5.1. Olivine-Phyric Shergottites
 Olivine-phyric shergottites [Goodrich, 2002] are extrusive pyroxene-plagioclase basalts with porphyritic textures of large olivine crystals (megacrysts). Related plutonic ultramafic rocks that also contain olivine are called lherzolitic shergottites, and related basalts without olivine megacrysts are called basaltic shergottites. Olivine-phyric shergottites provide the most direct analogy to the Gusev picritic basalts.
 The compositions and proportions of olivine in olivine-phyric shergottites and descriptive references for these meteorites are summarized in Table 2. A backscattered electron (BSE) image of the SAU 005 meteorite (Figure 12) shows a similar texture to those of Gusev basalts. The olivine megacrysts in these meteorites are strongly zoned with magnesian cores and ferroan rims, and their measured compositional ranges (bars in Figure 11a) generally overlap the average composition of olivine in Gusev basalts. This comparison is hampered by the fact that we do not know whether Gusev olivines are also zoned. If they are, as seems likely, their compositional ranges may be approximated by the compositions of olivines formed between the liquidus and solidus in a MELTS crystallization calculation (see section 6.1). This range (Fo81–55) is also illustrated by a gray bar labeled MELTS in Figure 11a. To allow a more straightforward comparison, the calculated normative olivine compositions for olivine-phyric shergottites (calculated assuming bulk rock Fe2+/Fe(total) = 0.84, as measured for Humphrey [Morris et al., 2004]) are illustrated by shaded circles in Figure 11a. The shergottite olivines appear to be more magnesian than those in Gusev basalts, but the differences are not pronounced. The compositions of olivines in lherzolitic shergottites (shaded box in Figure 11a) are also more magnesian. The volume proportions of olivine megacrysts in Gusev basalts and olivine-phyric shergottites also overlap (Figure 11b and Table 2). These similarities support the hypothesis that olivine-phyric shergottites and Gusev basalts may have had a similar petrogenesis. What insights can be gained from this comparison?
Figure 12. Backscattered electron (BSE) image of olivine-phyric shergottite SAU 005 showing olivine megacrysts in a groundmass of pyroxenes and plagioclase. Figure is 5.5 mm across [after Goodrich, 2003].
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Table 2. Properties of Olivine in Olivine-Phyric Shergotttites
|Meteorite||Compositional Range, % Fo||Abundance, vol%||Normative Composition,a % Fo||Referencesb|
|EETA 79001||81-52||7–13||62||(2), (3)|
|DaG 489c||79-59||18–20||66||(6), (8)|
 It is not clear whether the gray megacrysts (presumed to be olivine) in Gusev basalts are phenocrysts that grew from the magma or are foreign crystals added to the magma [McSween et al., 2004]. Likewise, there has been considerable controversy about the origin of the megacrysts in olivine-phyric shergottites. Some studies have focused on the ragged and sometimes embayed grain boundaries, the presence of occasional polymineralic grains, and the compositions of olivine cores that are generally too iron-rich to have been in equilibrium with the bulk-rock compositions. Such studies have also noted that ferroan rims occur on megacrysts where they are in contact with groundmass, suggesting disequilibrium with the enclosing magma. On the basis of such observations, the olivines have been suggested to be xenocrysts (foreign crystals incorporated into the magma) [Steele and Smith, 1982; McSween and Jarosewich, 1983; Wadhwa et al., 2001; Koizumi et al., 2004] or cumulates (crystals accumulated and concentrated from a large batch of magma) [Barrat et al., 2002; Koizumi et al., 2004]. In these models, olivine has also been suggested to have been partly resorbed and assimilated by the host magmas. The high thermal energy required to assimilate olivine crystals poses a considerable problem, and several alternatives have been proposed. Wadhwa et al.  suggested that the megacrysts might have been introduced by mixing an olivine phenocryst-bearing magma with a basaltic shergottite magma. Mittlefehldt et al.  favored the idea that the megacrysts were residual crystals from impact melting, and Folco et al.  proposed that they were restites from partial melting of lherzolitic shergottites.
