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Home , Allan Hills 84001, Bounce Rock, Magma, Martian meteorite, Nakhlite, Noachian

Water and the composition of

J. Brian Balta and Harry Y. McSween, Jr. Department of and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, Tennessee 37996, USA

ABSTRACT 52 Shergottites, the most abundant martian , represent the best source of informa- tion about ’ and its dissolved . If the mantle was wet, magmatic degassing 50 could have supplied substantial water to the early in its history. Research- ers have attempted to reconstruct the volatile contents of shergottite parental magmas, with 48 SP1 recent analyses confi rming that the shergottites contained signifi cant water. However, water is

(wt%) 46 SP2 not a passive tracer; it directly affects chemistry and physical properties. Deciphering 2 TP HPH the history of requires understanding how that water affected the chemistry of SiO 44 SM the shergottites and how they fi t within Mars’ geologic history. Both topics present diffi culties, SP3 AM1 EM as no shergottite-like has been found in stratigraphic context and there is debate over 42 AP the timing of eruptions of shergottite-like magmas. experiments on terrestrial OM AM2 PM and new data from orbiters and rovers on Mars provide the information needed to 40 overcome these diffi culties and explain the role of water in shergottite magmas. Here we show 14 16 18 20 22 that shergottite compositions and their martian geologic context can be explained by melting FeOT (wt%) of an originally wet mantle that degassed over time. We also demonstrate that models for the evolution of the martian mantle that do not consider water fail to account for the shergottite 52 compositions, surface distributions, and ages. Finally, we suggest that dehydration of the mar- 50 tian mantle has led to changes in magmatic chemistry over time, with shergottites represent- ing melts of water-bearing mantle and rocks similar to representing melts of other 48 mantle sources. SP1 SP2 (wt%) 46 2 HP INTRODUCTION whereas most earlier in Mars’ his- TP Post- (younger than 3.8 Ga) mag- tory is nearly unsampled by meteorites, a bias SiO 44 SP3 SM mas compose much of the martian , and if termed the shergottite paradox (Nyquist et EM AM1 they contained ~1 wt% water, they could have al., 1998). Craters possibly linked to shergottite 42 AP PM supplied the equivalent of several hundred me- ejection have been identifi ed within young vol- AM2 OM 40 ters water depth to the surface (McSween and canic sequences on the , suggest- 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Harvey, 1993). Determining the original water ing that the shergottites may sample only young, Th (ppm) contents of these magmas is currently possible coherent rocks from these recently active volca- only by using meteoritic samples. The shergottite noes (Tornabene et al., 2006; Lang et al., 2009). Figure 1. Shergottite whole-rock composi- martian meteorites are basaltic igneous rocks. If the shergottites are a biased sample, evaluat- tions (crosses are lherzolitic, diamonds are basaltic, and circles are -phyric) com- Other martian groups, the nakhlites, ing the mantle as a source for martian surface pared with gamma ray spectrometer (GRS) chassignites, and 84001(ALH water requires understanding how the shergot- data. Filled circles represent plausible paren- 84001) are cumulate rocks, and the water-rich tites relate to ancient magmatism. To address tal magma compositions (for a discussion of rock (NWA 7034) is a this question, we will assess how the shergottites parental magma choices, see the Data Re- crustal (Agee et al., 2013). As with ter- fi t into Mars’ history, combining mission results pository [see footnote 1]). Blue boxes show representative GRS compositions for older restrial basalts, the chemistry of martian basalts and mantle evolution models. () . Red boxes show com- can give information about their mantle sources. positions for younger () Tharsis However, the shergottites are not pristine, hav- SHERGOTTITES IN THE CONTEXT OF and Elysium volcanoes (Baratoux et al., 2011). ing undergone alteration on Mars, ejection in RECENT MISSION RESULTS Blue arrows show compositional changes from olivine accumulation (lherzolitic sher- energetic shock events, and residence in space Recent Mars missions have provided com- gottites). arrows represent shallow and on Earth prior to analysis. Only recently positional and mineralogical information that crustal assimilation and fractional crystalliza- have analyses of the established establishes geologic context for igneous rocks. tion (AFC) processes (basaltic shergottites). that some shergottite magmas contained water at First, a single erratic rock (named Bounce SP1—Sinai Planum; SP2—Solis Planum; the weight percent level (McCubbin et al., 2012) Rock) that geochemically resembles the sher- HP—; TP—Tyrrhena Pla- num; SP3—; SM—Syrtis Major; and indications of lower water content (e.g., gottites (Fig. 1) was discovered by the Mars EM—; AM1—; Usui et al., 2012) may refl ect degassing (Balta rover at (Zipfel AP—Alba Patera; AM2—; OM— et al., 2013). Despite confi rmation of the pres- et al., 2011). This rock was proposed to have ; PM—. X marks ence of water, current analyses are insuffi cient been excavated from beneath the late Noachian . to demonstrate that magmatic water contributed and/or early Hesperian Meridiani plains by an to martian because the shergottite impact (Arvidson et al., 2006), which would re- magmas are not fully representative of martian quire Bounce Rock to be Noachian (ca. 4 Ga). alkali basalts while the shergottites are tholei- magmatism. The shergottites are generally ac- This tenuous would require that itic, their textural and chemical properties sug- cepted to be relatively young (Amazonian, shergottite-like magmatism occurred through- gest that hydrous magmatism (Nekvasil et al., younger than 2 Ga), with K-Ar, Rb-Sr, and Sm- out Mars’ history. Second, the rover en- 2009; McSween et al., 2006) occurred early in Nd systems giving sometimes-concordant ages countered Hesperian-aged (ca. 3.5 Ga) basalts martian history. et al. (2013) proposed that of younger than 500 Ma (Nyquist et al., 2009), in crater. Although the Gusev rocks are the Gusev basalts derived from oxidized mantle,

