The early differentiation history of Mars from 182W-142Nd isotope systematics in the SNC meteorites (original) (raw)
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Geochimica et Cosmochimica Acta, 2012
Shergottite meteorites are a suite of mafic to ultramafic igneous rocks whose parental magmas probably derived from the martian mantle. In this study, a suite of 23 shergottites, spanning their known range in bulk compositions, Rb-Sr, Sm-Nd, and Lu-Hf isotopes, were measured for 187 Re-187 Os isotopic systematics and highly siderophile element abundances (HSE: including Os, Ir, Ru, Pt, Pd, Re). The chief objective was to gain new insight on the chemical evolution of the martian mantle by unraveling the long-term HSE budget of its derivative melts. Possible effects upon HSEs related to crustal contamination, as well as terrestrial and/or martian surface alteration are also examined. Some of the shergottites are hot arid-desert finds. Their respective acetic acid leachates and residues show that both Re and Os display open-system behavior during sample residence at or near the martian and/or terrestrial surfaces. In some meteorites, the alteration effects can be circumvented by analysis of the leached residues. For those shergottites believed to record robust Re-Os isotopic systematics, calculated initial 187 Os/ 188 Os are well correlated with the initial 143 Nd/ 144 Nd. Shergottites from mantle sources with long-term melt-depleted characteristics (initial e 143 Nd of +36 to +40) have chondritic initial c 187 Os ranging from À0.5 to +2.5. Shergottites with intermediate initial e 143 Nd of +8 to +17 have a range in initial c 187 Os of À0.6 to +2.3, which overlaps the range for depleted shergottites. Shergottites from long-term enriched sources, with initial e 143 Nd of $À7, are characterized by suprachondritic c 187 Os values of +5 to +15. The initial c 187 Os variations for the shergottites do not show a correlation with indices of magmatic differentiation, such as MgO, or any systematic differences between hot ariddesert finds, Antarctic finds, or observed falls. The strong correlation between the initial e 143 Nd and c 187 Os in shergottites from approximately +40 and 0 to À7 and +15, respectively, is assessed in models for mixing depleted mantle-derived melts with ancient crust (modeled to be similar to evolved shergottite in composition), and with assimilation-fractional crystallization. These models show that the correlation is unlikely to result from participation of martian crust. More likely, this correlation relates to contributions from depleted and enriched reservoirs formed in a martian magma ocean at ca. 4.5 Ga. These models indicate that the shergottite endmember sources were generated by mixing between residual melts and cumulates that formed at variable stages during solidification of a magma ocean. The expanded database for the HSE abundances in shergottites suggests that their martian mantle sources have similar HSE abundances to the terrestrial mantle, consistent with prior studies. The relatively high HSE abundances in both planetary
Early martian mantle overturn inferred from isotopic composition of nakhlite meteorites
Nature Geoscience, 2009
The early stages of planetary differentiation are characterized by the formation of magma oceans, which crystallize from the base up 1,2 . The final, iron-rich residues of crystallization are dense and therefore tend to sink into the mantle, whereas the deeper, magnesium-rich material tends to rise up 3,4 . The resultant mantle overturn would have had a profound influence on the evolution of the planets 3-6 . Such an event probably occurred on Mars, but its initiation, timing and geochemical consequences are poorly constrained. Here we use isotopic data for nakhlite meteorites-chunks of martian crust transported to the Earth-and numerical simulations to constrain the evolution of the early martian mantle. We interpret the isotopic composition of the meteorites as evidence for an episode that occurred relatively early in Mar's history, about 100 million years after the planet's formation, during which garnet was removed from material that rose up from the deep mantle. This episode implies large-scale reorganization in the martian mantle and thereby provides compelling support for overturn. We suggest that this event probably led to substantial re-melting in the deepest mantle, which may have influenced early martian processes such as the development of crustal dichotomy.
