Showing papers by "Fred Jourdan published in 2023"

Rubble pile asteroids are forever.

Abstract: Rubble piles asteroids consist of reassembled fragments from shattered monolithic asteroids and are much more abundant than previously thought in the solar system. Although monolithic asteroids that are a kilometer in diameter have been predicted to have a lifespan of few 100 million years, it is currently not known how durable rubble pile asteroids are. Here, we show that rubble pile asteroids can survive ambient solar system bombardment processes for extremely long periods and potentially 10 times longer than their monolith counterparts. We studied three regolith dust particles recovered by the Hayabusa space probe from the rubble pile asteroid 25143 Itokawa using electron backscatter diffraction, time-of-flight secondary ion mass spectrometry, atom probe tomography, and 40Ar/39Ar dating techniques. Our results show that the particles have only been affected by shock pressure of ca. 5 to 15 GPa. Two particles have 40Ar/39Ar ages of 4,219 ± 35 and 4,149 ± 41 My and when combined with thermal and diffusion models; these results constrain the formation age of the rubble pile structure to ≥4.2 billion years ago. Such a long survival time for an asteroid is attributed to the shock-absorbent nature of rubble pile material and suggests that rubble piles are hard to destroy once they are created. Our results suggest that rubble piles are probably more abundant in the asteroid belt than previously thought and provide constrain to help develop mitigation strategies to prevent asteroid collisions with Earth.

The origin of platinum group minerals in oceanic crust

Abstract: Highly siderophile elements (HSEs), including Re and Os, are used extensively as geochemical tracers and geochronometers to investigate the formation and evolution of Earth’s crust and mantle. Mantle rocks are commonly serpentinized, but the effect of serpentinization on the distribution of HSEs is controversial because HSEs are commonly hosted by rare, micrometer- to sub-micrometer-scale grains of platinum group minerals (PGMs) of ambiguous origin that are challenging to identify, characterize, and interpret. In this study, atom probe tomography (APT) is used to characterize two spatially close PGM grains hosted by a partially serpentinized harzburgite from Macquarie Island, Australia. The APT data reveal an extraordinary level of detail that provides insights into the origin of a complex Cu−Pt alloy grain (average composition ∼Cu4Pt). The grain hosts Fe-, Ni-, and Pt-rich sub-grains associated with Rh, variably overlapping networks of Pd- and Cd-enrichment, and OH-rich volumes identified as fluid inclusions. Osmium and Ru are hosted by an idioblastic laurite (RuS2) grain. Compositional, textural, and phase-diagram constraints are consistent with a modified pre-serpentinization origin for the PGMs, and a comparison between observed and calculated grain distributions indicate that while Os isotope ratios were probably unaffected by serpentinization, whole-rock and grain-scale HSE and isotopic ratios may have been decoupled during serpentinization.

Revised stratigraphy and first geochronology of the Miocene submarine volcanic succession at Kennaook/Cape Grim, northwestern Tasmania

Abstract: Abstract Kennaook/Cape Grim in far northwestern Tasmania, Australia, was a site of submarine intraplate basaltic volcanism during the Miocene. The succession is exceptionally well preserved and is dominated by pillow lava, massive lava and pillow fragment breccia. The total volume of volcanic products (1.1 km3) is relatively small on a global scale, but the preservation is world class. The oldest unit, the Woolnorth Tuff lies unconformably on the Neoproterozoic Rocky Cape Group. The first 40Ar/39Ar dating of the volcanic rocks reveals an early Miocene (24.5–23 Ma) age for the entire sequence. The Woolnorth Tuff is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments. The Woolnorth Tuff is overlain by the Slaughter Bluff Volcanic Breccia (23.12 ± 0.19 Ma) at Kennaook/Cape Grim. The Slaughter Bluff Volcanic Breccia is dominated by diffusely bedded pillow fragment breccia. North of Kennaook/Cape Grim, the Little Trefoil Basalt (ca 24 Ma) intrudes the Woolnorth Tuff. To the south of Kennaook/Cape Grim, the Woolnorth Tuff is overlain by the Studland Bay Basalts (23.73 ± 0.08 Ma) and intruded by a newly recognised unit, the Hippo Basalt (24.52 ± 0.12 Ma). The Studland Bay Basalts comprise mounds of basaltic pillow lavas followed by a succession of diffusely bedded, matrix-dominated, pillow fragment breccia and basaltic breccia. Detailed field mapping and sampling have revealed that the environment of deposition of all Kennaook/Cape Grim units was submarine and that they were emplaced in relatively rapid succession. KEY POINTS First 40Ar/39Ar geochronology for the Kennaook/Cape Grim volcanic succession. Little Trefoil Basalt has been re-interpreted from a subaerial extrusive unit to a submarine intrusive unit. A new intrusive unit, the Hippo Basalt, has been recognised. The stratigraphy of the Kennaook/Cape Grim volcanic succession has been revised.

