The parent bodies of CR chondrites and their secondary history     

Trygve Prestgard  Pierre Beck  Lydie Bonal  Jolantha Eschrig  Jérôme Gattacceca  Corinne Sonzogni  Lisa Krämer Ruggiu 《Meteoritics & planetary science》2023,58(8):1117-1148

Renazzo-type (CR) chondrites are a relatively rare group of carbonaceous chondrites with the vast majority having escaped thermal alteration. This means that CRs are composed of relatively unprocessed material, depending on the extent of aqueous alteration they have experienced. Hydration in CRs ranges from incipient alteration of matrix glass, up to nearly complete replacement of the rock by hydration products. The extent of secondary processes is often difficult to assess in these meteorites, due to their heterogeneity and diversity of alteration products. Yet, this is crucial in order to understand the extent of geological processing that occurred on the primary parent body. Additionally, the parent asteroids of CRs remain a mystery, mainly because terrestrial oxyhydroxide signatures dominate the reflectance spectra of CRs. In this work, we have conducted optical and IR reflectance and transmission spectra of 25 CR chondrites in order to (i) better evaluate the extent of aqueous alteration that occurred on the CR parent body, and (ii) find possible parent body candidates. Terrestrial oxyhydroxides were removed from 12 samples, as these tend to interfere with the optical-IR spectra of CRs. Our results suggest, among other, that (i) aqueous alteration in most of our CRs was limited to the matrix and (ii) most CRs may stem from a continuum of X-to-C complex asteroids, depending on their extent of aqueous alteration. More specifically, the endmembers being Xk/Xn types and Cgh/Ch types. This has strong implication in regard to what we can expect from the Psyche mission.  相似文献   

Role of vegetation in shaping Early Pennsylvanian braided rivers: Architecture of the Boss Point Formation,Atlantic Canada     

Alessandro Ielpi  Martin R. Gibling  Arden R. Bashforth  Corinne Lally  Michael C. Rygel  Saif Al‐Silwadi 《Sedimentology》2014,61(6):1659-1700

Vegetation is a major driver of fluvial dynamics in modern rivers, but few facies models incorporate its influence. This article partially fills that gap by documenting the stratigraphy, architecture and palaeobotany of the Lower Pennsylvanian Boss Point Formation of Atlantic Canada, which contains some of the Earth's earliest accumulations of large woody debris. Braided‐fluvial systems occupied channel belts of varied scale within valleys several tens of metres deep and more than 12 km wide, and their deposits predominantly consist of sandy and gravelly bedforms with subordinate accretionary macroforms, high flow‐strength sand sheets and rippled abandonment facies. Discrete accumulations of clastic detritus and woody debris are up to 6 m thick and constitute at least 18% of the in‐channel deposits; they represent lags at the base of large and small channels, fills of minor channels and sandy macroforms that developed in central positions in the upper parts of channel fills. Sandstones with roots and other remnants of in situ vegetation demonstrate that vegetated islands were present, and the abundance of discrete channel fills suggests that the formation represents an anabranching, island‐braided sandbed river, the earliest example documented to date. Although some sphenopsid and lycopsid remains are present, most woody fragments are derived from cordaitalean trees, and the evolution of this group late in the Mississippian is inferred to have exerted a significant influence on fluvial morphodynamic patterns. The formation records a landscape in which active channel belts alternated with well‐drained floodplains colonized by dense, mature forests and local patches of pioneering, disturbance‐tolerant vegetation. Lakes and poorly drained floodplains dominated by carbonate and organic deposition, respectively, were also present. A large supply of woody debris triggered channel blockage and avulsion, and active channel margins and islands within the channel belts were initially colonized by pioneer vegetation and subsequently stabilized by large trees. A similar alternation of stable and unstable conditions is observed in modern braided rivers actively influenced by vegetation.  相似文献