GEOLOGY, October 2013; v. 41; no. 10; p. 1–4; Data Repository item 2013309 | doi:10.1130/G34714.1 | Published online XX Month 2013 GEOLOGY© 2013 Geological | October Society 2013 of America.| www.gsapubs.org For permission to copy, contact Copyright Permissions, GSA, or [email protected]. 1 but these measurements were based on high-Ni Overall, recent production of shergottite mag- Yamato 980459 (Y 980459) and NWA 5789 contents not present in the interiors of igneous mas appears diffi cult to explain in the context are not elevated in the SiO2 activity proxy, but rocks such as Adirondak or Humphrey, and thus of surface measurements and this evolution- experiments suggest that the Y 980459 paren- do not represent igneous trends. ary model. An alternative possibility, that the tal magma could have been hydrous (Draper,

The gamma-ray spectrometer (GRS) on the shergottites are Noachian rocks, has been ar- 2007), and both are elevated in XSiO2 (Gross et Mars Odyssey orbiter mapped chemical varia- gued based on ancient Pb-Pb ages (Bouvier et al., 2011). To strengthen this argument, we used tions that further develop this chemostratigraphy. al., 2008). However, the interpretation of these the pMELTS algorithm to calculate the GRS has a large footprint (~250 km), but has ca. 4.5 Ga ages as magma ages at which magmas are multiply saturated with delineated compositions in the Tharsis and Ely- is inconsistent with chronometers interpreted to olivine and orthopyroxene, a function of SiO2 sium volcanic provinces (El Maarry et al., 2009; be resistant to alteration or shock resetting, and activity (Ghiorso et al., 1983, 2001). Martian Gasnault et al., 2010). GRS data show that the thus has not found wide acceptance (Nyquist et multiple saturation pressures are typically low young volcanoes previously proposed as shergot- al., 2009). Consequently, another explanation compared to terrestrial counterparts, indicating tite sources have low SiO2 and FeO and high Th for young shergottites is required. high SiO2 activity (Table 1). Low multiple satura- compared to older martian volcanoes (Baratoux tion pressures can be produced by low-, et al., 2011; El Maarry et al., 2009). However, SHERGOTTITES AS WET MELTS low-degree melting (Asimow and Longhi, 2004), shergottites are signifi cantly different from the As the presence of water has been demon- but this is impractical for Mars because its thick GRS measurements and no crustal assimilation strated in some shergottite magmas (McCubbin crust would increase the average melting pres- or fractionation path produces the GRS data from et al., 2012), its impact on magma composition sure. Thus, the high SiO2 contents measured in a shergottite-like parent (Fig. 1; Baratoux et al., offers an alternative. Experiments have estab- shergottites refl ect high silica activities consistent 2011). The best shergottite compositional match lished that partial melting of con- with derivation by wet melting and diffi cult to is found in ancient Noachian highlands terrains taining several hundred parts per million water produce without water. (Gasnault et al., 2010), which would accord with produces magmas with >1 wt% water that, after If shergottite-like compositions are the signa- the hypothesis of shergottite-like volcanism early degassing, are elevated in SiO2 (e.g., Balta et al., ture of magmas with weight percent level wa- in Mars’ history. Spacecraft-based mineralogi- 2011). Other such as Cl or CO2 have ter, we can propose a model for the evolution cal observations using the OMEGA (visible and been proposed as contributors to martian mag- of the martian mantle consistent with available infrared mineralogical mapping spectrometer) mas, but those volatiles give rise to silica-de- constraints. In this model, high-SiO2 magmas instrument on the orbiter similarly pleted melts (Dasgupta and Hirschmann, 2007; such as those that produced Bounce Rock, the fail to locate shergottite-like rocks outside of the Filiberto and Treiman, 2009). Gusev basalts, and the Noachian highlands were Noachian highlands, showing a lack of olivine To evaluate this possibility, we compared sher- produced by dehydration melting of