Multi-method dating constrains the diversification of early eukaryotes in the Proterozoic Mbuji-Mayi Supergroup of the D.R.Congo and the geological evolution of the Congo Basin

Abstract: We present a petrological and geochronological study of the Proterozoic Mbuji-Mayi sedimentary Supergroup (Sankuru-Mbuji-Mayi-Lomami-Lovoy basin, Democratic Republic of the Congo) and of the overlying subvolcanic doleritic rocks. Dating this Supergroup in Central Africa is crucial to reconstruct the geological history and evolution of the Congo Basin and because it contains a large diversity of organic-walled microfossils including early eukaryotes. For this study, we use the Re–Os dating method on kerogen from shales from the top of the lower Group BI (which contains the microfossils) and Ar–Ar, Sm–Nd and U–Pb methods on the dolerites emplaced near the top of the succession, defining the end of the sedimentation in the basin. The 187Re/187Os dating yield an age of 1041 ± 58 Ma. For the dolerites, 40Ar/39Ar dating provide a similar minimum age from 1006 ± 13 to 1009 ± 32 Ma in the whole basin. 147Sm/143Nd dating confirms a less precise but overlapping age of 926 ± 170 Ma for sample from the Western part of the basin. Some 40Ar/39Ar dating and U–Pb dating provide also inherited Archean ages. The weighted average age for igneous samples from the basin is 1006 ± 12 Ma. This confirms that the upper Group BII was deposited from 1030 to 1040 to 1006 Ma, consistent with previous U–Pb dating on diagenetic minerals and with biostratigraphy. This new dating provides constraints on the late Mesoproterozoic – early Neoproterozoic diversification of early eukaryotes in Central Africa. We also compare the petrology, geochemistry and geochronology of dolerites from different locations of the basin. Our new data evidence a common origin for all the subvolcanic rocks, emplaced in an intraplate setting and intruding the Archean basement, and indirectly date the end of the extensional stage that initiated the development of the Congo Basin.

Orogens and detritus: unravelling the Mesoproterozoic tectonic geography of northern Australia through coupled detrital thermo- and geo-chronometers

Abstract: This study presents detrital muscovite 40Ar/39Ar data from the Mesoproterozoic Roper Group and overlying informally named successions, in the Beetaloo Sub-basin, northern Australia. Detrital muscovite chronology reveals tectono-thermal processes within source regions and provides new constraints on the basin provenance, revising previous interpretations based on detrital zircon data. Detrital thermo- and geochronology, together demonstrate three main periods when the basin paleogeography was altered that correspond to the evolving tectonic history of the North Australia Craton (NAC) through the Mesoproterozoic. The first is characterised by an increased sediment contribution from source regions that lay along the eastern margin of Proterozoic Australia. These source regions are interpreted to have formed the uplifted rift-shoulders between Proterozoic Australia and Laurentia at ca 1.45 Ga. After that, sediments derived from eastern Proterozoic Australia sources become less voluminous up-section. The youngest analysed formation from the Roper Group, the Kyalla Formation, was predominately from sources to the south of the basin, representing another modification of basin geography. This is interpreted to result from the closure/subduction of the Mirning Ocean as the West Australian Craton (WAC) approached and collided with the NAC, resulting in an uplift of the southern margin of the NAC, at ca 1.35–1.31 Ga. The uppermost Mesoproterozoic to lower Neoproterozoic sandstone successions that overlie the Roper Group were derived from the Musgrave Province. Coupled detrital zircon and muscovite data imply a rapid cooling at ca 1.20–1.15 Ga that is interpreted to reflect syn-orogenic exhumation during the Musgrave Orogeny. Furthermore, data from the Beetaloo Sub-basin suggest that the changed basin tectonic settings reshaped basin geography and result in distinctive detrital zircon and muscovite geochronology records. In this study, we used the detrital U–Pb zircon and muscovite 40Ar/39Ar age data from the Beetaloo Sub-basin and a range of other basins deposited in different tectonic environments, including the convergent, collisional and extensional settings, to reconstruct the basin tectonic geography and illustrate various tectonic controls on basin formation in different tectonic backgrounds.KEY POINTSDetrital muscovite 40Ar/39Ar data provide thermochronological constraints on basin provenance, complementing previous interpretations based on detrital zircon data.Spatial and temporal variation of provenance reconstructs the basin tectonic geography, reflecting the Mesoproterozoic tectonic history of the North Australia Craton.Coupled thermo- and geochronology constrain tectonic settings of basin deposition.