large (50-1000 ^m), subhedral to anhedral inclusions within poikilitic pyroxenes, giving Lithology 1 an overall coarse- 7 grained, poikilitic texture. The large olivine in the center of Lithology 1 has a small K- rich glass inclusion in its center, as well as small Ca phosphates along intragranular cracks. Unlike Lithology 2, there are no high Ca-pyroxenes in Lithology 1 (except for the reaction zone between the two lithologies). Minor phases in Lithology 1 consist of small opaque minerals (10-100 [im) and include chromite, ulvospinel, ilmenite, and Fe sulfide.
Chromite and ulvospinel commonly occur within pyroxene and olivine crystals, while the ilmenite and Fe sulfide are only present in the large pyroxene crystals and are rare. The mineral modes in Lithology 1 based on point counting are: 20% olivine, 78% low-Ca pyroxene, 1% opaque minerals (chromite, ulvospinel, ilmenite, and Fe sulfide), and 1%
K-rich glass.
Lithology 1 ol core olrim pig core pig rim chrom ulv ilm melt incl n= 8 6 68 15 19 2 4 5
Si02 37.89 38.05 54.23 53.44 0.199 0.039 0.047 4.97
Ti02 0.025 0.024 0.123 0.200 1.44 15.95 52.39 7.56 A1203 0.040 0.037 0.901 1.13 7.39 3.84 0.049 0.210 Cr203 0.035 0.081 0.392 0.356 54.26 18.98 0.613 19.8 FeO 27.76 27.34 16.45 17.21 31.44 55.82 39.99 64.2 MnO 0.666 0.646 0.601 0.636 0.489 0.617 0.692 1.24 MgO 34.08 34.29 24.17 22.74 4.25 2.83 3.96 0.500 CaO 0.197 0.174 2.94 3.77 0.021 0.046 0.052 0.0158 Na20 0.013 0.0095 0.057 0.074 nm nm nm 0.0556 K20 0.0049 0.0028 0.0058 0.0017 nm nm nm 1.07 Total 100.7 100.7 99.88 99.54 99.58 98.20 97.79 99.6 Mg# 0.69 0.69 0.72 0.70 0.19 0.08 0.15 0.26 Table 1. Major element composition of Lithology 1. ol = olivine; pig = pigeonite; chrom = chromite; ulv = ulvospinel; ilm = ilmenite; melt incl = melt inclusion; n = number of measurements gone into average; nm = not measured; Mg# = molar Mg/(Mg+Fe). Figure 3. Al K Rx n ca o 11 T*- 1-; t-; Ja £ .39 8 0.13 .29 2 0.3 6 r- .06 2 sSt r- CO © u-i o •* o o ON CN >. o 3 "*• 00 4.2 1 2.5 5 14.2 4 53.9 8 0.62 4 0.02 5 22.6 9 0.032 2 ON S _ co NO O i_ •* ^-.OO^OOON — ^CN § o —i o __ H » ^ ir; „; ^i ^ o J3 NO CN N CN _: c o o o in cn 2 o S Ov o ~ in <=> 2 S OO — CN —i CN •* "* CN1 a m - >n r-. M S .-! CN c O in O O 2° CS O CN O O O O a o (O- ON O 00 — NO CN ON CO O ON ON •* CN '-': NO .V "-1 NO 00 NO o— Oi n o — o O CN «3 — n3 o o d o — ^ 2 ©' E CN 6£ ,—1 S CN Tf c- in in c-~ o ON O .—i OO NO OO in ON ^r TT ^— 00 •* CN rf l/-i in CN 5,* in O itho l o o •—' o — — J 3 o U4 NO •* ON ON NO ON o -f r- NO NO CN o T S P 60 '""' ^ in NO m o rt o —i C ON 3 O —i —• ON 03 £ ONinONcot--t-- — „o-<*- m .^ C NO —• — inr-Tj- — CNLJO— SQS s? rin o^ o» on —oid o">-•"- CN . P corf 'B. O O —i ° u kri oo o r-- ON NO I— — o £ 8 NO ON CN O NO m •* o o S °1 M l%! ON £P o o O —i O CN O C ON S, * s O m VI i" Tf W S ~ -: CN t~- CN m co oo NO 2 2 £ •c co o o o CN NO P P 2 CN "~ £ o NO o o o o CO O CN O sO g R 2 o u IH ON CN CN CO O '^f NO O Tj" NO O f- ^T CN NO CO NO r~ 2 K O NO « Tf COOOOCOOCNv-i o o o „; r- O p§ 2 © o II O O ^ ^ ^ « ap % °g o o S"cd tw^o a OH H 2 10 * #. 1000um Figure 4. Al Ka, Ca Ka, Fe Ka, and Mg Ka X-ray maps of Lithology 2. Colored scale bars are electron microprobe counts: red indicates high amounts of the element, while blue indicates low amounts of the element. Low-Ca pyroxenes are black in Al, dark blue in Ca, green in Fe, and orange in Mg. High-Ca pyroxenes are dark blue in Al, green in Ca, light blue in Fe, and light green in Mg. Olivines are black in Al, black in Ca, red in Fe, and red in Mg. Maskelynite is yellow to red in Al, light blue to dark blue in Ca, and black in Fe and Mg. Opaques are dark blue in Al, black in Ca and are red to orange in Fe and dark blue in Mg. Merrilites (Ca phosphates) are black in Al, red in Ca, and dark blue in Fe and Mg. Mineral chemistries are as follows. Lithology 1 low-Ca pyroxenes are slightly zoned, ranging from En6sFs32Wo3 cores to En64Fs2sWon rims (Fig. 5). Pyroxene Mg#s (molar Mg/(Mg+Fe)) decrease from 0.74 in the core to 0.68 in the rim. No exsolution exists in Lithology 1 pyroxenes. The slight rim-ward increase in Wo component in the 11 low-Ca pyroxene is likely to be related to the presence of the high Ca-pyroxene reaction rims separating Lithology 1 from the fine-grained matrix of Lithology 2. Figure 5. Ternary diagram of pyroxene compositions present in RBT04262. Lithology 1 is represented by blue squares, Lithology 2 by red triangles, and the reaction rim by green circles. Olivine grains appear to be homogeneous with no significant chemical zoning present, but larger olivine crystals have slighty higher Mg#s (0.70) than the smaller olivines (0.67). Coexisting olivine and pyroxene MgO and FeO contents are plotted in Figure 6 along with modeled Martian mantle compositions for comparison (Ringwood, 1981; Agee and Draper, 2005). Chromites in Lithology 1 have an average composition of ChrcioHeio (Chr = chromite, He = hercynite) while ulvospinels have an average composition of Usp46Chr57 (Usp = Ulvospinel). Ilmenites in Lithology 1 have an average composition of Ilm^Hems (Fe3+ components estimated from stoichiometry). All oxides in Lithology 1 are homogenous with no zoning. 12 40 O Lithology 1 bulk Mg# = 0.5 / • Lithology 2 bulk 30 0.7^ Olv & 20 O v HH 10 Martian mantle 40 Figure 6. Plot of FeO and MgO content of RBT04262. Dashed lines indicate different molar Mg#s (Mg/(Mg+Fe)). Lithology 1 compositions are shown as blue triangles; Lithology 2 compositions are shown as red squares. An average of the olivine and low- Ca pyroxene compositions in the two lithologies are marked, as is high-Ca pyroxene in Lithology 2. Error bars on the points represent the range of compositions in the averages. Bulk compositions of Lithology 1 and Lithology 2 are plotted as blue and red circles, respectively, and are calculated using oxide concentrations in the minerals and the modal abundances of the minerals in the lithologies. The gray triangle is the estimated range of values for the Martian mantle (Agee and Draper, 2005, Minitti et al., 2006). 3.2 Lithology 2 Lithology 2 (Fig. 4) represents the finer-grained matrix that dominates the RBT04262 section and surrounds the cumulate-textured clasts represented by Lithology 1 (Fig. 1-2). Although over-all more fine-grained than Lithology 1, the crystal size distribution in Lithology 2 is bimodal. Lithology 2 is characterized by large, anhedral to euhedral olivine grains (0.5-2 mm) surrounded by a finer-grained matrix (<100 \xm) of 13 olivine, high Ca-pyroxene, low Ca-pyroxene, maskelynite (plagioclase that has been shocked to glass; (Milton and de Carli, 1963)), and accessory minerals such as Ca- phosphates, oxides (chromite, ulvospinel, ilmenite), and Fe-sulfides (Fig. 7). Figure 7. Backscattered electron image of a portion of Lithology 2. Minerals labeled are olivine (ol), pyroxene (px), maskelynite (mask), merrillite (merr), chromite (chrm), ilmenite (ilm), and Fe sulfide (Fe sulf). Scale bar is 500 \xm. As for the accessory minerals, the opaques (oxides and sulfides) are largely associated with maskelynite, either along the former plagioclase grain boundaries or within the maskelyenite itself, but not in the olivines or pyroxenes. Ca-phosphates are also found in the maskelynite, occurring as needle-shaped crystals along maskelyenite grain boundaries. Mineral modes in Lithology 2 are 46% olivine, 22% low-Ca pyroxene, 14 11% high-Ca pyroxene, 19% maskelynite, 1% merrillite (Ca phosphate), and 1% opaques (chromite, ulvospinel, ilmenite, and Fe sulfide). Detailed mineral compositions are as follows. Pyroxene compositions fall into two distinct groups: pigeonite (En57Fs37Wo6 to En57Fs3iWoi2) and augite (En5oFs2iWo29 to E1143FS18W039). Rare orthopyroxene (CaO <2 wt%) is also present. Lithology 2 pyroxenes are more ferroan than those in Lithology 1, with Mg#s of 0.63 to 0.65 for low- Ca pyroxene and 0.68 to 0.70 for high-Ca pyroxene. There is also a much larger range in both Ca, Mg, and Fe contents in Lithology 2 pyroxenes than Lithology 1 pyroxenes (Fig. 5). Olivines in Lithology 2 are less magnesian than Lithology 1 olivines with Mg#s of 0.61 for the coarse-grained crystals and 0.58 for the fine-grained crystals. The majority of the maskelynite compositions range from An7o to An63 (Fig. 8) but there are a few areas with compositions of Aihu to Ango- Trace amounts of K-feldspar-like glass (7-10 wt% K2O) occur in the groundmass (only one point on the ternary plot in Figure 8). 