 Recent recovery of new olivine-phyric shergottites with distinct textures and more magnesian olivine core compositions supports another interpretation. Some of these (Dhofar 019 and Yamato 980459) contain euhdral olivine megacrysts having magnesian core compositions (Fo83-86) that are in equilibrium with the bulk-rock compositions [Taylor et al., 2002; Greshake et al., 2004; Mikouchi et al., 2004]. Crystal size distribution (CSD) analyses of olivines for the most part produce linear arrays [Taylor et al., 2002; Goodrich, 2003; Greshake et al., 2004], supporting the hypothesis that most olivines formed by continuous cooling without interruption. These data support the hypothesis that the olivine megacrysts are phenocrysts.
 A complicating factor is that even Dhofar 019 and Yamato 980459 contain a few corroded, Fe-rich olivines, and their CSD patterns deviate from the linear trend at the largest grain sizes [Goodrich, 2003; Greshake et al., 2004]. These unusual grains are possibly xenocrysts or cumulates, but they comprise only a small portion of the olivine megacryst population in each meteorite. A further complication is that, for most olivine-phyric shergottites, the earliest crystallizing olivine phenocrysts (those with compositions in equilibrium with the bulk-rock magma composition) are missing, apparently removed by crystal fractionation [Goodrich, 2003]. Taking all the data into account, the most plausible model is that the bulk of olivine crystals in olivine-phyric shergottites are phenocrysts. In many of these meteorites, the earliest-formed phenocrysts have been lost and some small proportion of xenocrysts or cumulates have been added. However, the most magnesian olivine-phyric shergottites like Yamato 980459 appear to represent liquid compositions, or nearly so.
 As expected, olivine-phyric shergottites have higher contents of magnesium and nickel than do basaltic shergottites (Figure 13a). Given the model above, this correlation can be interpreted as reflecting the higher magnesium and nickel abundances in primitive magmas, with the trend representing an olivine-control fractionation line. The Gusev basalts Humphrey, Adirondack, and Mazatzal plot near the olivine-phyric shergottites in Figure 13a. All the Martian rocks have higher magnesium contents at a given nickel content than terrestrial mafic and ultramafic rocks (Figure 13a). Gusev basalts also have magnesium-chromium ratios that are similar to olivine-phyric shergottites (Figure 13b). In this case, chromium occurs dominantly in associated spinel (also an early crystallizing phase; see section 6.1) rather than being hosted in olivine.
 The tight clustering of Gusev basalt compositions may provide a further argument that the olivines they contain are phenocrysts. Incorporation of xenocrysts or cumulates would likely be a random process, producing variable proportions of megacrysts and varying bulk rock chemistry. Despite apparent differences in modal olivine contents in Adirondack, Humphrey, and Mazatzal, their chemical compositions are nearly uniform.
5.2. Orbital Spectroscopy and Regional/Global Context
 The Thermal Emission Spectrometer (TES) on Mars Global Surveyor has provided unprecedented insights into the composition of broad areas of the Martian surface. A detailed study of atmospherically corrected spectra [Smith et al., 2000] in Cimmeria Terra by Christensen et al.  identified basaltic surface compositions dominated by plagioclase (45% and 53%) and high-calcium pyroxene (26% and 19%) with detectable amounts of olivine (12%). Further analyses [Hoefen et al., 2003; Hamilton and Christensen, 2005] of TES spectra using several intermediate olivine compositions suggested that olivine is widespread and abundant in Nili Fossae, a volcanic terrain northeast of Syrtis Major. Mapped olivine compositions in Nili Fossae overlap olivine compositions in Gusev basalts, but olivines in Syrtis Major are more iron-rich (Figure 11a). The olivine composition becomes progressively more iron-rich from west to east across the 30,000 km2 Nili Fossae area, and linear deconvolutions indicate an olivine abundance of ∼30%. A study by Hamilton et al.  also found high abundances of olivine in Nili Fossae and local concentrations including but not limited to Gangis Chasma, Aurorae Planum, the Argyre and Hellas basin rims, and Eos Chasma. Rogers et al.  also found several olivine-bearing bedrock units, containing 25% olivine of composition Fo60, in Ares Vallis using data from TES and the Thermal Emission Imaging System (THEMIS). These authors noted that olivine-phyric shergottites provide the closest compositional match for the Ares Vallis rocks. Their exposure patterns suggest formation as several distinct flows.