Earth and Planetary Science Letters, 2008
Chemical heterogeneities in the Martian mantle are believed to result from the crystallization of a magma ocean in the first 100 million years of its history. Shergottite meteorites from Mars are thought to retain a compositional record of such early differentiation and the resulting mineralogy at different depths. The coupled 176 Lu-176 Hf and 147 Sm-143 Nd isotope systematics in 9 shergottites are used here to investigate these issues. Three compositional groups in the shergottites display distinct isotope systematics. One group, commonly termed as depleted, is characterized by positive ɛ 176 Hf i from + 46.2 to +50.4 and ɛ 143 Nd i from + 36.2 to + 39.1. Another, termed as enriched, has negative ɛ 176 Hf i = − 16.5 to −13.2 and ɛ 143 Nd i = −7.0 to − 6.5. The third group is intermediate between the depleted and enriched groups with positive ɛ 176 Hf i = + 30.0 to + 33.4 and ɛ 143 Nd i = +16.9. Together, they describe mixing curves between 176 Hf/ 177 Hf, 143 Nd/ 144 Nd, Lu/Hf, and Sm/Nd, implying that they sample two distinct sources in the Martian mantle. All shergottites are characterized by (Sm/Nd) source b (Sm/Nd) sample , but (Lu/Hf) source N (Lu/Hf) sample . This decoupling can be explained by two successive partial melting episodes in the depleted shergottite source and localized in the Martian upper mantle. The genesis of shergottites can be modeled using non-modal equilibrium partial melting in a source initially composed of 60% olivine, 21% clinopyroxene, 9% orthopyroxene, and 10% garnet, with degrees of partial melting of 8.8% and 3.9%, respectively, for the two successive events. The enriched end-member of the shergottite mixing curve is best modeled by late-stage quenched residual melt resulting from the crystallization of a magma ocean. The depleted shergottite source may be modeled as a mixture of cumulates and residual melt, as convection in the Martian magma ocean is expected to reduce the incompatible trace element heterogeneity in the final solidified layers. Consequently, equilibrium crystallization is preferred to model the crystallization of the Martian magma ocean. The models that best explain the shergottite data are those where the magma ocean is at a depth of at least 1350 km in Mars.
Geochimica et Cosmochimica Acta, 2004
The timescale of accretion and differentiation of asteroids and the terrestrial planets can be constrained using the extinct 182 Hf-182 W isotope system. We present new Hf-W data for seven carbonaceous chondrites, five eucrites, and three shergottites. The W isotope data for the carbonaceous chondrites agree with the previously revised 182 W/ 184 W of chondrites, and the combined chondrite data yield an improved ⑀ W value for chondrites of Ϫ1.9 Ϯ 0.1 relative to the terrestrial standard. New Hf-W data for the eucrites, in combination with published results, indicate that mantle differentiation in the eucrite parent body (Vesta) occurred at 4563.2 Ϯ 1.4 Ma and suggest that core formation took place 0.9 Ϯ 0.3 Myr before mantle differentiation. Core formation in asteroids within the first ϳ5 Myr of the solar system is consistent with the timescales deduced from W isotope data of iron meteorites. New W isotope data for the three basaltic shergottites EETA 79001, DaG 476, and SAU 051, in combination with published 182 W and 142 Nd data for Martian meteorites reveal the preservation of three early formed mantle reservoirs in Mars. One reservoir (Shergottite group), represented by Zagami, ALH77005, Shergotty, EETA 79001, and possibly SAU 051, is characterized by chondritic 142 Nd abundances and elevated ⑀ W values of ϳ0.4. The 182 W excess of this mantle reservoir results from core formation. Another mantle reservoir (NC group) is sampled by Nakhla, Lafayette, and Chassigny and shows coupled 142 Nd-182 W excesses of 0.5-1 and 2-3 ⑀ units, respectively. Formation of this mantle reservoir occurred 10-20 Myr after CAI condensation. Since the end of core formation is constrained to 7-15 Myr, a time difference between early silicate mantle differentiation and core formation is not resolvable for Mars. A third early formed mantle reservoir (DaG group) is represented by DaG 476 (and possibly SAU 051) and shows elevated 142 Nd/ 144 Nd ratios of 0.5-0.7 ⑀ units and ⑀ W values that are indistinguishable from the Shergottite group. The time of separation of this third reservoir can be constrained to 50-150 Myr after the start of the solar system. Preservation of these early formed mantle reservoirs indicates limited convective mixing in the Martian mantle as early as ϳ15 Myr after CAI condensation and suggests that since this time no giant impact occurred on Mars that could have led to mantle homogenization. Given that core formation in planetesimals was completed within the first ϳ5 Myr of the solar system, it is most likely that Mars and Earth accreted from pre-differentiated planetesimals. The metal cores of Mars and Earth, however, cannot have formed by simply combining cores from these pre-differentiated planetesimals. The 182 W/ 184 W ratios of the Martian and terrestrial mantles require late effective removal of radiogenic 182 W, strongly suggesting the existence of magma oceans on both planets. Large impacts were probably the main heat source that generated magma oceans and led to the formation metallic cores in the terrestrial planets. In contrast, decay of short-lived 26 Al and 60 Fe were important heat sources for melting and core formation in asteroids.