15 Figure 8. Ternary diagram of feldspar compositions present in Lithology 2. Chromites are rare in Lithology 2, so only one was analyzed. Its composition is Chr92He8. Ulvospinels have a large range of compositions, from Usp5iChr49 to Usp2oChr8o with an average of Usp37Chr63, and are zoned from chromium-rich cores to titanium-rich rims (Fig. 9). Ilmenites in Lithology 2 have an average composition of Ilm94Hem6 and are homogenous with no zoning. As shown in Figure 10, there is a large range of ulvospinel compositions, whether the mineral is in olivine, pyroxene, or maskelynite. There does not appear to be a compositional bias dependent on host mineral. The average composition of the Ca phosphates is Ca4Mgo.3Nao.4(P04)3. The phosphates are classified as merrillites (Rubin, 1997). 16 60 rim core rim 50 1 -0-Cr203 «40 -"-Ti02 -*-FeO 13 0 i :£ * 20 i HH 0 —i—i—i—i—i—i—i—i—i—i—i—i—i—i—i 0 50 100 150 Distance (jim) Figure 9. Profiles of Cr2C>3, Ti02, and FeO across an ulvospinel crystal in Lithology 2. 18 n 16 " A A A AA 14 • iA A A • i4 A A AA A ^ 12 " A A % io : A » »» d 8" H 6 : OL1 ulvosp- inpx 4 • AL2ulvosp- inpx AL2ulvosp- inol 2 • AL2 ulvosp - in mas 0 : 15 20 25 30 35 410 Cr203 (wt%) Figure 10. Cr2C>3 and Ti02 contents of ulvospinels within minerals in Lithologies 1 (Ll) and 2 (L2). Ulvosp = ulvospinel, px = pyroxene, ol = olivine, and mas = maskelynite. 17 3.3 Reaction zone between Lithologies 1 and 2 Reaction between Lithology 1 and 2 occurs as a thin rim of augitic pyroxene separating Lithology 1 low-Ca pyroxenes from Lithology 2. We observed no reaction zones between the olivines of Lithology 1 and groundmass of Lithology 2. Pyroxenes in the reaction zone between Lithologies 1 and 2 range between En53Fsi9Wo28 to E1145FS17W038, and are more magnesian than augites in Lithology 2, with Mg#s of 0.71 to 0.74. 3.4 Inclusions in megacrysts The olivine megacryst in Lithology 1 has a large, glassy melt intrusion in the center. No minerals are present in this inclusion, although there are cracks in the olivine leading to the melt inclusion that contain small amounts of Ca phosphates. The major element composition of this inclusion is listed in Table 1. In Lithology 2, several olivine megacrysts contain melt inclusions. These melt inclusions contain pyroxene, chromium- rich spinel, and iron sulfide, along with glass (Fig. 11). 18 Figure 11. BSE image of melt inclusions in olivines in Lithology 1 (A) and Lithology 2 (B). 3.5 Shock-related features The olivines and pyroxenes in both lithologies and the merrillites in Lithology 2 are severely fractured, with some fractures completely cross-cutting the minerals (i.e., Fig. 11). Olivines display undulatory extinction and weak to moderate mosaicism, and have some radial fracturing. Both low- and high-Ca pyroxenes have moderate to strong mosaicism, and some orthopyroxenes contain planar fractures that may represent cleavage planes of the minerals. The plagioclase present in RBT04262 has undergone complete isotropization to maskelynite glass with few instances of remnant birefringence. Strong grain boundaries and twinning of the original crystals have been preserved. 19 4. Trace element geochemistry All trace element concentrations are listed in Tables 3 and 4. Representative rare earth element (REE) concentrations for each mineral phase are plotted in Figure 12 normalized to Cl-chondrite (Anders and Grevesse, 1989). Below, we compare and contrast the trace element chemistry of minerals in the different lithologies. 1UUU.UU -• •- -o-Llol 100.00 -*-Ll pig -»-L2ol ~*~L2 pig -*-L2 aug ~*~L2 mask -*-L2merr "*-rxnrim 001 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu "igure 12. Representative REE patterns of phases in RBT04262. LI indicates Lithology 1 (blue); L2 is Lithology 2 (red); rxn rim is the reaction rim between the lithologies (gray). 01 = olivine, pig = pigeonite, aug = augite, mask = maskelynite, and merr = merrilite (Ca phosphate). 20 O OO ~r-- ~ NO B^ E v> CN S £ S 2 5 £ 5 5 s s £ £ £ O2 £- £- e*-—> in > B ^ B B Bf—. B co B — B in £ £ 2 ON B B ~ 5§ B S B _• B ^ E S E °° B E B B B B B 2 NO co BE. £££2,££g£ ££°,££o£££.. ^ . S r< g g g p B a B NO E >-i R H •E t- E B E BBB^BBCC O o CO ON B NO s s s 53 s £§ £ ^ 5 o o ,-, o (fi00^. ooh^ooor^ooON ooo„-^ooo oo n £ 3 o •q-O — 12 — 0"*CN SS^O^-S^^CN'S- m r< O — ~ o rt N m m M ^ o d C-- •* ^ ro ON —' O O O o 00 OO o NO O O — oc?oo<^vot-^-r-SP •* —. o ts 5 t~- o o ^ o es O CS 1 o . 1«2 SO C»3 MMf! "o - o -5 «" o "3 « m ^ © "3 °° "S <=> o d o o ^ o © o.OOOO^.oo^OO O CN f ?5 nr r~- o CNOOCO^^^SSCN 1- •* ^^^SOO^SCN ooococNcot--^i-cNco •* w-> i/-> o O ^ ^ ^ «• « o\ ^o CN CN o o o o o Oi»l„gg o O _ CN t- o ^r ON ~- 00 © O in O O _ O NoONr~-oocNO.u-iON222 o — — "• • n vo f- 1] co ^r 12 oo 0 —'— *+_• *-*00 m o. cK - O —: _• NOr-,--CNro O O in o NO V) 00 —i •* o co vO — U-l CN p-; NO ° O O CN >0 CN 'Tf — 00 NO CN NO r-. <^ =2 •* •<*• o "^ o TI >n O 5j! —, O 00 O CN •* —i OO ii —• CN r~- ON co " ~ O ON i_J r~- \o so vo t^ •^ CO « o o CN —i £ « -° CN <* "~~ 2 d d ~pi S"P?^ c? <* S ^ •J, _ ^ _ O 5^ ON ir-'co in OO'ON'S i^ 00 r- o oo SO m o\ a o oo —i r- o CM CM o t- CM o o o o o o O —' — O O O o o o o o o o o o o o o o o; o o o 0 0=? d d d o o o o r- r- 3 CM O so •* so -S u O en CM — o ^H f~ oo en x; en vo •" T N ^ P CM_ in CM CM d d 0 —; — ^ —' d ° c o ^ ^r t 0 Os in rN-oooog^OsggS in co t~- o 00 " o o o so © © . © „ cn^MD(No OS so in _"_ •—* '_;'^"t!II'f2r^«sl'sl-^ ~~: o o i o oOJ5O©J5PSPPSPS°°P P ° d d ° PPPOPOPOOPOPPPPO d o o o o o o o o oo^o^^o-co^^o^o - > n . t^ 1— ^ o _ so M ^ M w ^O \0 2 ^o so -*• H B £ 2 2 o £ P so (N S ^ - H '- © S o o p p o o o o o o _: o o d d d d d d d .a r- so en p r- so in t-~ Tf — o m CN (N CM CM en T >—i . 2 ^o o o o o o o o § o o o O O p p o o o o p p p o d a o °. d d d o o d d d d d O 00 ono«iM>J;SS2 a^ oo^en'-r,"00rfCs|'or~-T:l" o•no "'-'oooooooo o o , o 0E^^^C^^^ooo0 o o o . .°.°.PPoooo d °0-0-oooooooo0- OOOOOOQQQQ La o 3 s-'mt^t^aOn'm^^oaniolfleiin^Hoots^' 5^o o _ o o O O o o 000 oo •>*• ,_ o — co © o o o oo 2t--©©©iMS ,©©,corsiTi-roOs m >s o n; t-- CN o oo o o . d 00 .§ O o o o o o o o o o o o o o o lO M o o o o o rt o o o o •* co — 00 o o o os ^o oo oo . ** — <~i -• ©. ^ r- © — © o o o o h •* VO VO •» r- 2 © i. OOOO^Ocs)©©© mcocooH--osr--.MS MS ©.