Jens Hopp

Structure and thermal history of the H-chondrite parent asteroid revealed by thermochronometry

Our Solar System formed ∼4.6 billion years ago from the collapse of a dense core inside an interstellar molecular cloud. The subsequent formation of solid bodies took place rapidly. The period of <10 million years over which planetesimals were assembled can be investigated through the study of meteorites. Although some planetesimals differentiated and formed metallic cores like the larger terrestrial planets, the parent bodies of undifferentiated chondritic meteorites experienced comparatively mild thermal metamorphism that was insufficient to separate metal from silicate. There is debate about the nature of the heat source as well as the structure and cooling history of the parent bodies. Here we report a study of 244Pu fission-track and 40Ar–39Ar thermochronologies of unshocked H chondrites, which are presumed to have a common, single, parent body. We show that, after fast accretion, an internal heating source (most probably 26Al decay) resulted in a layered parent body that cooled relatively undisturbed: rocks in the outer shells reached lower maximum metamorphic temperatures and cooled faster than the more recrystallized and chemically equilibrated rocks from the centre, which needed ∼160 Myr to reach 390K.

Neon isotopes in mantle rocks from the Red Sea region reveal large-scale plume–lithosphere interaction

Abstract Helium and neon isotopes are ideal tracers to quantify contributions of primitive mantle plumes, which are characterized by a higher proportion of primordial solar-type noble gases compared to lithospheric or asthenospheric mantle sources. This property was used to investigate the role of the Afar mantle plume (a high 3 He/ 4 He plume of up to 20 R A ; 1 R A =atmospheric composition) during continental breakup of the Red Sea rift. We analyzed ultramafic rocks from Zabargad Island and mantle xenoliths from the Quaternary volcanic fields of Al Birk and Jizan (Saudi Arabia) that are representative of the local subcontinental lithospheric mantle. 3 He/ 4 He ratios range from 6.1 to 8.3 R A , similar to results of worldwide lithospheric and asthenospheric mantle, and therefore are not indicative of a plume component. In contrast, we observe significant contributions of plume-derived neon, which is characterized by a higher proportion of primordial solar-type neon than typical mid-ocean ridge basalts (MORB). Considering both helium and neon isotope systematics reveals mixing of a deep mantle plume, a pre-rift MORB-like, and a more radiogenic pre-rift lithospheric mantle component. As the deep mantle plume component has a higher Ne/He ratio when compared to the shallow mantle components, it is more prominent in Ne than in He, with up to 57% plume-derived neon and 11% plume-derived helium present in the investigated samples. This further underlines the importance of high-precision neon measurements. It demonstrates that the Afar plume source contributed primordial noble gases to an intrinsically more radiogenic and nucleogenic lithospheric and asthenospheric component up to a distance of >1800 km (Zabargad), even in the early stages of continental rifting, ∼20 Ma ago. These results present the first unambiguous geochemical evidence suggesting an active role for the Afar mantle plume during the evolution of the Red Sea rift, supporting geochronological and geodynamic evidence.

A common mantle plume source beneath the entire East African Rift System revealed by coupled helium‐neon systematics

We report combined He-Ne-Ar isotope data of mantle-derived xenoliths and/or lavas from all segments of the East Africa Rift System (EARS). Plume-like helium isotope (3He/4He) ratios (i.e., greater than the depleted MORB mantle (DMM) range of 8 ± 1RA) are restricted to the Ethiopia Rift and Rungwe, the southernmost volcanic province of the Western Rift. In contrast, neon isotope trends reveal the presence of an ubiquitous solar (plume-like) Ne component throughout the EARS, with (21Ne/22Ne)EX values (where (21Ne/22Ne)EX is the air-corrected 21Ne/22Ne ratio extrapolated to Ne-B) as low as 0.034, close to that of solar Ne-B (0.031). Coupling (21Ne/22Ne)EX with 4He/3He ratios indicates that all samples can be explained by admixture between a single mantle plume source, common to the entire rift, and either a DMM or subcontinental lithospheric mantle source. Additionally, we show that the entire sample suite is characterized by low 3He/22NeS ratios (mostly < 0.2)—a feature characteristic of oceanic hot spots such as Iceland. We propose that the origin of these unique noble gas signatures is the deeply rooted African Superplume which influences magmatism throughout eastern Africa. We argue that the Ethiopia and Kenya domes represent two different heads of this common mantle plume source.

Annama H chondrite—Mineralogy, physical properties, cosmic ray exposure, and parent body history

The fall of the Annama meteorite occurred early morning (local time) on April 19, 2014 on the Kola Peninsula (Russia). Based on mineralogy and physical properties, Annama is a typical H chondrite. It has a high Ar-Ar age of 4.4 Ga. Its cosmic ray exposure history is atypical as it is not part of the large group of H chondrites with a prominent 7 - 8 Ma peak in the exposure age histograms. Instead, its exposure age is within uncertainty of a smaller peak at 30 \pm 4 Ma. The results from short-lived radionuclides are compatible with an atmosperic pre-entry radius of 30 - 40 cm. However, based on noble gas and cosmogenic radionuclide data, Annama must have been part of a larger body (radius >65 cm) for a large part of its cosmic ray exposure history. The 10Be concentration indicates a recent (3 - 5 Ma) breakup which may be responsible for the Annama parent body size reduction to 30 - 35 cm pre-entry radius.