 Figure 14a shows a THEMIS daytime infrared mosaic of Gusev Crater. Superimposed on the mosaic are the MER Spirit landing ellipse (∼83 km long by ∼10 km wide) and a TES orbital track (ock 5367, ick 1600–1603) with different colors showing derived surface temperatures (3 × 6 km spatial resolution). Differential heating of slopes produces a clear picture of the crater rim, Ma'adim Vallis, and mesas in the south. The floor of Gusev Crater, however, displays temperature variations that are independent of topography. Two prominent streaks trending SSE are warmer in the day than the rest of the crater floor and correspond to relatively low albedo, dust-free streaks that are evident in visible images. Their warmer temperatures are due to the differential heating that result from albedo contrast.
Figure 14. (a) THEMIS daytime infrared mosaic of Gusev Crater with superimposed MER Spirit landing ellipse and TES orbital track showing derived surface temperatures. Temperature variations on the floor of Gusev are due to differential heating that results from albedo contrast. (b) Average atmospherically corrected emissivity spectrum of six of the warmest TES pixels outlined in the THEMIS mosaic with a linearly deconvolved modeled spectral fit. The composition of the warm, low-albedo streak in Gusev Crater agrees well with MiniTES-derived compositions of dark disturbed soils.
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 An average atmospherically corrected emissivity spectrum of six of the warmest TES pixels (294–296 K) outlined in the THEMIS mosaic is shown in Figure 14b with a linearly deconvolved modeled spectral fit. The abundances of derived minerals that fit the measured spectrum have been summed into mineral groups with accuracies of 5–10 volume%. The end-member spectra shown are scaled by these abundances to show how each contributes to the measured spectrum. The composition of the warm, low-albedo streak is dominated by plagioclase (45%) and clinopyroxene (30%) with 15% olivine. This bulk mineralogy agrees well with the MiniTES-derived composition (section 4.2) of dark disturbed soils (50% plagioclase, 40% clinopyroxene, and 10% olivine) and indicates a regional similarity in basaltic compositions in Gusev Crater.
 Figure 15 shows a new global map of olivine abundances on Mars derived from atmospherically corrected TES data, overlain on a color mosaic. Recent work [Hamilton and Schneider, 2005] has led to four additional intermediate to ferroan olivine composition (Fo68, Fo60, Fo35, and Fo10) spectral end-members that can be used to detect and model olivine abundances more accurately. TES emissivity spectra were binned and averaged at 4 pixels/degree (15 km/pixel) [Bandfield, 2002] and linearly deconvolved from 1301–825 cm−1 and 508–233 cm−1 using a spectral end-member set that includes a broad range of igneous and sedimentary minerals, Martian atmospheric dust and water ice [Bandfield et al., 2000b], and epf (emission phase function)-derived Martian surface dust [Bandfield and Smith, 2003]. The new TES olivine map shows detectable amounts of olivine (10–15 volume%) in broad near-equatorial regions and corresponds well with the highest mapped abundances of the global TES basalt unit [Bandfield et al., 2000a]. This result is consistent with previous work by Christensen et al.  which modeled 12% olivine for basaltic surface compositions. The global olivine map also resolves higher olivine abundances (35–50 volume%) detected in the Nili Fossae region [Hoefen et al., 2003; Hamilton and Christensen, 2005] but does not resolve olivine abundance in Gusev Crater because of the decreased spatial resolution (15 km/pixel) of the binned data set and high amount of surface dust.
 Results from individual TES spectra of Gusev Crater and globally mapped olivine abundances indicate that large expanses of the Martian surface are characterized by olivine-bearing basalts and/or soils with similar compositions.