mantle with and an overabundance of high-Ca in gottite parental magma compositions with terres- several hundred parts per million water. With no younger rocks (Poulet et al., 2009). Thus, mul- trial melting experiments, using the parameters , the mantle would desiccate over tiple, independent spacecraft measurements sug- XSiO2 and XSiO2/(XMgO + XFeO) (effectively time, leading to water-poor, low-SiO2 magmas gest that shergottite-like rocks are only found in a proxy for SiO2 activity; SiO2 compared to the building the young volcanoes. If fl uids rich in older terrains, not in the terrains with ages similar other cations in olivine and orthopyroxene; Balta other volatiles such as CO2 were able to migrate to those of the shergottites. et al., 2011). Figure 2 shows that martian parental within and metasomatize parts of Mars’ mantle, A proposed planetary evolution model can magma compositions are consistent with hydrous they could also be involved in younger mag- explain this compositional evolution assum- melting, as all samples have elevated XSiO2 rela- matism. But, any surviving high-H2O mantle ing that the lack of tectonic recycling leads to tive to the experimental dry basalts. Shergottites could cause occasional resumption of shergot- crustal thickening over time (Baratoux et al., tite magmatism if that mantle became entrained 2011; Poulet et al., 2009). The petrologic conse- in the Tharsis or Elysium upwellings (Fig. 3). quences of crustal thickening should be increas- 2 These shergottite magmas could be uncommon High SiO2 ing melting pressures and decreasing melt frac- activity enough to avoid detection by GRS, effectively 1.8 tions, leading to lower SiO2 and FeO and higher hidden from orbiting spectrometers. This mod- Th in younger primary magmas, matching GRS el implies that the mantle source of the recent trends (Fig. 1; Baratoux et al., 2011). Even if 1.6 Low SiO2 shergottites is directly related to the source of surface igneous rocks are not primary magmas, activity ancient volcanism, consistent with isotopic sim- this model can explain the chemical evolution as 1.4 ilarities between shergottites and the Noachian- /(XFeO + XMgO) Th could also be elevated by increasing crustal 2 aged meteorite ALH 84001 (Lapen et al., 2010). assimilation. However, this model predicts that 1.2 An earlier model similarly proposed a drying XSiO all young igneous rocks should be high in Th mantle, but suggested that early water-bearing 1 and low in SiO2 whether they are primary mag- magmas were calc-alkaline while the younger mas or not. Both primary magma compositions 0.42 0.44 0.46 0.48 0.5 0.52 shergottites were tholeiitic (McSween et al., and a crustal contamination trend have been rec- X SiO2 2003), inconsistent with the chemostratigraphy ognized among the shergottites (Fig. 1; see the Figure 2. Proxies for silica activity: silica mole discussed here. GSA Data Repository1), yet they match neither fractions plotted against ratio of and mag- The elevated Th measured by GRS is also con- the surface evolution trend nor the predictions of nesium mole fractions to silica mole fraction sistent with this model. If recent, water-bearing, (effectively, red arrow shows increase in oliv- the thickening-crust model. ine component, blue arrow shows increase shergottite magmas underwent little interaction in silica activity). Circles are martian parental with the martian crust, they would erupt with 1GSA Data Repository item 2013309, data and ci- magma compositions as in Figure 1. Red el- the low measured Th contents, whereas mag- tations used in Figure 1, and discussion of parental lipse shows dry magma compositions, blue mas contaminated by the crust would show Th magma choices, is available online at www.geosociety ellipse shows water-bearing compositions .org/pubs/ft2013.htm, or on request from editing@ (Balta et al., 2011, and references therein). enrichment. Crustal contamination is a particu- geosociety.org or Documents Secretary, GSA, P.O. Box Molar quantities are used to compare larly likely explanation for the GRS measured 9140, Boulder, CO 80301, USA. with differing iron abundances. Th abundances, as the 5–10× enrichment in Th

