Large impact‐melt pockets in shergottites contain both Martian regolith components and sulfide/sulfite bleb clusters that yield high sulfur concentrations locally compared to bulk shergottites. The regolith may be the source of excess sulfur in the shergottite melt pockets. To explore whether shock and release of secondary Fe‐sulfates trapped in host rock voids is a plausible mechanism to generate the shergottite sulfur bleb clusters, we carried out shock recovery experiments on an analog mixture of ferric sulfate and Columbia River basalt at peak pressures of 21 and 31 GPa. The recovered products from the 31 GPa experiment show mixtures of Fe‐sulfide and Fe‐sulfite blebs similar to the sulfur‐rich bleb clusters found in shergottite impact melts. The 21 GPa experiment did not yield such blebs. The collapse of porosity and local high‐strain shear heating in the 31 GPa experiment presumably created high‐temperature hotspots (~2000 °C) sufficient to reduce Fe3+ to Fe2+ and to decompose sulfate to sulfite, followed by concomitant reduction to sulfide during pressure release. Our results suggest that similar processes might have transpired during shock production of sulfur‐rich bleb clusters in shergottite impact melts. It is possible that very small CO presence in our experiments could have catalyzed the reduction process. We plan to repeat the experiments without CO.

Martian regolith components are found in some impact melts ( IM ) containing Martian atmospheric gases in the shergottites Elephant Moraine ( EET ) 79001, Tissint, Zagami, and Shergotty. Excess sulfur abundances provide strong indicators for the presence of an exogenous component. High sulfur abundances and the SO 3 ‐SiO 2 correlation in polished thin section ( PTS ) EET 79001,507 (here #507) are comparable to those in Martian soils. Correlations of SO 3 with FeO in #507 from Lithology B and of CaO and Al 2 O 3 in EET 79001,506 (here #506) from Lithology A suggest the possible occurrence of two varieties of sulfate‐bearing phases in impact‐melt precursors. Fe/S (atomic) ratios of 1.02–1.34 determined in several sulfide blebs in #507 differ from those determined in igneous sulfides (Fe/S = 0.92), and suggest that most sulfide blebs in #507 are not related to igneous sulfides. Fe/S (atomic) ratios in a Tissint glass range from ~0.5 (pyrite) to >1.1 suggesting a mixture of sulfur‐bearing phases. S K‐ XANES spectra of the blebs in EET 79001 and Tissint glasses show that sulfur occurs as mixed amorphous sulfide and sulfite. The δ 34 S values and the 87 Sr/ 86 Sr (I) ratios determined in EET 79001 impact melts are consistent with the proposition that the sulfide blebs result from decomposition of secondary sulfates into sulfites during shock heating followed by reduction to sulfides by isentropic cooling. These results suggest the presence in some shergottites of extraneous regolith components containing oxidized S‐bearing species resembling sulfur species present in Martian soils.

We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N-Particle codes. Assuming a one-stage irradiation of a meteoroid with a pre-atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (< 6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one-stage cosmic-ray exposure (CRE) age of 115 Ma. Argon-36 released at intermediate temperatures, 36Ar(n), is attributed to production by thermal neutrons. From the values of 36Ar(n), an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1-4 x 1016 neutrons cm-2. We infer comparable values from epsilon 149Sm and epsilon 150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2-1.4 x 1016 neutrons cm-2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto-Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma.

It is widely assumed that ferroan anorthosites (FANs) formed as flotation cumulates on a global lunar magma ocean (LMO). A corollary is that all FANs are approximately contemporaneous and formed with the same initial Nd-143/Nd-144 ratio. Indeed, a whole rock isochron for selected FANs (and An93 anorthosite) yields an isochron age of 4.42 +/- 0.13 Ga and initial Nd-143/Nd-144, expressed in epsilon-units, of epsilon(sub Nd,CHUR) = 0.3+/-0.3 relative to the CHondritic Uniform Reservoir , or epsilon(sub Nd,HEDPB)=-0.6+/-0.3 relative to the HED Parent Body. These values are in good agreement with the age (T) = 4.47+/-0.07 Ga, and epsilon(sub Nd,HEDPB) =-0.6 +/- 0.5 for FAN 67075. We also have studied anorthositic clasts in the Dhofar 908 and 489 lunar highland meteorites containing clasts of magnesian anorthosites (MAN) with Mg# approximately 75. Because of their relatively high Mg#, magnesian anorthosites should have preceded most FANs in crystallization from the LMO if both are LMO products. Thus, it is important to determine whether the Nd-isotopic data of MAN and FAN are consistent with a co-magmatic origin. We previously reported Sm-Nd data for white clast Dho 908 WC. Mafic minerals in this clast were too small to be physically separated for an isochron. However, we estimated initial Nd-143/Nd-144 for the clast by combining its bulk ("whole rock") Sm-Nd data with an Ar-39-Ar-40 age of 4.42+/-.04 Ga. Here we report additional Sm-Nd data for bulk samples of Dho 908 and its pair Dho 489.

