Accessory mineral microstructure and chronology reveals no evidence for late heavy bombardment on the asteroid 4-Vesta

A long-standing paradigm in planetary science is that the inner Solar System experienced a period of intense and sustained bombardment between 4.2 and 3.9 Ga. Evidence of this period, termed the Late Heavy Bombardment is provided by the 40Ar/39Ar isotope systematics of returned Apollo samples, lunar meteorites, and asteroidal meteorites. However, it has been largely unsupported by more recent and robust isotopic age data, such as isotopic age data obtained using the U-Pb system. Here we conduct careful microstructural characterisation of baddeleyite, zircon, and apatite in six different eucrites prior to conducting SIMS and LA-ICP-MS measurement of U, Th, and Pb isotopic ratios and radiometric dating. Baddeleyite, displaying complex internal twinning linked to reversion from a high symmetry polymorph in two samples, records the formation of the parent body (4554 ± 3 Ma 2σ; n = 8), while structurally simple zircon records a tight spread of ages representing metamorphism between 4574 ± 14 Ma and 4487 ± 31 Ma (n = 6). Apatite, a more readily reset shock chronometer, records crystallisation ages of ∼4509 Ma (n = 6), with structurally deformed grains (attributed to impact events) yielding U-Pb ages of 4228 Ma (n = 12). In concert, there is no evidence within the measured U-Pb systematics or microstructural record of the eucrites examined in this study to support a period of late heavy bombardment between 4.2 and 3.9 Ga

The shocking state of apatite and merrillite in shergottite Northwest Africa 5298 and extreme nanoscale chlorine isotope variability revealed by atom probe tomography

The elemental and chlorine isotope compositions of calcium-phosphate minerals are key recorders of the volatile inventory of Mars, as well as the planet’s endogenous magmatic and hydrothermal history. Most martian meteorites have clear evidence for exogenous impact-generated deformation and metamorphism, yet the effects of these shock metamorphic processes on chlorine isotopic records contained within calcium phosphates have not been evaluated. Here we test the effects of a single shock metamorphic cycle on chlorine isotope systematics in apatite from the highly shocked, enriched shergottite Northwest Africa (NWA) 5298. Detailed nanostructural (EBSD, Raman and TEM) data reveals a wide range of distributed shock features. These are principally the result of intensive plastic deformation, recrystallization and/or impact melting. These shock features are directly linked with chemical heterogeneities, including crosscutting microscale chlorine-enriched features that are associated with shock melt and iron-rich veins. NanoSIMS chlorine isotope measurements of NWA 5298 apatite reveal a range of δ37Cl values (-3 to 1 ‰; 2σ uncertainties 37Cl values can be readily linked with different nanostructural states of targeted apatite. High spatial resolution atom probe tomography (APT) data reveal that chlorine-enriched and defect-rich nanoscale boundaries have highly negative δ37Cl values (mean of -15 ± 8 ‰). Our results show that shock metamorphism can have significant effects on chemical and chlorine isotopic records in calcium phosphates, principally as a result of chlorine mobilization during shock melting and recrystallization. Despite this, low-strain apatite domains have been identified by EBSD, and yield a mean δ37Cl value of -0.3 ± 0.6 ‰ that is taken as the best estimate of the primary chlorine isotopic composition of NWA 5298. The combined nanostructural, microscale-chemical and nanoscale APT isotopic approach gives the ability to better isolate and identify endogenous volatile-element records of magmatic and near-surface processes as well as exogenous, shock-related effects

Lunar samples record an impact 4.2 billion years ago that may have formed the Serenitatis Basin

Impact cratering on the Moon and the derived size-frequency distribution functions of lunar impact craters are used to determine the ages of unsampled planetary surfaces across the Solar System. Radiometric dating of lunar samples provides an absolute age baseline, however, crater-chronology functions for the Moon remain poorly constrained for ages beyond 3.9 billion years. Here we present U–Pb geochronology of phosphate minerals within shocked lunar norites of a boulder from the Apollo 17 Station 8. These minerals record an older impact event around 4.2 billion years ago, and a younger disturbance at around 0.5 billion years ago. Based on nanoscale observations using atom probe tomography, lunar cratering records, and impact simulations, we ascribe the older event to the formation of the large Serenitatis Basin and the younger possibly to that of the Dawes crater. This suggests the Serenitatis Basin formed unrelated to or in the early stages of a protracted Late Heavy Bombardment

Evidence of extensive lunar crust formation in impact melt sheets 4,330 Myr ago

Accurately constraining the formation and evolution of the lunar magnesian suite is key to understanding the earliest periods of magmatic crustal building that followed accretion and primordial differentiation of the Moon. However, the origin and evolution of these unique rocks is highly debated. Here, we report on the microstructural characterization of a large (~250-μm) baddeleyite (monoclinic-ZrO2) grain in Apollo troctolite 76535 that preserves quantifiable crystallographic relationships indicative of reversion from a precursor cubic-ZrO2 phase. This observation places important constraints on the formation temperature of the grain (>2,300 °C), which endogenic processes alone fail to reconcile. We conclude that the troctolite crystallized directly from a large, differentiated impact melt sheet 4,328 ± 8 Myr ago. These results suggest that impact bombardment would have played a critical role in the evolution of the earliest planetary crusts