2 www.gsapubs.org | October 2013 | GEOLOGY TABLE 1. OLIVINE-ORTHOPYROXENE MULTIPLE SATURATION PRESSURES from the shergottites or from older, low-Ca FOR VARIOUS LIQUIDS pyroxene-rich rocks in the highlands. As Meteorite Multiple saturation pressure shown previously, younger rocks are enriched (calculated using pMELTS) in Th and low in silica, properties that gener- (kbar) ally resemble the nakhlites (high-Ca pyroxene NWA 1068 9.5 cumulates formed from early-depleted mantle LAR 0631 9.2 Yamato 980459 11.0 sources but Th enriched; Treiman, 2005). Al- Gusev 12.1 though nakhlites formed from a single magma NWA 2990 11.3 and could be unique, given these similarities it NWA 5789 13.3 is tempting to suggest that nakhlites, with ages of ca. 1.3 Ga, could more closely represent Terrestrial magmas Multiple saturation pressure recent magmatism than the shergottites. How- (measured experimentally) ever, establishing this connection is diffi cult as (kbar) estimates of the parent magma silica MORB (Kinzler,1997) 19 content vary signifi cantly (Treiman, 2005). (Maaløe, 2004) 20.5 Baratoux et al. (2011) proposed that low melt fractions could explain elevated Th contents, Calculated integrated terrestrial magmas from pMELTS (Asimow and Longhi, 2004) similar to some models for nakhlite generation Initial melting pressure Average melting pressure Multiple saturation pressure (Treiman, 2005). Alternatively, if a volatile-en- (kbar) (kbar) (kbar) riched reservoir remains in the martian mantle, 21.6 11.7 11.7 then by fl uids rich in volatiles 35.0 17.9 17.9 such as CO2 or Cl could be involved in their 15.0 7.4 7.5 production. The presence of both water and Cl Note: MORB—mid-oceanic ridge basalt. NWA—Northwest Africa; LAR—Larkman Nunatak (). in nakhlite magmas is required by recent mea- surements (Hallis et al., 2012; McCubbin et al., 2013); however, Th is immobile in water-rich time Noachian Hesperian Amazonian fl uids (Johnson and Plank, 2000), requiring

Low SiO2, high Th, low H2O Nakhlites some addition of a water-poor fl uid. A carbon-

High SiO2, low Th, high H2O Shergottites ate-rich component in nakhlite magma is con- sistent with their fugacities (Stanley et al., 2012) and high Ca/Al ratios (Dasgupta and Crust Crust Crust Hirschmann, 2007), and could provide a mech- MeltingMelilting anism for sampling a high-pressure, garnet- Melting zonezone Melting zone rich source required by some trace elements zones (Treiman, 2005). Alternatively, McCubbin et al. (2013) proposed late addition of a Cl-rich crustal brine to the nakhlites (and chassignites) Dry mantle Dry mantle during crystallization. If these fl uids carried Th and affected the crystallization sequences of most recent magmas, they could similarly

explain the SiO2-Th variation. This model implies a large fl ux of mantle- Wet mantle ABCWet mantle Wet mantle derived water to the surface early in Mars his- tory, within the Noachian or early Hesperian Figure 3. Schematic history of martian magmatism. A: Noachian ; wet mantle produces periods. Shergottite-like magmas could have voluminous high-SiO , low-Th, shergottite-like magmas. B: Hesperian era; previously de- 2 supplied even more water to the surface than hydrated mantle with magmatism focused into isolated upwellings. Melting of previously previously estimated, as those estimates were depleted mantle beneath thick crust creates low-SiO2, high-Th magmas. Dehydrated up- per mantle with possible hydrous material in lower mantle. C: Amazonian era; occasional based on the volume of post-Noachian magma- entrainment of hydrous material in upwelling resumes production of shergottite-like mag- tism (McSween and Harvey, 1993). This water matism, while previously dehydrated mantle, low melt fractions, fl uid metasomatism, and could have driven the formation of clays and crustal contamination give rise to nakhlite parental magmas and gamma-ray spectrometer– possibly given rise to an active hydrosphere measured compositions. capable of supporting . between GRS data and shergottites (Fig. 1) is , which can hold substantial structur- ACKNOWLEDGMENTS too large to be produced by changing degrees of al water, are stable phases (Bercovici and Karato, Support for this work was provided by NASA Cosmochemisty grant NNX13AH86G to McSween. melting alone. 2003). A hydrous lower mantle could serve as a We thank Francis McCubbin and three anonymous stable reservoir over geologic time, and occasion- reviewers for comments that improved the quality of Implications for Mars’ Geologic History al entrainment of such material in an upwelling this manuscript. This model has important implications for mar- that brings it into a melting zone could generate tian evolution. First, it requires that portions of recent shergottite magmatism (Fig. 3). REFERENCES CITED Agee, C.B., and 15 others, 2013, Unique meteorite the martian mantle still retain water (McCubbin Second, this model implies that high-Ca py- from early Amazonian Mars: Water-rich basaltic et al., 2012). Much of the lower mantle on Mars roxene-rich rocks analyzed by OMEGA near breccia Northwest Africa 7034: Science, v. 339, is at pressures where the or the young volcanoes have a different source p. 780–785, doi:10.1126/science.1228858.

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