* ^* © ~-" MS ' <-o m r<, ro C- ^oot-~'-<;Z"coOsoo»nco co^l-mco£'n <= © © O ^ ?S vo § o o o ro © § O O O O H o N » 0\ h oo •* ^t S O (N O ^ O S ~J — «") O ° ° ° o —' o 3 m I"- OS ^ (N >g m co CM r- ro ^ ^ —i co o o o O o o o o o © CM O MS OOOr-OOOOOoooOmo J Oi t * « »1 os MS o MS O MS •<*• ^ r~. in **• n ri (N ^ ^ t os (rN- oc _o ^ o S -5 "^^ CN 2 00 "o ^ 2 ri S CO ^^ c J2 *• oo > § a K MS d ° o o o o O o o o o o •q- (N OS MS m oo vo oJo o"o in 0\ "~i Oi "* in "° "! t m M " os cs S o ^ o ^ •* ^5 CM c* «n •5 O m 1° |of; O O O 2 oo o — n O O o -5 o o o OS o o omesiOOrn^oo MS OS — OS ^f 00 CO O Tt 00 2! o o m o 2 (sivpfi-^mNg^^, -^MSMSSr-TT -• - 2 2 § ^ •* so in "**" 0 : (N CO t-. r-- rN r- — -^MS°(Nrn Pi Pi g g *3 ! | hJ t^' OS os O o - S. ie- id CN (N (v> gMS^MSCgoOoog "O "8 "2 >^nl in u->., MS S « on a 2 !2 m n 5 "S "S s s S is "c < E/3 OH ZS2 02i£u6S 23 O O O §0^0,^0^0000 O OS SO OS CM O O go >n (s I-; Ifl » N O) N _• _• CM 00 CN M o " m' H "' n ri 9 co co O O m CM 8 d o O O o \pi\ os so o oc so O O O O so os v-> o •* o in oo r- so CM t-~ rZ oo rf oo OO^-^^^OOOooOOO^v O O 5 £ £ £ £ S £ ££§££p££ ~ £ °°- £ £ £ £ Os s in 5 o R R R C S R R CM ^ m R R K _• S CM O £ £ £ § £ £ £ s s s s s.. - £ £ g £ £ SO £ so R OO £ o £ g S R R CM c R R CCCRRRrORRooR C R R R R CO R —' I <=>. R H CM O O 00 SO 5 £ £ •* £ s s s s s ££££°.££°.££ o CM R R R iC C CM OS S R R R R RRRR^SC^jRR >n so R o K ££££§£ £ £ £ o 52g ° S § £ 2 £ g SS*° s» s C R R R —< R R R R in R R c so £ R 2 co S CM R CM "^ co O 2* 00 o> co ££o££££§££°|£ R R RRO\RRS:C^RR„'^-~ OS §3 |? |2 S § S 2* CO £- £- £. £ £ g £ £ £ £ R so in fc °° £o £>^ g •* £ ro oo £ R " R R R cR o^ R r- RCO RCO R -* s oo " ^ I R R R R R t^ R ~ Ocj R o OS •c o £ £ £ £ £ £ "o £ OS ~ O £ so £ s a » § s s 2 s •n r- CN) R R R R R R c d OS c SO 94. 3 •n •» —I K c s R o £ SO - l> ~ d £ £ o £ ££££o££££°££m^£ oo £ «n £ ^9 £ co - <* s R £-• K RCCROOKRCRi^RC-^^R in R co s —; Os CM CO; Q\ ~ 00 ff. CM so ^ S S " 5 ££££°.££££S£_ S £ £ £ 0 ^ £ £ "°- £ os £ r-~ c S o os °° R R $j! R R so R R R CO in" R ^ 2 ^ r- R ° *• co ^coin^o^os'osi^cM'rr>lS!nt::^S;0^0o' CO ro' so so £> oo as SO \o so so °° oo oo 3 3 C ^ 5 » H H H > U O 2 | £ O 3 Z Z U O N 3 r H g H „S r *< wa o H o w g z "0 O a o Z N tt,~:Ui©oo-o. o o b b p p o o ii o oi ; o ^ b b o o o • o o o o v 00 Oi W K) OO to NO >— ON ^ \o w o o\ oo m Ag *> Ul I— M Ul Ol o M Ui to o NO -U oo •— NO o\ oo o i— £ *- ON Ul ON W H- ^O OO NO 0PPPPPPoooo©o P P P o o o O O O k, o o '<=> o P toooooooiijr;^;^;:zl- U) 00 i— N Ul -fc. O O Ul 00 U> -fc* — ON Ui to ON ^ u> NO to Ul NO U> U> <1 Ul .^ NO L»J NO oPgggPPooPoPoPPoPooPgP iS-SS2ww5;stgc-5r:aS*tiE^2o r o S: f° _ o o o o o o l-1 o o o o l_, ° ° b ^ o o o • o • o o ° § b b P ON to S-J o o ON •° NJ -" S ~ —' -J U> ^ ^- Ul NO o N >o " oo oo ~~ O -J _ _ P o oP°,P°,oP©oooP. o<- J P.— ©N_ J o © o o o o o o o ^ ^ o o o • to o o '- ^ (A O, 2 ° y, oo to to i.i - i K -i .—. *^ NiNJ Ul to tO ON to 00 NO O Lft -U O Poo O O o0oPooooP O O O © p gob k)owSuwH*.g ^ Ul i— ON P to to touii— i£uitoooji ^ O UJ ON ~J 2: *. OJ IN) " W O to °° ~ o o o o „ o _ o _ g O O © P P — — p © p p o © © ON ^ O 4^ K- to :, i-- Ul O0 '*—' NO tO ON '<-* Ul o Ul tO ONw \o ^i ~ NO ON to •-» -fc* O -J NO o to oo -j OJ ~j -j *- Ul NO o o o o ooooooooooooo o ^o ^o ^o o u "-NO"-ON^-tOUl—' -j to ooo ^, OOtOtO-JONOOOO NO o O „ o H o o ooo^^o^^o^^ooooooo o o Ul OO ^— O b g b NO ~J 2 -. N_O -U -J -1^ ON o oo to oo oo ON to oo s s g* > pppopoooooul 22 io ^ •-0^-000000000 ^1 bbbbbUi^^jbouj'^tobi-^.^- ^-j i-^ ON "to to b i— to VI Uii— K-toO-J^-«4^>— OJOOU)^ ONUJWOUlUlO-J *-< o 3 I: H 2 H K r >< CD O m g1 ffl X o v; o r O 2 ISJ I £ re a o m » E2, cr E-? o& . rse Wt 0\ ^1 ^ -^ w w « ^S re o *-- ^o UJ o ^o 1 B o r r r 5 -p 'r t- ET 73 70 70 •& fO jo ^ ?fl !* 7° r P p P ^5 I o !» .^. 5-3 7° 3 o si *&• a.* 11 O © ; o o p o O O O O O •— p o o OV O b io a M a « N b\ ©• © N 00 o u> ~j ^ b '_ 'h— ° C)o oo K> --J <-r\ to -— U) © O 4^ bP o U) <-L/5i ON B 3 5* K) K> ^O >— ON -J © H^ to re o B ° B|. m ° O: "« •P 3 3 re co O re : o o p p p _ Pre re 3' ? o o o o o ^^ © . ° ° Ln K) to «-> w '>- ig oo '>- ;. CO ea s 0©0© © © 3 O B,- © © © 0 0 0 s s © © © © © © o © b b © •;-, © o © a a oo -IO fa ftl © © © 2 M r 3 5 Ul \D W [T ©©©©2©2© 3 3 — J-» Ui-Jt^lNO^^C^^-J >^ B II a 3 B O re 3 P -ft* o* 4.1 Silicate minerals REE contents in olivines in both lithologies are an order of magnitude higher than olivines in terrestrial peridotites (Agranier and Lee, 2007). RBT04262 olivines are slightly light REE (LREE) depleted relative to the heavy REEs (HREEs), but not to the extent seen in terrestrial olivines in peridotites. Olivines in Lithology 1 have lower REE contents than Lithology 2 by a factor of ~5. Olivines in both lithologies have a slight negative Eu anomaly (Eu/Eu* = 0.472 ± 0.17 in Lithology 1 olivine cores, 0.586 ± 0.20 0 5 in Lithology 2 cores, where Eu/Eu* = EuN/(SmNGdN) ' and EUN, SmN, and GdN are CI- normalized abundances), consistent with equilibrium in the presence of plagioclase (shocked to maskelynite). Olivines in Lithology 1 also have lower concentrations of Ca, Al, and P than Lithology 2 olivines, but have higher Ni contents and similar Cr contents (Fig. 13). 27 60 : •ol X E jmas A 55 - mig Lithology pig P«g£,PJ6-— - • xaug J^^aug 2 £ 50 z • • Rxnrim -— x maskelynite H Lithology A 33 40 : mer 1 _• 35 : 'ol x ^n : 0.1 1 10 5 10 15 20 Eu/Eu* CaO (wt%) mer Anthology 1 •Lithology 2 XRxnrim OL1 bulk •L2 bulk 10 20 30 40 0 10 20 30 MgO (wt%) A1203 (wt%) Figure 13. Chemical compositions of silicate phases in RBT04262. A. Eu/Eu*. B. SiC>2 (wt%) and CaO (wt%). C. Ni (ppm) and MgO (wt%). D. Lu (ppm) and A1203 (wt%). Blue squares indicate Lithology 1, red triangles represent Lithology 2, and green open circles are the reaction rim between the lithologies. Phases are labeled (ol = olivine, pig = pigeonite, aug = augite, mas = maskelynite, and mer = merrillite). Tie lines between the phases of each lithology constrain the bulk composition of the lithologies (labeled as LI and L2 bulk). The positions of the bulk composition points are dictated by the modal abundances of the minerals in each lithology. Differences in trace-element compositions between pigeonites from the two lithologies are analogous to the differences in olivines. Lithology 2 pigeonites have higher REE contents than Lithology 1 pigeonites. For comparison, the REE contents in these pigeonites are similar in absolute abundance to those in terrestrial peridotites or primitive magmas (Agranier and Lee, 2007). Overall, normalized REE abundance patterns are similar in the two lithologies. Both pigeonites are LREE-depleted relative to 28 the HREEs. Both pigeonite types have similar negative Eu anomalies (Eu/Eu* = 0.604 ± 0.020 in Lithology 1 pigeonite cores, 0.539 ± 0.066). Pigeonites in Lithology 1 have lower concentrations of Ca, similar Al and Cr, and higher Ni and P than Lithology 2 pigeonites (Fig. 14). LuYb ErHoDyTbOdEu Sm Nd Pr Ce La B LuYb ErHoDyTbGdEu Sm Nd Pr » La —«•• >»»>>•« *—•—•—•— 10 "•^ol/aug core 1 "••ol/aug rim 5 0.1 4 0.01 o.ooi 0.001 0.95 1.00 1.05 1.10 1.15 1.20 0.95 1.00 1.05 1.10 1.15 1.20 Ionic radius (A) Ionic radius (A) LuYb Er Ho Dy Tb Gd Eu Sm Nd Pr Ce La 1 —»—• • » • • • • a— -^-Lithology 1 "•"•Lithology 2 0.1 0.95 1.05 1.1 1.15 1.2 Ionic Radius (A) Figure 14. Plot of REE compositions of olivine divided by augite (A) and pigeonite divided by augite (B) in Lithology 2, both cores and rims, and olivine divided by pigeonite in Lithologies 1 and 2 (C). ol = olivine, pig = pigeonite, and aug = augite. Equilibrium lines at temperatures of 800, 1000, 1200, and 1400°C are plotted in A (Lee et al., 2007) and B (Agranier and Lee, 2007). 