Light noble gas data in Guli massif carbonatites reveal the subcontinental lithospheric mantle as primary fluid source

For better understanding of the fluid phase sources of carbonatites of Guli alkaline-ultrabasic intrusion (Maymecha-Kotuy complex) we have studied isotope composition of He and Ne in the carbonatites of different formation stages. The data definitely point to the subcontinental lithospheric mantle (SCLM) as a primary source of fluid phase of Guli carbonatites. The absence of plume signature in such a plume-like object (from petrological point of view) could be explained in terms that Guli carbonatites have been formed at the waning stage of plume magmatic activity with an essential input of SCLM components.

Evaluation of neutron sources for ISAGE—in-situ-NAA for a future lunar mission

For a future Moon landing, a concept for an in-situ NAA involving age determination using the (40)Ar-(39)Ar method is developed. A neutron source (252)Cf is chosen for sample irradiation on the Moon. A special sample-in-source irradiation geometry is designed to provide a homogeneous distribution of neutron flux at the irradiation position. Using reflector, the neutron flux is likely to increase by almost 200%. Sample age of 1Ga could be determined. Elemental analysis using INAA is discussed.

He, Ne, Ar stepwise crushing data on basalt glasses from different segments of Bouvet Triple Junction

Here we present the first data on He, Ne, Ar isotopic and elemental composition in fluid phases of tholeiitic chilled glasses from the Bouvet Triple Junction (BTJ). The chilled glasses from several dredging stations situated at different segments of BTJ have been investigated: Spiess Ridge, Mid Atlantic Ridge (MAR) and in a valley of the Southwest Indian Ridge (SWIR). The data allow to distinguish within BTJ three segments characterized by different geochemical behavior of He, Ne and Ar. MAR and Spiess samples contain MORB-like helium and neon while SWIR is characterized by addition of plume type He and Ne. The strong atmospheric contamination is typical of all segments, but for MAR it is less pronounced. The Ne-Ar isotope systematics suggests that the atmospheric component was most probably introduced into the mantle source of the fluids with fragments of oceanic crust/sediments.

What happened to the moon? A lunar history mission using neutrons

The ages of lunar rocks can be determined using the 40Ar−39Ar technique that can be used in-situ on the moon if a neutron source, a noble gas mass spectrometer and a gas extraction and purification system are brought to the lunar surface. A possible instrument for such a task is ISAGE, which combines a strong 252Cf neutron source and a compact spectrometer for in-situ dating of e.g. the South Pole Aitken impact basin or the potentially very young basalts south of the Aristachus Plateau. In this paper, the design of the neutron source will be discussed. The source is assumed to be a hollow sphere surrounded by a reflector, a geometry that provides a very homogeneous flux at the irradiation position inside the sphere. The optimal source geometry depending on the experimental conditions, the costs of transportation for the reflector and the costs of the source itself are calculated. A minimum 252Cf mass of 1.5mg is determined.

40Ar-39Ar dating of volcanic rocks from the Fernando de Noronha Archipelago

The Fernando de Noronha Archipelago is situated345 km off the Brazilian coast. In terms of tectonics, thearchipelago is confined to a transform fracture zone. TheFernando de Noronha lineament extends a considerabledistance westward and is traced by the Atol das Rocas, aseamount chain, and occurrences of alkaline magmatismin the continent. According to tectonic conceptions, thisstructure is interpreted as a hotspot track.The universally accepted differentiation of the volcanic rocks of the Fernando de Noronha Archipelago wasproposed by Almeida [1]. According to his data, three formations can be distinguished: Remedios, Quixaba, andSao Jose. The igneous rocks of the Remedios Formationare dominated by phonolites and trachytes. The pyroclastic material was produced by explosive eruptions of phonolitic and trachytic magmas. The rocks of the QuixabaFormation are dominated by intercalating ankaratriteflows and pyroclastic layers of the same composition andlie on an erosion surface. Sedimentary rocks were notfound at the stratigraphic boundary between the Remedios and Quixaba formations. Therefore, the sequence offormation of these complexes cannot be established bygeological methods. The nepheline basanites of Sao JoseIsland were distinguished into a separate formation.Almeida argued that these basanites are younger than theankaratrites of the Quixaba Formation.The dating of igneous rocks from the Fernando deNoronha (FN) Islands is important for the understandingof the magmatic evolution of this hot spot. The K–Ar agesreported by Cordani [2] and Kogarko et al. [3] showedthat the alkali basanites of the Sao Jos e Formation and thetrachytes of the Remedios Formation are the oldest igneous rocks of the archipelago (11.7 ± 0.6 Ma and 11.8–11.7 Ma, respectively). According to the same data, thephonolites of the Remedios Formation are somewhatyounger (9.5–8.6 Ma), and the olivine melanephelinitesof the Quixaba Formation are the youngest rocks. In contrast to this model, Almeida [1] supposed previously thatthe Sao Jose basanites are younger than the Quixabaankaratrites (note that the contact between these rocks islocated below the sea level).The classical K–Ar method ignores the possible presence of excess Ar (i.e., Ar absorbed and accumulated during the existence of the mantle block producing the magmatism of the island, rather than radiogenic Ar formed