New Mineral Names (August - September 2012)

In This Issue In this New Mineral Names, we present alexandrovite, arsenohopeite, \ue5skagenite-(Nd), bassoite, beaverite-Zn, carlosbarbosaite, cryptophyllite, cuprokalininite, davidlloydite, florencite-(Sm), natrotitanite, and shlykovite from journals around the world

Veblenite, K2□2Na(Fe2+5Fe3+4Mn2+7□)Nb 3Ti(Si2O7)2(Si8O22)2O6(OH)10(H2O) 3, a new mineral from Seal Lake, Newfoundland and Labrador : mineral description, crystal structure, and a new veblenite Si8O2 ribbon

Veblenite, ideally K2\u25a12Na(Fe2+5Fe3+4Mn2+7\u25a1)Nb 3Ti(Si2O7)2(Si8O22) 2O6(OH)10(H2O)3, is a new mineral with no natural or synthetic analogues. The mineral occurs at Ten Mile Lake, Seal Lake area, Newfoundland and Labrador (Canada), in a band of paragneiss consisting chiefly of albite and arfvedsonite. Veblenite occurs as red brown single laths and fibres included in feldspar. Associated minerals are niobophyllite, albite, arfvedsonite, aegirine-augite, barylite, eudidymite, neptunite, Mnrich pectolite, pyrochlore, sphalerite and galena. Veblenite has perfect cleavage on 001 and splintery fracture. Its calculated density is 3.046 g cm-3. Veblenite is biaxial negative with \u3b1 1.676(2), \u3b2 1.688(2), \u3b3 1.692(2) (\u3bb 590 nm), 2Vmeas = 65(1)\ub0, 2Vcalc = 59.6\ub0, with no discernible dispersion. It is pleochroic in the following pattern: X = black, Y = black, Z = orange-brown. The mineral is red-brown with a vitreous, translucent lustre and very pale brown streak. It does not fluoresce under short and long-wave UV-light. Veblenite is triclicnic, space group P1\u304, a 5.3761(3), b 27.5062(11), c 18.6972(9)\uc5, \u3b1 140.301(3), \u3b2 93.033(3), \u3b3 95.664(3)\ub0, V = 1720.96(14)\uc53. The strongest lines in the X-ray powder diffraction pattern [d(\uc5)(I)(hkl)] are: 16.894(100)(010), 18.204(23)(01\u3041), 4.271(9)(14\u3041, 040, 120), 11.661(8)(001), 2.721(3)(19\u3045), 4.404(3)(1\u3043\u3042, 14\u3042), 4.056(3)(031, 11\u3042; 15\u3042, 1\u3044\u3043), 3.891(2)(003). The chemical composition of veblenite from a combination of electron microprobe analysis and structural determination for H2O and the Fe2+/Fe3+ratio is Nb2O5 11.69, TiO2 2.26, SiO2 35.71, Al2O3 0.60, Fe2O3 10.40, FeO 11.58, MnO 12.84, ZnO 0.36, MgO 0.08, BaO 1.31, SrO 0.09, CaO 1.49, Cs2O 0.30, K2O 1.78, Na2O 0.68, H2O 4.39, F 0.22, O = F -0.09, sum 95.69 wt.%. The empirical formula [based on 20 (Al+Si) p.f.u. is (K0.53Ba0.28Sr0.03\u25a10.16) \u3a31(K0.72Cs0.07\u25a11.21) \u3a32(Na0.72Ca0.17\u25a11.11) \u3a32(Fe2+5.32Fe3+4.13Mn2+5.97Ca0.70Zn 0.15Mg0.07\u25a10.66) \u3a317(Nb2.90Ti0.93Fe3+0.17) \u3a34(Si19.61Al0.39)S20O 77.01H16.08F0.38. The simplified formula is (K,Ba,\u25a1)3(\u25a1,Na)2(Fe2+,Fe 3+,Mn2+)17(Nb,Ti)4(Si 2O7)2(Si8O22) 2O6(OH)10(H2O)3. The infrared spectrum of the mineral contains the following bands (cm-1): 453, 531, 550, 654 and 958, with shoulders at 1070, 1031 and 908. A broad absorption was observed between ~3610 and 3300 with a maximum at ~3525. The crystal structure was solved by direct methods and refined to an R1 index of 9.1%. In veblenite, the main structural unit is an HOH layer, which consists of the octahedral (O) and two heteropolyhedral (H) sheets. The H sheet is composed of Si2O7 groups, veblenite Si8O22 ribbons and Nb-dominant D octahedra. This is the first occurrence of an eight-membered Si 8O22 ribbon in a mineral crystal structure. In the O sheet, (Fe2+, Fe3+, Mn2+) octahedra share common edges to form a modulated O sheet parallel to (001). HOH layers connect via common vertices of D octahedra and cations at the interstitial A(1,2) and B sites. In the intermediate space between two adjacent HOH layers, the A(1) site is occupied mainly by K; the A(2) site is partly occupied by K and H 2O groups, the B site is partly occupied by Na. The crystal structure of veblenite is related to several HOH structures: jinshanjiangite, niobophyllite (astrophyllite group) and nafertisite. The mineral is named in honour of David R. Veblen in recognition of his outstanding contributions to the fields of mineralogy and crystallography