The Lu–Hf isotope geochemistry of shergottites and the evolution of the Martian mantle–crust system
Earth and Planetary Science Letters, 1999
We report the first Lu-Hf isotopic data from SNC (Martian) meteorites with high-precision analyses of five shergottites by plasma sector mass spectrometry. Hf isotopic compositions indicate the presence of both geochemically enriched and depleted components, reflecting early segregation of Martian crust and mantle. Comparison with Sm-Nd and U-Th-Pb isotopic systems suggests that the enriched component reflects the extraction of very small-degree melts at depths in excess of 200 km in the presence of garnet, while the depleted component could correspond to large-degree melts from mantle already depleted by the extraction of enriched liquids. The general lack of correlation between major element composition and isotopic properties of SNC meteorites argues against the segregation of the lithophile elements into Earth-like continents or Moon-like highlands on Mars, although early magma ocean fractionation may explain many of the primary isotopic properties of SNC meteorites. Recent (0.18-0.33 Ga old) fractionation of Lu=Hf and Sm=Nd in shergottites is inconsistent with purely magmatic partitioning of these elements and appears more likely to be a product of metasomatic processes involving the circulation of P 2 O 5 -rich fluids, triggered either by distant magmatic activity or, more probably, by impacts.
Meteoritics & Planetary Science, 2004
20-25 mg whole rock samples of the nakhlites Lafayette and Nakhla have been analyzed via the 40 Ar-39 Ar technique, in part to verify their formation ages, but primarily, in an attempt to determine the timing of aqueous alteration in these martian meteorites. As in previous studies, plateaus in apparent age are observed at about 1300 Ma (1322 ± 10 for Lafayette, 1332 ± 10 and 1323 ± 11 for Nakhla), presumably corresponding to crystallization ages. The plateaus are not entirely flat, perhaps reflecting the effects of recoil during creation of 39 Ar in the nuclear irradiation. The first 5-20% of the K-derived Ar released from all three samples give apparent ages <1300 Ma. Coupled with the fact that chronometric isotopic studies of nakhlites typically show some disturbance, we believe the low temperature pattern represents more recent (than 1300 Ma) formation of martian aqueous alteration products such as iddingsite. No low temperature plateaus are observed. This is consistent with petrographic evidence for multiple formation events, although the lack of low temperature plateaus is far from conclusive. On the other hand, if there was a single time of alteration, we believe that it will be difficult, if not impossible, to determine it using the K-Ar system.
Geochimica et Cosmochimica Acta, 2002
Detailed Rb-Sr and Sm-Nd isotopic analyses have been completed on the lherzolitic shergottites ALH77005 and LEW88516. ALH77005 yields a Rb-Sr age of 185 Ϯ 11 Ma and a Sm-Nd age of 173 Ϯ 6 Ma, whereas the Rb-Sr and Sm-Nd ages of LEW88516 are 183 Ϯ 10 and 166 Ϯ 16 Ma, respectively. The initial Sr isotopic composition of ALH77005 is 0.71026 Ϯ 4, and the initial Nd value is ϩ11.1 Ϯ 0.2. These values are distinct from those of LEW88516, which has an initial Sr isotopic composition of 0.71052 Ϯ 4 and an initial Nd value of ϩ8.2 Ϯ 0.6. Several of the mineral and whole rock leachates lie off the Rb-Sr and Sm-Nd isochrons, indicating that the isotopic systematics of the meteorites have been disturbed. The Sm-Nd isotopic compositions of the leachates appear to be mixtures of primary igneous phosphates and an alteration component with a low 143 Nd/ 144 Nd ratio that was probably added to the meteorites on Mars. Tie lines between leachate-residue pairs from LEW88516 mineral fractions and whole rocks have nearly identical slopes that correspond to Rb-Sr ages of 90 Ϯ 1 Ma. This age may record a major shock event that fractionated Rb/Sr from lattice sites located on mineral grain boundaries. On the other hand, the leachates could contain secondary alteration products, and the parallel slopes of the tie lines could be coincidental. Nearly identical mineral modes, compositions, and ages suggest that these meteorites are very closely related. Nevertheless, their initial Sr and Nd isotopic compositions differ outside analytical uncertainty, requiring derivation from unique sources. Assimilation-fractional-crystallization models indicate that these two lherzolitic meteorites can only be related to a common parental magma, if the assimilant has a Sr/Nd ratio near 1 and a radiogenic Sr isotopic composition. Further constraints placed on the evolved component by the geochemical and isotopic systematics of the shergottite meteorite suite suggest that it (a) formed at ϳ4.5 Ga, (b) has a high La/Yb ratio, (c) is an oxidant, and (d) is basaltic in composition or is strongly enriched in incompatible elements. The composition and isotopic systematics of the evolved component are unlike any evolved lunar or terrestrial igneous rocks. Its unusual geochemical and isotopic characteristics could reflect hydrous alteration of an evolved Martian crustal component or hydrous metasomatism within the Martian mantle.