29 High-Ca clinopyroxene (augite) is present only in Lithology 2 and the reaction zone between the two lithologies. REE patterns of clinopyroxenes in Lithology 2 and the reaction zone are remarkably similar, and, as expected, REE contents are distinctly higher than seen in pigeonites. Clinopyroxenes are all LREE-depleted relative to the HREEs. The Eu anomaly present in the pigeonite and olivine patterns also exists for 0 5 clinopyroxene with Eu/Eu* values (Eu/Eu* = EuN/(SniNGdN) ' and EUN, SHIN, and GdN are Cl-normalized abundances) of 0.605 ± 0.084 for Lithology 2 clinopyroxenes and 0.643 ± 0.044 for the reaction zone clinopyroxenes. Clinopyroxenes in Lithology 2 have higher Ca, similar Cr and Al, but lower P and Ni than the reaction zone clinopyroxenes. Maskelynite, present only in Lithology 2, has a typical plagioclase REE pattern with a significant positive Eu anomaly (Eu/Eu* value of 33.9 ± 9.2). Except for Eu, the concentrations of the REEs are near chondritic and lie between the concentrations for clinopyroxene and pigeonite. The maskelynite also has the typical enrichments of Sr and Ba seen in plagioclases, as evidenced by the super-chondritic Sr/Nd and Ba/La ratios. 4.2 Ca-phosphate (merrillite) REE contents in the Ca-phosphates (merrillite) are -1000 times higher than the silicate phase with the highest REE contents. Thus, even though the Ca-phosphates make up only <1 % of the phases in the meteorite (recall the Ca-phosphate is found only in Lithology 2), the Ca-phosphates still dominate the REE budget of the meteorite. The normalized REE abundance pattern of the phosphates is slightly LREE-enriched, indicating that Lithology 2's bulk rock REE abundance pattern is also slightly LREE- enriched. Like all the olivines and pyroxenes, the Ca-phosphate also exhibits a negative 30 Eu anomaly (Eu/Eu* = 0.0660 ± 0.044), consistent with formation in the presence of plagioclase. 4.3 Oxides Chromite is present in Lithology 1 and rarely in Lithology 2. In Lithology 1, chromites have average Zr/Hf=113, Lu/Hf=0.0317, and Nb/Ta=13,100. Ulvospinel is rare in Lithology 1 and has a Zr/Hf=25 and a Nb/Ta=33.8. In Lithology 2, ulvospinel has a Zr/Hf=24, Lu/Hf= 0.000455, and Nb/Ta=21. Zr, Hf, and Ta concentrations are higher in Lithology 1 ulvospinels, whereas Lu and Nb concentrations are higher in Lithology 2, with Lu being below the detection limit in Lithology 1. Ilmenite in Lithology 2 has Zr/Hf=15.8, Lu/Hf=0.000118, and Nb/Ta= 25. 5. Crystallization sequence The crystallization history of Lithology 1 is as follows. Subhedral chromites crystallized first, followed by olivine, which completely enclosed the chromite crystals. The megacrysts formed first, as evidenced by their higher Mg#s, followed by the smaller olivines. Chromite continued crystallizing and ilmenite began forming after olivine crystallized as no olivines contain ilmenite. Finally, large, subhedral pigeonites crystallized, enclosing all the previously-formed minerals. In Lithology 2, olivine appears to be the earlier crystallizing phase because pyroxenes and maskelynite tend to form on the grain boundaries of olivine crystals. Merrillites cross-cut pyroxene and maskelynite, indicating the phosphates crystallized before the other two minerals. Chromite, ulvospinel, ilmenite, and Fe sulfides are fully 31 enclosed in pyroxene and maskelynite, indicating the pyroxenes and maskelynite crystallized last. Maskelynite appears to be the latest major phase to crystallize as it fills the interstices between subhedral to anhedral olivine and pyroxene crystals. The bimodal grain size distribution, made up by large euhedral olivines set within a fine-grained groundmass, along with the apparent sequence of crystallization, suggests a magmatic origin for Lithology 2. The large olivines may represent relict phenocrysts while the groundmass may represent the crystallized liquid. 6. Temperature and oxygen fugacity Calculation of intensive parameters, such as equilibration temperatures, pressures and oxygen fugacities, is always difficult in samples that show extensive evidence of disequilibrium. Thus, any calculations presented here are qualitative. Because both Lithology 1 and Lithology 2 contain olivine and chromium-rich spinel, we were able to use an olivine-spinel geothermometer to calculate temperatures (Sack and Ghiorso, 1991). Lithology 1 has a calculated equilibration temperature of 1081°C at a pressure of 5 kbar. In Lithology 2, we can use two-pyroxene thermometry based on augite-pigeonite pairs, as well as the olivine-spinel geothermometer. The Sack and Ghiorso (1991) thermometer gives 1026°C, while the Wells (1977) thermometer gives 1180 °C, and the Brey and Kohler (1990) thermometer gives 1050 °C at 5 kbar (no significant difference if rim compositions are considered). We also applied two-pyroxene thermometry using the QUILF program (Andersen et al., 1995). Equilibrium of diopside component between augite and pigeonite gives a temperature of 1075 °C (P=5 kbar). 32 We also used QUILF to estimate apparent oxygen fugacities from coexisting oxide chemistries. We considered the following two reactions for estimating T and fiC^: 4Fe3C>4 + 02 = 6Fe2C>3 and Fe3C>4 + FeTiC^ = Fe2Ti04 + Fe2C>3. Although no magnetite or hematite was present in the meteorite, there were magnetite and hematite components in the minerals that were present. Simultaneous solutions of these two equations (at 5 kbar) yield temperatures ranging from 483 to 902°C and^/Ch values from 2.632 log units below to 0.467 log units above the quartz-fayalite-magnetite (FMQ) buffer, e.g., DlogFMQ2, in Lithology 2. Only one ilmenite crystal was present in Lithology 1, so oxygen fugacity estimates were calculated using the ilmenite composition with both chromite and ulvospinel compositions. The calculated temperatures for Lithology 1 are between 403 and 594°C and oxygen fugacity ranges from QFM - 2.388 log units to QFM + 2.557 log units. Temperatures and oxygen fugacities were also calculated using the Ghiorso and Evans (2008) method with an online web calculator at http://ctserver.ofm- research.org/OxideGeothrm/OxideGeothrm.php. The calculated temperatures for Lithology 2 using Ghiorso and Evans' (2008) method ranged from 776 to 1227°C, while oxygen fugacities were between QFM - 3.44 and QFM - 0.36. In Lithology 1, temperatures calculated ranged from 801 to 897°C, and/D2 values were between QFM - 3.11 and QFM + 1.05. Temperatures and JO2 values calculated using the QUILF (Andersen et al. 1995) and Ghiorso and Evans (2008) methods are listed in Table 5. The low calculated temperatures could reflect the fact that Fe-oxide re-equilibration closes at lower temperatures than silicates. However, given the textural evidence for disequilibria, these numbers should be examined with caution. 