Though brecciated, the Norton County (NC) aubrite contains little or no trapped noble gas and has been widely assumed to have a simple if unusually long cosmic ray exposure (CRE), 115 Ma. One goal of this ongoing study of NC has been to search for signs of pre-irradiation as proposed. One may test for multiple stages of CRE by comparing thermal neutron fluences inferred from Ca-41 (t(sub 1/2)=0.1 Ma) activities, which reflect irradiation conditions over the last approximately 0.3 Ma, with those inferred from (stable) Sm isotope abundances, which integrate over the entire CRE history. In the case of a one-stage exposure the fluences should agree. We focus on these particular comparisons because the properties of NC - its long CRE exposure, relatively large size, and low iron concentration - all promised high production rates and ease of measurement. Previously, we reported on several cosmogenic nuclides in NC. Here we present new Ca-41 data, Sm isotope measurements, and comparisons with model calculations of cosmic ray production.

Introduction: GRA06128 and GRA06129 (hereafter GRA 8 and GRA 9) are partial melts of a parent body of approximately chondritic composition [1-3]. We [4,5] reported a conventional Sm-Nd isochron age of 4.559±0.096 Ga and a Sm-Nd model age of 4.549±0.036 for combined data for the two rocks. Plagioclase plus whole rock and leachate (~phosphate) samples gave a secondary Sm-Nd age of 3.4±0.4 Ga [5]. An Ar-Ar age of 4.460±0.028 Ga [3,6] was interpreted by [3] as dating metamorphism in GRA 9. We report Ar-Ar ages in the range ~4344-4366 Ma for GRA 8, establishing similar but different Ar-Ar ages for the two rocks, consistent with their different Sr-istopic systematics [5], and discuss these ages in the context of the complex sequence of events that affected these samples (cf. [3]). Ar-Ar Stepped Ar-Release Ages for GRA 8: We did a stepped-temperature Ar extraction (49 steps) of a plagioclase separate of GRA 8 (12.7 mg) of low magnetic susceptibility (Fig. 1). Small age variations occurred among “phases” with different K/Ca at low, intermediate, and high extraction temperatures, and the summed age is 4354 Ma. Partitioned according to the fraction of Ar released, the calculated ages varied from 4326±18 to 4344±14 to 4362±18 Ma (1 ) for 4-14%, 14-47%, and 52-96% Ar released, resp. A single temperature step for 47-52% of the Ar release appeared to mark a transition in the gas release mechanism as also seen in an Arrhenius plot constructed from the data (Fig. 2).

We have studied anorthositic clasts in the Y-86032 and Dhofar 908 meteorites by the Rb-Sr, Sm-Nd, and Ar-39-Ar-40 techniques combining isotopic studies with mineralogical/petrological studies of the same clasts. As a result of these studies, we conclude that the lunar crust is composed of a variety of anorthosites, at least some of which must have formed as plutons in the earliest formed ferroan anorthosite crust.

Anorthosites occur ubiquitously within the lunar crust at depths of ~3-30 km in apparent confirmation of the Lunar Magma Ocean (LMO) hypothesis. We have dated lunar anorthosite 67075, a Feldspathic Fragmental Breccia (FFB) collected near the rim of North Ray Crater by the Sm-Nd and Rb-Sr techniques. We also have dated an anorthositic white clast (WC) in lunar meteorite Dhofar 908 by the Ar-39-Ar-40 technique and measured whole rock (WR) Sm-Nd data for a companion sample. We discuss the significance of the ages determined for these and other anorthosites for the early magmatic and bombardment history of the moon.

Yamato 984028 (Y984028) was discovered by the Japanese Antarctic Research Expedition (JARE) in 1998 and recently classified as a lherzolitic shergottite with large pyroxene oikocrysts enclosing rounded olivine and chromites. It also contains shock veining and maskelynite. Y984028 is paired with the more recent lherzolitic shergottite finds 27/47/97 based on similarities in mineralogy and chemistry, as well as isotopic composition. We present here the studied Ar-39-Ar-40 of Y-984028 whole rock (WR) and pyroxene (Px), in order to gain better understanding of trapped Ar components with a comparison of the possibly-paired 97 Ar release.