The age of SNC meteorites and the antiquity of the Martian surface
Earth and Planetary Science Letters, 2005
We report new Sm-Nd, Lu-Hf, and Pb-Pb mineral and whole-rock isotope data for the basaltic shergottite Zagami, as well as Pb-Pb whole-rock isotope data for the basaltic shergottite Los Angeles, the lherzolitic shergottite Dar-al-Gani 476 (DaG 476), and the clinopyroxenite Nakhla. In agreement with previous findings, our new Sm-Nd and Lu-Hf mineral ages on the Martian meteorite Zagami are young (155 and 185 Ma, respectively). The 207 Pb/ 206 Pb-204 Pb/ 206 Pb compositions of the insoluble fractions of shergottites (Zagami, Los Angeles, and literature data for Shergotty and EETA79001) form an excellent alignment indicative of a 4.0 Ga crystallization age. The range of Pb isotope compositions observed in the leachates of these samples attests to negligible contamination of the shergottites by terrestrial Pb and argues against mixing relationships. The age of 4.048 F 0.017 Ga (MSWD = 1.5) provided by the Pb isotope compositions of the Zagami whole-rock and residues is therefore taken to date the crystallization of this rock, which, so far, was believed to be only~180 Ma old. Based on this result, we argue that the lithosphere of Mars is extremely old and that most mineral ages were reset recently by acidic aqueous solutions percolating through the Martian surface. This interpretation is consistent with photographic interpretations of erosional features on Mars. It also relieves the constraint imposed by the presence of anomalies of 142 Nd and 182 W (both products of extinct radioactive nuclides) that the Martian mantle should have preserved primordial isotopic heterogeneities, thus allowing for the planet interior to be actively convecting.
Geochimica et Cosmochimica Acta, 2003
Samarium-neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 Ϯ 11 Ma and an initial Nd 143 value of ϩ36.6 Ϯ 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87 Sr/ 86 Sr ratio of 0.701249 Ϯ 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace elementisotopic systematics observed in the martian meteorite suite.
–We report precise triple oxygen isotope data of bulk materials and separated fractions of several Shergotty–Nakhla–Chassigny (SNC) meteorites using enhanced laser-assisted fluorination technique. This study shows that SNCs have remarkably identical D 17 O and a narrow range in d 18 O values suggesting that these meteorites have assimilated negligibly small surface materials (<5%), which is undetectable in the oxygen isotope compositions reported here. Also, fractionation factors in coexisting silicate mineral pairs (px-ol and mask-ol) further demonstrate isotopic equilibrium at magmatic temperatures. We present a mass-dependent fractionation line for bulk materials with a slope of 0.526 AE 0.016 (1SE) comparable to the slope obtained in an earlier study (0.526 AE 0.013; Franchi et al. 1999). We also present a new Martian fractionation line for SNCs constructed from separated fractions (i.e., pyroxene, olivine, and maskelynite) with a slope of 0.532 AE 0.009 (1SE). The identical fractionation lines run above and parallel to our terrestrial fractionation line with D 17 O = 0.318 AE 0.016& (SD) for bulk materials and 0.316 AE 0.009& (SD) for separated fractions. The conformity in slopes and D 17 O between bulk materials and separated fractions confirm oxygen isotope homogeneity in the Martian mantle though recent studies suggest that the Martian lithosphere may potentially have multiple oxygen isotope reservoirs.