33 Lithology 1 chrm/ulv ilm ol px T(°C) JQ2 Method chrm 20 (ulv in px) ilm 20 (px) ol 7 px 6 (pig) 594 -2.388 Anderson et al., 1995 chrm 15(chrminpx) ilm 20 (px) ol 7 px 6 (pig) 403 2.557 Anderson et al., 1995 chrm 20 (ulv in px) ilm 20 (px) 801 -3.11 Ghiorso & Evans, 2008 chrm 15 (chrm in px) ilm 20 (px) 897 1.05 Ghiorso & Evans, 2008 Lithology 2 chrm/ulv ilm ol px T(°C) /D2 Method chrm 1 (ulv in px) ilm 1 (px) ol8 px 12 (pig) 902 -0.075 Anderson et al., 1995 chrm 1 (ulv in px) ilm 1 (px) ol8 px 12 (aug) 848 0.467 Anderson et al, 1995 chrm 2 (ulv in mas) ilm 3 (mas) ollO px 7 (pig) 669 -1.657 Anderson et al, 1995 chrm 9 (ulv in ol) ilm 9 (ol) ollO px 2 (aug) 632 -1.635 Anderson et al, 1995 chrm 9 (ulv in ol) ilm 9 (ol) ollO pxl0(pig) 631 -1.641 Anderson et al., 1995 chrm 14 (ulv in ol) ilm 14 (ol) ollO px 2 (aug) 581 -2.632 Anderson et al., 1995 chrm 14 (ulv in ol) ilm 14 (ol) ollO pxlO(pig) 483 -0.295 Anderson et al., 1995 chrm 11 (ulv in px) ilm 11 (px) ol8 px 12 (pig) 495 -0.301 Anderson et al., 1995 chrm 11 (ulv in px) ilm 11 (px) ol8 px 12 (aug) 593 -2.505 Anderson et al., 1995 chrm 1 (ulv in px) ilm 1 (px) n/a n/a 1227 -0.36 Ghiorso & Evans, 2008 chrm 9 (ulv in ol) ilm 9 (ol) n/a n/a 868 -2.39 Ghiorso & Evans, 2008 chrm 2 (ulv in mas) ilm 3 (mas) n/a n/a 847 -2.37 Ghiorso & Evans, 2008 chrm 11 (ulv in px) ilm 11 (px) n/a n/a 785 -3.27 Ghiorso & Evans, 2008 chrm 14 (ulv in ol) ilm 14 (ol) n/a n/a 776 -3.44 Ghiorso & Evans, 2008 Table 5. Calculated temperatures and oxygen fugacities. chrm = chromite; ulv = ulvospinel; ilm = ilmenite; ol = olivine; px = pyroxene; pig = pigeonite; aug = augite. JO2 is in log units relative to the Quartz-Fayalite Magnetite (QFM) buffer. Parentheses in the chromite/ulvospinel column indicate which spinel was used in the calculations and in which mineral it was located. Parentheses in the ilmenite column indicate in which mineral the ilmenite crystal used for calculations was located, and parentheses in the pyroxene column indicate which pyroxene was used in the calculations. Despite the uncertainties in temperature calculations, it is probably safe to conclude that Lithology 1 retains a record of higher equilibration temperatures than Lithology 2. This is also manifested in the apparent distribution of heavy REEs between olivine and pigeonite: olivine/pigeonite heavy REE ratios are higher in Lithology 1 than in Lithology 2, indicating qualitatively higher equilibration temperatures. 34 7. Comparison of RBT04262 with other shergottites RBT04262 is most similar to Lithology A of EETA79001, another olivine-phyric shergottite. EETA79001A contains megacrysts of olivine and low-Ca pyroxene, similar to Lithology 1 of RBT04262. However, most of the megacrysts in EETA79001A are composed of either olivine or low-Ca pyroxene with few composite megacrysts, while Lithology 1 of RBT04262 contains both low-Ca pyroxene and olivine (Goodrich, 2003). The other meteorites in the olivine-phyric shergottite class have olivine megacrysts only, and are similar in texture and mineralogy to Lithology 2. Like most SNCs (McSween, 1994), RBT04262 is a cumulate. The major element composition of RBT04262 is most similar to Dar al Gani (DaG) 476 (Zipfel et al., 2000) and Sayh al Uhaymir (SaU) 005 (Goodrich, 2003), with almost identical FeO/MgO ratios, AI2O3, Si02, Na20, and K2O contents. The bulk composition of EETA79001A is more SiC>2- and A^Ch-rich and has a higher FeO/MgO ratio (Goodrich, 2003). RBT04262's Fe/Mn ratio is 39, most similar to Sayh al Uhaymir 005 (Goodrich, 2003), and in the low to middle of the range of olivine-phyric shergottites (36 to 45). The bulk rock Mg# of RBT04262 is 0.65, lower than SaU 005 (Goodrich, 2003) and Yamato 98045 (Shirai and Ebihara, 2004), both of which have Mg#s of 0.67, and DaG 476 with an Mg# of 0.68 (Zipfel et al., 2000), and higher than NWA1068 (Mg# = 0.59; Barrat et al, 2002), Dhofar 019 (Mg# = 0.59; Taylor et al., 2002), and EETA79001A (Mg# = 0.61; Goodrich, 2003). The REE pattern of RBT04262 is nearly flat, similar to other enriched shergottites, such as Shergotty (basaltic shergottite) and NWA1068 (olivine-phyric shergottite). However, RBT04262 has a lower concentration than those meteorites by 35 approximately a factor of four. Lithology 2 alone contains the majority of the REE budget of the meteorite, and has concentrations and a REE pattern similar to the enriched shergottites, as shown in Figure 15. RBT04262 has a Ni content of 260 ppm, higher than Dar al Gani 476 (Zipfel et al., 2000), EETA79001A (Goodrich, 2003), NWA1068 (Barrat et al., 2002), Dhofar 019 (Taylor et al., 2002), and Yamato 980459 (Shirai and Ebihara, 2004), but lower than Sayh al Uhaymir 005 (Goodrich, 2003). "•"Lithology 1 "•"Lithology 2 -*"RBT04262 bulk -t-Shergotty -*-ALH77005 -®"EETA79001A "•"NWA 1068 -Q-SaU 005 La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu Figure 15. Comparison of REE concentrations in RBT04262 with other shergottites relative to CI chondrite reference (Anders and Grevesse, 1989). Shergotty and EETA79001A concentrations are from Lodders, 1998. ALH77005 concentrations are from Wanke, 1986 and Smith, 1984 (Pr and Er only), NWA1068 concentrations are from Barrat, 2002, and SaU 005 concentrations are from Dreibus, 2000. The bulk compositions of Lithology 1, Lithology 2, and the RBT04262 whole rock were calculated using concentrations in minerals and modal abundances of those minerals. The oxygen fugacity estimates for RBT04262 are more oxidizing on average than most olivine-phyric shergottites. Yamato 980459 has an oxygen fugacity of QFM - 2.6 36 (McKay et al., 2004), the EETA79001A groundmass has anJ02 of QFM - 1.8 log units, DaG 476 has an/}2 of QFM - 2.5, SaU 005 has an/)2 of QFM - 3.4, and Dhofar 019 has any02 of QFM - 1.7 log units (Herd, 2003). However, the groundmass of NWA 1068/1110 is more oxidizing than RBT04262, with anyD2 of QFM + 0.5 log units (Herd, 2006). RBT04262's calculated equilibration temperatures are in the middle of the range for olivine phyric shergottites, with EETA79001A having temperatures between 860°C and 926°C, DaG 476 having an average temperature of 1018°C, SaU 005 giving an average temperature of 1058°C, and Dhofar 019 having an average temperature of 1099°C (Herd, 2002). 8. Interpretations 8.1 RBT04262 represents a crystallized magma containing cumulate clasts RBT04262 is a composite meteorite. The only other composite Martian meteorite is Elephant Moraine 79001 (EETA79001), consisting of two separate lithologies juxtaposed against each other, and was interpreted to represent the contact of two different lavas (McSween and Jarosewich, 1983) or a basalt and an impact melt (Mittlefehldt et al., 1999). However, the origin of RBT04262 is fundamentally different as we discuss below. The features most relevant to the petrogenesis of RBT04262 are summarized below. Lithology 1 is coarse-grained, cumulate-textured, has high Ni, high Mg#, low REE content, and higher equilibration temperatures than Lithology 2. This is characteristic of a primitive crystallized melt. Most of the cracks in the large olivine grain in Lithology 1 appear to be late stage, and hence likely related to impact 37 deformation. Olivine and pyroxene Mg#s in Lithology 1 are just slightly lower than estimates of the primitive Martian mantle (0.7-0.78; Agee and Draper, 2005; Minitti et al., 2006), and thus too low for Lithology 1 to represent residual Martian mantle and too high for Lithology 1 to represent a magma composition. The presence of a negative Eu anomaly in Lithology 1 olivines and pyroxenes indicates that the olivines and pyroxenes formed in the presence of plagioclase even though there is no maskelynite visible in Lithology 1. Since olivine and pyroxene are the only silicate minerals in Lithology 1, the bulk rock composition of Lithology 1 is also characterized by a negative Eu anomaly. This means that Lithology 1 has a magmatic origin, and in particular, it has been completely separated or segregated from its plagioclase-bearing complement. Collectively, these geochemical signatures and textural relationships argue that Lithology 1 represents cumulate clasts that fully segregated from a relatively primitive magma. In contrast, Lithology 2, which surrounds the