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Water and the Composition of Martian Magmas

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Water and the Composition of Martian Magmas Water and the composition of Martian magmas J. Brian Balta and Harry Y. McSween, Jr. Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, Tennessee 37996, USA ABSTRACT 52 Shergottites, the most abundant martian meteorites, represent the best source of informa- tion about Mars’ mantle and its dissolved water. If the mantle was wet, magmatic degassing 50 could have supplied substantial water to the martian surface 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 magma chemistry and physical properties. Deciphering 2 TP HPH the history of water on Mars 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 rock has been found in stratigraphic context and there is debate over 42 AP the timing of eruptions of shergottite-like magmas. Partial melting experiments on terrestrial OM AM2 PM basalts 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 nakhlites representing melts of other 48 mantle sources. SP1 SP2 (wt%) 46 2 HP INTRODUCTION whereas most magmatism earlier in Mars’ his- TP Post-Noachian (younger than 3.8 Ga) mag- tory is nearly unsampled by meteorites, a bias SiO 44 SP3 SM mas compose much of the martian crust, and if termed the shergottite age 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 Tharsis plateau, 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 olivine-phyric) com- Other martian meteorite groups, the nakhlites, ing the mantle as a source for martian surface pared with gamma ray spectrometer (GRS) chassignites, and Allan Hills 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 Northwest Africa 7034 (NWA 7034) is a this question, we will assess how the shergottites parental magma choices, see the Data Re- crustal breccia (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. (Hesperian) volcanism. Red boxes show com- can give information about their mantle sources. positions for younger (Amazonian) 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). Green 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 mineral apatite 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—Hesperia Planum; TP—Tyrrhena Pla- num; SP3—Syria Planum; SM—Syrtis Major; and indications of lower water content (e.g., gottites (Fig. 1) was discovered by the Mars EM—Elysium Mons; AM1—Ascraeus Mons; Usui et al., 2012) may refl ect degassing (Balta rover Opportunity at Meridiani Planum (Zipfel AP—Alba Patera; AM2—Arsia Mons; OM— et al., 2013). Despite confi rmation of the pres- et al., 2011). This rock was proposed to have Olympus Mons; PM—Pavonis Mons. X marks ence of water, current analyses are insuffi cient been excavated from beneath the late Noachian Bounce Rock. to demonstrate that magmatic water contributed and/or early Hesperian Meridiani plains by an to martian surface water 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 stratigraphy 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 Spirit 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. Tuff et al. (2013) proposed that of younger than 500 Ma (Nyquist et al., 2009), in Gusev 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 pressures GRS has a large footprint (~250 km), but has ca. 4.5 Ga ages as magma crystallization 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-pressure, 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 peridotite 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).
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