The (80)Kr excesses determined in the gas-rich impact-melt (GRIM) glasses in EET79001 and Shergotty correlate poorly with the (129)Xe(M) (Martian atmosphere) suggesting that the majority of the neutron-capture (80)Kr(n) was not shock-implanted along with (129)Xe(M) into these glasses during impact. This inference is consistent with the variations in delta (80)Kr(n) excesses observed in these samples. The results reported here indicate that the (80)Kr(n) excesses in these glasses were produced in the same way as the (149)Sm isotopic deficits, i.e., by thermal neutron (n) capture on Br and Sm occurring in the glass-precursor regolith materials on Mars. The thermal neutron fluences calculated from (80)Kr(n) excesses (similar to 0.3-1.0 x 10(15) n/cm(2)) and from (149)Sm deficits (1.0 +/- 0.4 x 10(15) n/cm(2)) agree with each other confirming that (80)Kr(n) was mostly produced in situ. In the Martian regolith, thermal (n) and fast (N) neutrons occur together. Also, in the GRIM glasses, the (83)Kr/(86)Kr ratios correlate positively with (84)Kr/(86)Kr indicating that the cosmogenic Kr contains a fast neutron-produced component in addition to Kr produced by galactic cosmic and solar cosmic irradiation. Using (83)Kr(N) and (84)Kr(N) excesses produced by fast neutron reactions on Rb and Sr targets in some of these glasses, we determine fast neutron fluences of similar to 3-47 x 10(15) N/cm(2). The integrated fluences of thermal and fast neutrons in GRIM glasses suggest that the glass-precursor materials were irradiated at different depths in the top few meters of the water-ice-bearing regolith near the shergottite source regions on Mars.

Antarctic Martian meteorites Yamato (Y) 984028 and 27/47/97 have similar textures, mineralogy, chemistry, and isotopic composition and are possibly paired. We analyzed the argon isotopic composition of Y984028 whole rock (WR) and pyroxene mineral separates (Px) in order to evaluate their trapped Ar components and compare with 97 Ar data. WR and Px yield an apparent Ar-39-Ar-40 age spectra of roughly 2 Ga, much older than the crystallization age determined by other isotopic techniques. Sm-Nd and Rb-Sr ages for Y984028 are approximately 170 Ma. This discrepancy is likely the byproduct of several coexisting Ar components, such as radiogenic 40Ar*, cosmogenic Ar, and trapped Ar from the multiple minerals, as well as multiple source origins. Similarly, the reported Ar-39-Ar-40 age of 97 is approximately 260 Ma with a Rb-Sr age of 147+/- 28 Ma and a Sm-Nd age of 152 +/- 13 Ma [4]. Apparently Ar-Ar ages of both Y984028 and 97 show trapped Ar components. Stepwise temperature extractions of Ar from Y984028 Px show several Arcomponents released at different temperatures. For example, intermediate temperature data (800-1100 C) are nominally consistent with the Sm-Nd and Rb-Sr radiometric ages (approximately 170 Ma) with an approximately Martian atmosphere trapped Ar composition with a Ar-40-Ar-36 ratio of approximately 1800. Based on K/Ca distribution, we know that Ar-39 at both lower and intermediate temperatures is primarily derived from plagioclase and olivine. Argon released during higher temperature extractions (1200-1500 C), however, differs significantly. The thermal profile of argon released from Martian meteorites is complicated by multiple sources, such as Martian atmosphere, Martian mantle, inherited Ar, terrestrial atmosphere, cosmogenic Ar. Obviously, Ar release at higher temperatures from Px should contain little terrestrial atmospheric component. Likewise, Xe-129/Xe-132 from high temperature extractions (1200-1800 C) gives a value above that of terrestrial Xe ratio of 0.98. The most plausible explanation of the high temperature argon data is that it contains a Martian mantle Ar-40 component as well as excess Ar-40 assimilated from inherited magma.

We studied the inter-relationships between the major anions (SO3, Cl, and Br) and cations (FeO, CaO and MgO) using elemental abundances determined by APXS in salt assemblages of RATted (abraded) rocks at Meridiani to characterize the behavior of fluids that infiltrated into this region on Mars. A plot of SO3 versus Cl for the abraded rocks yielded an unusual pattern, whereas the SO3/Cl ratios versus Cl for the same rocks showed a monotonically decreasing trend represented by a hyperbola. The systematic behavior of the SO3 and Cl data in the documented rocks at Meridiani suggests that these anions behaved conservatively during fluid–rock interactions. These results further indicate that two kinds of fluids, referred to as SOL-I and SOL-II, infiltrated into Endurance/Eagle/Fram craters, where they underwent progressive evaporative concentration. SOL-I is a low pH fluid consisting of high SO3 and low Cl and high Br, (this fluid infiltrated all the way to the crater-top region), whereas SOL-II fluid of high pH with low SO3 and high Cl and low Br reached only an intermediary level known as the Whatanga contact at Endurance. Based on the FeO/MgO as well as CaO/MgO versus SO3/Cl diagram for rocks above the Whatanga contact, the cation and anion relationships in this system suggest that the Fe2+/SO4 and Ca2+/SO4 ratios in SOL-I fluids at Meridiani were >1 before the onset of evaporation based on the “chemical divide” considerations. Below the Whatanga contact, relatively dilute SOL-II fluids seem to have infiltrated and dissolved/flushed away the easily soluble Mg-sulfate/chloride phases (along with Br) without significantly altering the SO3/Cl ratios in the residual salt assemblages. Further, Cl/Br versus Br in rocks above the Whatanga contact show a hyperbolic trend suggesting that Cl and Br behaved conservatively similar to SO3 and Cl in the SOL-1 fluids at Meridiani. Our results are consistent with a scenario involving two episodes (SOL-I and SOL-II) of groundwater recharge at Meridiani Planum.