cumulate clasts of Lithology 1, is much more evolved, with lower Mg#s, lower bulk Ni, higher bulk Ca, higher bulk REE contents, abundant maskelynite, and occurrence of Ca-phosphates. Equilibration temperatures are also lower in Lithology 2 than Lithology 1. These systematics indicate that Lithology 2 represents an evolved magma or cumulates from an evolved magma. The textures of nearly euhedral, coarse-grained olivines set within a finer-grained matrix of olivine, pyroxene, and plagioclase in the interstices are reminiscent of a cooled magma, wherein the large euhedral olivines represent phenocrysts and the finer-grained matrix represents a crystallized liquid groundmass. We note, however, that the high abundance of olivine crystals in Lithology 2 would result in very high bulk MgO contents if the modal abundances in our polished section are representative; these high MgO 38 contents are much higher than any reasonable magma compositions. Thus, Lithology 2 is most likely an evolved magma containing accumulated olivine. The closest terrestrial analog to Lithology 2 would be an olivine-gabbro troctolite, representing a liquid with some cumulate component mixed in. The fine-grained nature of Lithology 2 suggests that this olivine-laden liquid cooled more rapidly then Lithology 1. However, the fully crystallized nature of Lithology 2 indicates that it was not quenched and hence it cannot represent a lava flow; instead, it must represent a magma that crystallized at depth, perhaps in a shallow-level sill. Olivines and pyroxenes all have slightly LREE-depleted abundance patterns, but in detail, the REE contents in these minerals do not reflect equilibrium partitioning between the phases. Because of the much stronger preference of LREEs in clinopyroxene compared to pigeonite and olivine, equilibrium olivine/augite and pigeonite/augite REE abundance ratios should decrease monotonically with increasing ionic radius (Lee et al, 2007; Agranier and Lee, 2007). However, the observed olivine/augite and pigeonite/augite REE ratios plotted against ionic radius are U-shaped in Lithology 2, suggesting disequilibrium in the LREEs (Fig. 14). We can also compare Lithology 1 and 2 directly by examining olivine/pigeonite ratios in the two lithologies. It can be seen that Lithology 2 has a more U-shaped pattern, indicating Lithology 2 is more out of equilibrium than Lithology 1. 8.2 Petrogenetic model and evidence for a deep magma chamber The textural, geochemical and temperature differences between Lithologies 1 and 2 are interpreted as follows. Lithology 1 represents a cumulate, crystallized at high 39 temperatures from a primitive magma, while Lithology 2 represents a magma, containing accumulated olivine, that cooled and crystallized at lower temperatures and faster rates. Although there is no way to show that Lithology 1 and 2 are petrogenetically related, that is, ultimately derived from the fractionation trend of a common parental magma, two circumstantial observations are at least consistent with a petrogenetic link. First, Lithology 2 is consistently more evolved than Lithology 1. Second, Lu/Hf and I76Hf/177Hf systematic of different mineral phases in RBT04262, taken without regard to the presence of different lithologies, yield an internal isochron (Lapen et al., 2008), suggesting an indistinguishable temporal relationship between the lithologies. We interpret the composite relationship to indicate that the evolved melt entrained fragments of the early-formed cumulates as the melt ascended towards the surface (e.g., Lithology 1 represents the "antecrysts" of Lithology 2). These liquids (Lithology 2) must have formed shallow-level sills in the upper crust, allowing them to cool and crystallize rapidly (but not quench as would be the case if they were lava flows), preventing the Lithology 1 cumulate fragments from fully re-equilibrating with the host liquid. The greater degree of internal LREE disequilibria in Lithology 2 compared to Lithology 1 (more U-shaped pattern in Fig. 14) is consistent with the qualitative suggestion of different cooling rates. Our interpretations are fully consistent with terrestrial analogs. For example, many terrestrial basalts contain xenoliths of earlier formed cumulates from many different stages of the crystallization history. Many terrestrial basalts also contain significant amounts of accumulated olivine phenocrysts. Finally, we speculate that Lithology 1 cumulates were formed in a deep crustal or lithospheric mantle magma chamber (Fig. 16). The negative Eu anomaly of Lithology 1 40 in the absence of any visible maskelynite indicates that Lithology 1 must have been completely segregated from the complementary plagioclase-bearing liquid. Assuming that such segregation was controlled primarily by differential gravity settling, a magma chamber rather than a thin sheet-like sill is required as there needs to be enough of a settling distance to yield efficient segregation (Hoink et al., 2008 and references therein). Unfortunately, we cannot quantify how big or deep this magma chamber was because we are unable to estimate pressure or any lengthscales of cumulate layering from our meteorite section. Nevertheless, Lithology 1 must have formed in a magma chamber large enough and deep enough to prevent rapid cooling because rapid cooling would have quickly arrested gravitational segregation (as can be seen in Lithology 2). 41 Figure 16. Cartoon of crystallization sequence of RBT04262, with locations of Lithologies 1 and 2 labeled. 8.3 Implications for Martian crust-formation, rejuvenation, atmospheric evolution We now return to the issue of crust-forming scenarios on Mars. At least for RBT04262, addition of new magmas to the Martian crust occurred via underplating or intrusion and not by lava flows. Although EETA79001 and a few other SNC meteorites may represent lava flows (i.e. Lentz and McSween, 2000; Mikouchi and Miyamoto, 2002), the great majority of SNC meteorites represent cumulates or crystal-laden liquids, the former undoubtedly representing deep-level intrusions and the latter also representing deep intrusions or shallow level sills. If these samples are representative, the suggestion is that Martian crust-formation is controlled primarily by intrusions and not by surface 42 lava flows. Indeed, gravity evidence indicates the presence of large, extinct magma chambers beneath Syritis Major (Kiefer, 2004), Olympus Mons (Zuber and Mouginis- Mark, 1992), Tyrrhena Patera, and Hadriaca Patera (Kiefer, 2003). If the above generalization is correct, the question arises as to why is magmatism on Mars dominated by intrusions? On Earth, extrusive magmatism is common (as evidenced at mid-ocean ridges and island arcs), but Earth has a mobile lid (plate tectonics), whereas Mars appears to have a more sluggish or perhaps even a stagnant lid even when its interior was active in the Noachian (Solomon et al., 2005). Could the lack of plate tectonics prevent magmas from escaping to the surface? The lack of a mobile lid implies that on average the thickness of the lithosphere (as well as the crust) might be greater than that on Earth. On Earth, lithosphere thicknesses are zero at mid-ocean ridges as this is where the mobile, upper thermal boundary layer is rejuvenated. Furthermore, without efficient means of recycling the surface (e.g., via subduction), did continued magmatism result in a thick Martian crust, further frustrating the eruption of magmas to the surface? An