Asteroids, Comets, Meteors (2012) 6206.pdf

ASTEROIDS AND COMETS SAMPLED DIFFERENT WATER RESERVOIRS. L. Bonal1,2,3*, C.M.O’D. Alexander1, G.R. Huss2, K. Nagashima2, E. Quirico3, P. Beck3. 1DTM, Carnegie Institution of Washington, Wash- ington DC 20015, USA; 2HIGP/SOEST, University of Hawai`i at Mānoa, Honolulu, HI 96822, USA; 3IPAG, OSUG, UJF/CNRS 38041 Grenoble FRANCE (*[email protected]).

Introduction: The isotopic composition of asteroi- Each of the studied CR chondrites escaped long dura- dal water is: (i) an important input in nebular models tion thermal metamorphism and experienced some [e.g., 1, 2]; (ii) a key parameter to better understand the fluid circulation [11, 12]. We have measured the H asteroid-comet continuum; and (iii) allows a better isotopic compositions of the altering fluids in situ, by understanding of asteroidal alteration. SIMS, by analyzing fine-grained hydrated silicates in Many chondrites experienced aqueous alteration, as bulk and individual coarse-grained hydrated silicate revealed by the presence of phyllosilicates that are grains in CRs. The main observations [11, 12] are that often associated with carbonates, magnetite, sulfides, (i) the water is systematically enriched in D in each CR and sulfates. It is generaly accepted that the alteration chondrite (up to δDwater = 1980‰) compared to Earth; of chondrites took place over extended periods of time, (ii) the isotopic composition of the water is character- and probably occurred almost entirely in asteroidal ized by highly variable D-enrichment at the microme- rather than nebular settings [e.g., 3]. Our understanding ter scale; (iii) there is no clear trend observed in the of the conditions of alteration remains imperfect. For isotopic composition of the water (maximum D- example, the origin, composition, and evolution of the enrichment, range of variation) along the aqueous fluids during alteration remain poorly constrained. The alteration sequence in the studied CR chondrites. modification induced by the fluids on the pristine char- The high variability of the isotopic composition of acteristics of the original constituents of the chondrites the water/OH in CR chondrites and the rather low D- is only partially understood. enrichment of bulk water (e.g., δDwater-bulk (Renazzo)~ Aqueously altered chondrites are composed of two 200‰, [7]) is most easily reconciled with a secondary main H-bearing phases: organics and hydrated miner- origin of the D-enrichments, rather than with the accre- als. Phyllosilicates and silicates in some aqueously tion and incomplete mixing of two or more ices with meteorites are enriched in deuterium (D) relative to distinct isotopic compositions/sources. The isotopic Earth [e.g., 4-6], but the enrichments are smaller than compositions of the water as measured today in situ in in the respective organics [7]. If chondritic water hydrated chondrites do not reflect the isotopic compo- formed in the outer region of the solar nebula or was sition of the water initially accreted on their respective inherited from the chemistry that took place in the parent body, but largely result from secondary modifi- presolar molecular cloud, one might expect it to be D- cation [11, 12]. The estimated initial water composi- rich compared to the solar H isotopic composition [1, tion (δDwater ≤ – 450 ‰) shows that most or all of it 2], and similar to that in comets [8, 9]. The water formed/re-equilibrated in the inner Solar Sytem [7, 12]. measured so far in six Oort cloud is indeed D-enriched Conclusion: the H isotopic composition of the wa- compared to the solar composition [8]. Hartley-2, a ter as observed today in chondrites has been substan- Jupiter-family comet, has a water H isotopic composi- tially modified through secondary processes on their tion that is similar to Earth’s, but still D-enriched com- parent asteroids. Moreover, the water accreted by the pared to the solar composition [9]. An “interstellar” hydrated chondrites most likely formed or reequilibrat- origin was attributed to the asteroidal water based on ed in the inner Solar System. Chondrites and comets its D enrichment [4, 5]. However, these studies did not may then have sampled different water reservoirs. This consider alternative interpretations. The D/H ratio of has significant implications for the transport and the chondritic molecular water could also represent a later isotopic evolution of materials in the nebula. signature produced through some secondary repro- References: [1] Drouart et al. (1999) Icarus 140, 139. cessing. [2] Mousis et al. (2000) Icarus 148, 513. [3] Brearley (2006) By selecting a suite of CR chondrites that experi- MESSII, 584. [4] Deloule and Robert (1995) GCA 59, 4695. enced different degrees of aqueous alteration, and [5] Deloule et al. (1998) GCA 62, 3367. [6] Grossman et al. determining the isotopic composition of the water in (2000) MPS 37, 49. [7] Alexander et al. (2010) GCA 74, different hydrated minerals, our objective was to place 4417. [8] Jehin et al. (2009) EMP 105, 167. [9] Hartogh et al. new constraints on the composition, evolution, and (2011) Nature 478, 218. [10] Bonal et al. (2011) LPSC origin of asteroidal water [10, 11]. #1287. [11] Bonal et al. subm. to GCA. [12] Alexander et al. H isotopic composition of the water in chon- (2011) LPSC #1869. drites: CR chondrites are considered to be in many ways the most primitive chondrites in our collections.