intrusive-dominated mechanism for Martian crust formation has many implications. Progressive underplating and intrusion should lead to an ever-thickening Martian crust, resulting in the lower crust becoming progressively hotter as the crust thickened. Intrusion/underplating could generate extensive partial melting of pre-existing lower crust, leading to felsic magmas. On Earth, the most evolved magmas (such as granites) tend to occur where magma ascent is frustrated by thick lithosphere or pre existing crust, such as in continental arcs. However, formation of felsic rocks does not have to be restricted to subduction zones. Early continental crust on Earth has been 43 suggested to have been formed by re-melting of thickened basaltic crusts with or without subbduction zones (Rollinson, 2008). Indeed, felsic rocks such as granite have been found on Mars, though the amount of such rock types is still poorly constrained (Christensen et al., 2005). Spectral data of the Martian crust has recently been interpreted to indicate the presence of basaltic andesite to andesite compositions, suggesting that intracrustal differentiation may be important on Mars (McSween et al., 2003), though there is debate as to whether the spectra indicate the crustal composition is simply weathered basalt (Michalski et al., 2006). An intrusion-dominated crust-forming process also has implications for the ongoing debate on the age of Martian SNC meteorites (Bouvier et al., 2008). Although more than 40% of the Martian surface is older than 3 Gy (Solomon et al., 2005), of the fifty or so SNC meteorites, only one has an internal isochron age of ~4.5 Ga while all others give internal isochron ages of less than ~1 Gy (Nyquist et al., 2001; Walton et al., 2008; Righter et al., 2009). However, whole-rock Pb-Pb apparent isochrons on the same SNC meteorites give -4.1 Ga ages (Bouvier et al., 2008), consistent with the cratering- based estimates of the age of the Martian crust. One resolution to this debate is if the young internal isochron ages reflect an origin by either complete thermal resetting induced by magmatic intrusions or that the SNC meteorites actually derive from parental liquids generated by re-melting of ancient crust. In such a scenario, whole-rock isotopic compositions could preserve isochronous relationships that relate to the age of the source region, but internal isochrons reflect the latest magmatic event. In any case, internal isotopic re-equilibration requires the presence of young (<1 Ga) magmas. Indeed, it has 44 been suggested that magmatic intrusions may have been responsible for de-magnetization of much of the Martian crust (Ogawa and Manga, 2007). Finally, intrusive magma emplacement should substantially decrease the efficiency of volatile degassing compared to eruptive emplacement. This implies that if the mechanisms of magma emplacement are not considered in coupled geodynamic and geochemical evolution models of planetary interiors (e.g., Xie and Tackley, 2004), predicted degassing rates could be in serious error. Similarly, interpreting the significance of noble gas elemental ratios and isotopic ratios in the Martian atmosphere will also be in error if crust-formation mechanisms are ignored. 9. Conclusions RBT04262 is rare among Martian meteorites in that it represents a composite meteorite, composed of early formed cumulate clasts hosted within a finer-grained, more evolved picritic magma. The cumulate clasts, made up of large olivine and low Ca- pyroxene grains, have negative Eu anomalies but there is no visible plagioclase, which we have interpreted to indicate formation of the cumulates in the presence of plagioclase followed by efficient density segregation of the cumulates from the plagioclase-bearing complement. This suggests that the cumulates formed in a deep, crustal or lithospheric magma chamber, wherein cooling was slow enough to allow for crystallization of large crystals and efficient segregation. In contrast, the host lithology is interpreted to represent a magma, laden with accumulated olivine phenocrysts, that cooled and crystallized at shallow crustal levels, but not at the surface. The composite meteorite may 45 have formed by the entrainment of cumulate clasts into the host magma at depth, followed by cooling and crystallization in a shallow sill. Collectively, the textural and geochemical observations lead us to conclude that RBT04262 originates from deep level magma chambers or intrusions. Because the majority of SNC meteorites also have a cumulate origin, we speculate here that the dominant process of crust-formation on Mars occurs via intrusion or underplating from below rather than by subaerial lava flows. An intrusive origin for the Martian crust has many implications for interpreting the age of Martian meteorites, degassing history of the Martian interior, resetting of magnetic properties of the crust, and the extent of intracrustal differentiation and the generation of felsic liquid compositions. 46 References Agee C. B. and D. S. Draper. High pressure melting of H-chondrite: A match for the Martian basalt source mantle. 36' Lunar and Planetary Science Conference, Abstract #1434, 2005. Agranier, A. and C.-T. A. Lee. Quantifying trace-element disequilibria in mantle xenoliths and abyssal peridotites. Earth and Planetary Science Letters 257, 290- 298, 2007. Allegre, C. J., A. Hofrnann, and K. O'Nions. The Argon constraints on mantle structure, Geophysical Research Letters 23 (24), 3555-3557, 1996. Anders, E. and N. Grevesse. Abundances of the elements: Meteoritic and solar. Geochimica et Cosmochimica Acta 53, 197-214, 1989. Andersen, D. J., D. H. Lindsley, and P. M. Davidson. QUILF: a PASCAL program to assess equilibria among Fe-Mg-Ti oxides, pyroxenes, olivine, and quartz. Computers in Geosciences 19, 1333-1350, 1993. Barrat, J. A., A. Jambon, M. Bohn, Ph. Gillet, V. Sautter, C. Gopel, M Lesourd, and F. Keller. Petrology and chemistry of the Picritic Shergottite North West Africa 1068 (NWA 1068). Geochimica et Cosmochimica Acta 66 (19), 3505-3518, 2002. Bogard, D. D., R. N Clayton, K. Marti, T. Owen, and G. Turner. Martian volatiles: Isotopic composition, origin, and evolution. Chronology and Evolution of Mars 96,425-458,2001. Bouvier, A., J. Blichert-Toft, J. D. Vervoort, P. Gillet, and F. Albarede. The case for old basaltic shergottites. Earth and Planetary Science Letters 266, 105-124, 2008. Brey, G. P. and T. Kohler Geothermobarometry in four-phase lherzolites II. New thermobarometers, and practical assessment of existing thermobarometers. Journal of Petrology 31, 1353-1378, 1990. Christensen, P. R., H. Y. McSween, Jr., J. L. Bandfield, S. W. Ruff, A. D. Rogers, V. E. Hamilton, N. Gorelick, M. B. Wyatt, B. M. Jakosky, H. H. Kieffer, M. C. Malin, and J. E. Moersch. Evidence for magmatic evolution and diversity on Mars from infrared observations. Nature 436, 504-509, doi:10.1038/nature03639, 2005. Dreibus, G., B. Spettel, R. Haubold, K. P. Jochum, H. Palme, D. Wolf, and J. Zipfel. Chemistry of a new shergottite: Sayh al Uhaymir 005. Meteoritics and Planetary Science 37, A43, 2000. Gao, S., X. M. Liu, H. L.Yuan, B. Hattendorf, D. Gunther, L. Chen, and S. H. Hu. Determination of forty two major and trace elements in USGS and NIST SRM glasses by laser ablation-inductively coupled plasma-mass spectrometry. Geostandards and Geoanalytical Research 26 (2), 181-196, 2002. Ghiorso, M. S. and B. W. Evans. Thermodynamics of rhombohedral oxide solid solutions and a revision of the Fe-Ti two-oxide geothermometer and oxygen-barometer. American Journal of Science, 308, 957-1039, doi: 10.2475/09.2008.01, 2008. Goodrich, C. A. Petrogenesis of olivine-phyric shergottites Sayh al Uhaymir 005 and Elephant Moraine A79001 lithology A. Geochimica et Cosmochimica Acta 67, 3735-3772, 2003. Herd, C. D. K. The oxygen fugacity of olivine-phyric martian basalts and the components within the mantle and crust of Mars. Meteoritics and Planetary Science 38 (12), 1793-1805, 2003. 47 Herd, C. D. K. Insights into the redox history of the NWA 1068/1110 martian basalt from mineral equilibriua and vanadium oxybarometry. American Mineralogist 91,1616-1627, 2006. Hoink, T., C.-T. A. Lee, J. Hawthorne, and A. Lenardic. Paleo-viscometry of magma bodies. Earth Planetary Science Letters 267, 100-106, 2008. Kiefer, W. S. Gravity evidence for extinct magma chambers on Mars: Tyrrhena Patera and Hadriaca Patera. 34' Lunar and Planetary Science Conference, Abstract #1234,2003. Kiefer, W. S. Gravity evidence for an extinct magma chamber beneath Syrtis Major, Mars: A look at the magmatic plumbing. Earth and Planetary Science Letters 222,349-361,2004. Lapen, T. J., A. D. Brandon, B. L. Beard, A. H. Peslier, C-T. A. Lee, H. A. Dalton. Lu-Hf age and isotope systematics of the olivine-phrice shergottite RBT-04262 and implications for the sources of enriched shergottites. 39th Lunar and Planetary Science Conference, Abstract #2073, 2008. Lee, C.-T. A., A. Harbert, W. P. Leeman. Extension of lattice strain theory to mineral/mineral rare-earth element partitioning: a tool for assessing disequilibrium and developing internally consistent partition coefficients between olivine, orthoyproxene, clinopyroxene and basaltic melt. Geochimica et Cosmochimica Acta 71, 481-496, doi:10.1016/j.gca.2006.09.014, 2007. Lentz, R. C. F. and H. Y. McSween, Jr. Crystallization of the basaltic shergottites: Insights from crystal size distribution (CSD) analysis of pyroxenes. Meteoritics and Planetary Science 35, 919-927, 2000. Lodders, K. A survey of shergottite, nakhlite and chassigny meteorites whole-rock compositions. Meteoritics and Planetary Science 33, A183-190. Longerich, H. P., D. Gunther, S. E. Jackson. Elemental fractionation in laser ablation inductively coupled plasma mass spectrometry. Fresenius' Journal of Analytical Chemistry 355, 538-542, 1996. Mathew, K. J., B. Marty, K. Marti, and L. Zimmermann. Volatiles (nitrogen, noble gases) in recently discovered SNC meteorites, extinct radioactivities and evolution. Earth and Planetary Science Letters 214, 27-42, 2003. McKay, G., L. Le, C. Schwandt, T. Mikouchi, E. Koizumi, and J. Jones. Yamato 980459: The most primitive shergottite? 35th Lunar and Planetary Science Conference, Abstract #2154, 2004. McSween, H. Y., Jr. and E. Jarosewich. Petrogenesis of the Elephant Moraine A79001 meteorite: Multiple magma pulses on the shergottite parent body. Geochimica et Cosmochimica Acta 47', 1501-1513, 1983. McSween, H. Y., Jr. What we have learned about Mars from SNC meteorites. Meteoritics 29, 757-779, 1994. McSween, H. Y., Jr., T. L. Grove, and M. B. Wyatt. Constraints on the composition and petrogenesis of the Martian crust. Journal of Geophysical Research 108 (E12), 5135, doi:10.1029/2003JE002175, 2003. Michalski, J. R., M. D. Kraft, T. G. Sharp, and P. R. Christensen. Effects of chemical weathering on infrared spectra of Columbia River Basalt and spectral interpretations of martian alteration. Earth and Planetary Science Letters 248, 822-829, doi:10.1016/j.epsl.2006.06.034, 2006. 48 Mikouchi, T. and M. Miyamoto. Mineralogy and olivine cooling rate of the Dhofar 019 shergottite. Antarctic Meteorite Research 15, 122-142,2002. Milton, D. J. and P. S. de Carli. Maskelynite: Formation by explosive shock. Science 140 (3567), 670-671, DOI: 10.1126/science.l40.3567.670, 1963. Minitti M. E., Y. Fei, and C. M. Bertka. New, geophysically-constrained Martian mantle compositions. LPI Workshop on Early Planetary Differentiation, Abstract #4057, 2006. Mittlefehldt, D. W., D. J. Lindstrom, M. M. Lindstrom, and R. R. Martinez. An impact melt origin for Lithology A of Martian meteorite EETA79001. Meteoritics and Planetary Science 34 (3), 357-367 Nyquist L.E., Bogard D.D., Shih C.Y. Greshake A., Stoffler D., and Eugster O. Ages and geologic histories of Martian meteorites. Space Science Reviews 96, 105-164, 2001. Ogawa, Y., and M. Manga. Thermal demagnetization of Martian upper crust by magma intrusion. Geophysical Research Letters 34, L16302, doi:10.1029/2007GL030565, 2007. Righter, M., T. J. Lapen, A. D. Brandon, B. L. Beard, J. T. Shatter, and A. H. Peslier. Lu-Hf age and isotope systematics of ALH84001. 40th Lunar and Planetary Science Conference, Abstract #2256, 2009. Rollinson, H. Secular evolution of the continental crust: Implications for crust evolution models. Geochemistry Geophysics Geosystems 9, Q12010, doi:10.1029/2008GC002262, 2008. Rubin, A. E. Mineralogy of meteorite groups: An update. Meteoritics and Planetary Science 32, 733-834, 1997. Satterwhite C. and K. Righter. Antarctic Meteorite Newsletter, 30 (1), 1-20, 2007. Sack, R. O. and M. S. Ghiorso. Chromian spinels as petrogenetic indicators: Thermodynamics and petrological applications. American Mineralogist 76, 827- 847, 1991. Shirai, N. and M. Ebihara. Chemical characteristics of a Martian meteorite, Yamato 980459. Antarctic Meteorite Research 17, 55-67, 2004. Smith, J. V. and I. M. Steele. Achondrite ALHA77005: Alteration of chromite and olivine. Meteoritics 19, 121-133. Solomon, S. C, O. Aharonson, J. M. Aurnou, W. B. Banerdt, M. H. Carr, A. J. Dombard, H. V. Frey, M. P. Golombek, S. A. Hauck, II, J. W. Head, III, B. M. Jakosky, C. L. Johnson, P. J. McGovern, G. A. Neumann, R. J. Phillips, D. E. Smith, and M. T. Zuber. New perspectives on ancient Mars. Science 307 (5713), 1214-1220, DOI: 10.1126/science.l 101812, 2005. Taylor, L. A., M. A. Nazarov, C. K. Shearer, H. Y. McSween, Jr., J. Cahill, C. R. Neal, M. A. Ivanova, L. D. Barsukova, R. C. Lentz, R. N. Clayton, and T. K. Mayeda. Martian meteorite Dhofar 019: A new shergottite. Meteoritics and Planetary Science 37, 1107-1128, 2002. Walton, E. L., S. P. Kelley, and C. D. K. Herd. Isotopic and petrographic evidence for young Martian basalts. Geochimica et Cosmochimica Acta 72 (23), 5819-5837, 2008. Wells, P. R. A. Pyroxene thermometry in simple and complex systems. Contributions to Mineralogy and Petrology 62 (2), 129-139, DOI: 10.1007/BF00372872, 1977. 49 Witt-Eickschen, G. and H. St.C. O'Neill. The effect of temperature on the equilibrium distribution of trace elements between clinopyroxene, orthopyroxene, olivine and spinel in upper mantle peridotite. Chemical Geology 221, 65-101, doi:10.1016/j.chemgeo.2005.04.005,2005. Xie, S. and P. J. Tackley. Evolution of helium and argon isotopes in a converting mantle. Physics of the Earth and Planetary Interiors 146, 417-439, 2004. Wanke, H., G. Dreibus, E. Jagoutz, H. Palme, B Spettel, and G. Weckwerth. ALHA77005 and on the chemistry of the Shergotty parent body (Mars). 17th Lunar and Planetary Science Conference, 919-920, 1986. Zipfel, J., P. Scherer, B. Spettel, G. Dreibus, and L. Schultz. Petrology and chemistry of the new shergottite Dar al Gani 476. Meteoritics and Planetary Science 35, 95- 106, 2000. Zuber, M. T. and P. J. Mouginis-Mark. Caldera subsidence and magma chamber depth of the Olympus Mons volcano, Mars. Journal of Geophysical Research 97 (Ell), 18295-18307, 1992.