40Ar/39Ar geochronology of volcanic clasts and pumice in CRP-1 core, Cape Roberts, Antarctica
40Ar/39Ar geochronology of volcanic clasts and pumice in CRP-1 core, Cape Roberts, Antarctica
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南极洲罗伯茨角 CRP-1 岩心火山碎屑和浮石的 40Ar/39Ar 年代学
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发表时间：
1998
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通讯作者：
W. Mcintosh
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作者：
W. Mcintosh
火山碎屑和浮石的Ar/Ar定年为CRP-1岩心的沉积年龄提供了制约。激光熔融法分析来自116 mbsf附近的浮石碎屑的斜长岩斑晶，其沉积年龄为18.40 ± 1.12 Ma（± 2 σ）。从五个改造熔岩碎屑的斜长岩斑晶和基质浓缩物的结果提供了CRP-1的61至114 mbsf间隔的最大沉积年龄为19.73 ± 0.86 Ma至17.15 ± 0.80 Ma。最精确的年龄测定为19.32 ± 0.14 Ma的粗面碎屑近105 mbsf。总的来说，从CRP-1的下部的Ar/Ar结果表明，没有超过116 mbsf的沉积物是年龄超过19.5 Ma，和数据是最兼容的快速积累的61至116 mbsf的CRP-1段近18 Ma。来自33 mbsf富碳酸盐层的玄武岩碎屑的Ar/Ar结果提供了该层位的最大沉积年龄为1.2 ± 0.1 Ma。通过沉积（主要是冰川）过程，它们只提供沉积时间的最大年龄。除了再造熔岩和火山渣的碎屑外，CRP-1的114至117 mbsf间隔中还存在0.5至3 mm圆形粗面浮石碎屑的浓度。虽然这些浮石已被改造，但其脆弱的性质和在有限地层间隔中的集中表明它们在喷发后不久作为火山碎屑喷出物沉积，因此推断其喷发年龄与其沉积时间非常接近（Armienti等人，这卷; Smellie，这卷）。它们的圆形可能是由于风的输送和在海冰上沉积后的磨损。火山碎屑从CRP-1样本中分离出来，CRP-1样本是由约翰·斯梅利（John Smellie）在到达德国不来梅港（Bremerhaven）后从工作的一半岩芯中收集的。根据从麦默多站CRP-1收集的样品薄切片的观察结果，选择了桑迪至含砾火山碎屑的取样间隔。一些较大的火山碎屑，在半个核心的外表面或切割面上可见，是直接从核心手工挑选的。大多数碎屑是从100至200克富含沙子的沉积物样品中手工挑选出来的，经过超声波分解和洗涤。从每个碎屑中凿出小的子样品，并通过电子探针进行检查，以确认其火山性质，然后进行测年。火山碎屑的成分从碧玄岩到过碱性粗面岩不等。两个碎屑和浮石被发现含有斜长石，富钾的火山长石非常适合40 Ar/39 Ar分析。采用HF酸蚀和手工挑选的方法从这些样品中分离出斜长石。通过在稀盐酸中洗涤，以及对于一些富含斑晶的碎屑，通过破碎和手工挑选以去除贫钾斑晶相，制备了缺乏斜长石的熔岩碎屑。介绍CRP-1核心附近的罗伯茨角，南极洲，作为一个国际努力的一部分，采样海洋沉积物在麦克默多声音，重建新生代和可能的白垩纪古环境和古气候的南极洲的目标。由于海冰破裂，CRP-1仅穿透147.9 mbsf，不到计划深度的20%。对CRP-1岩心取样的古环境记录的准确解释需要岩心的准确年代学。硅藻地层学的初步研究（Cape Roberts Science Team，1998; Harwood et al.，该卷）确定CRP-1的上部层段（19-43 mbsf）为第四纪，下部层段的大部分（43-147.9 m）为下中新统。一些其他的定年方法，包括古地磁分析（罗伯茨等人，本卷）和壳体材料的锶年代测定（拉威尔，本卷）随后被应用于CRP-1年代学问题。本文报道了从33 mbsf和62 ~ 117 mbsf岩芯中回收的火山浮石和熔岩碎屑的~（40）Ar/~（39）Ar定年结果。40 Ar/39 Ar分析是确定火山矿物和基质喷发年龄的一种行之有效的方法（McDougall &Harrison，1988）。CRP-1中的火山碎屑CRP-1中观察到的大多数火山碎屑是玄武质至粗面质熔岩和火山渣的碎片，通常为圆形，直径一般为1 - 10 mm，但在少数情况下直径可达2-3 cm（Smellie，本卷）。由于这些熔岩碎屑被运输并沉积在西南极裂谷系WAIS CRP-1 60 S东南极冰盖© Terra Antartica Publication
Ar/Ar dating of volcanic clasts and pumice provide constraints for depositional ages of the CRP-1 core. Laser-fusion analyses of anorthoclase phenocrysts from pumice clasts concentrated near 116 mbsf suggest a depositional age of 18.40 ± 1.12 Ma (± 2 σ). Results from anorthoclase phenocrysts and groundmass concentrates from five clasts of reworked lava provide maximum depositional ages of 19.73 ± 0.86 Ma to 17.15 ± 0.80 Ma for the 61 to 114 mbsf interval of CRP-1. The most precise age determination is 19.32 ± 0.14 Ma for a trachytic clast near 105 mbsf. Taken together, the Ar/Ar results from the lower part of CRP-1 suggest that none of the sediment above 116 mbsf is older than 19.5 Ma, and the data are most compatible with rapid accumulation of the 61 to 116 mbsf segment of CRP-1 near 18 Ma. Ar/Ar results from a basaltic clast from the carbonate-rich layer at 33 mbsf provide a maximum depositional age of 1.2 ± 0.1 Ma for this horizon. by sedimentary (dominantly glacial) processes, they only provide maximum ages for the time of deposition. In addition to the clasts of reworked lava and scoria, a concentration of 0.5 to 3 mm clasts of rounded trachytic pumice are present in the 114 to 117 mbsf interval of CRP-1. Although these pumice have been reworked, their fragile nature and concentration in a restricted stratigraphic interval suggests that they were deposited soon after their eruption as pyroclastic ejecta, therefore their eruption age is inferred to closely approximate the time of their deposition (Armienti et al., this volume; Smellie, this volume). Their rounding may be due to wind transport and abrasion after their deposition on sea ice. Volcanic clasts were separated from CRP-1 samples collected by John Smellie from the working half of the core after its arrival in Bremerhaven, Germany. Sandy to pebbly volcanic-clast-bearing sampling intervals were selected on the basis of observations from thin sections of samples collected from CRP-1 at McMurdo Station. A few larger volcanic clasts, visible on the outer surface or cut face of the half core, were hand picked directly from the core. Most clasts were hand picked from 100 to 200 g samples of sand-rich sediment, following ultrasonic disaggregation and washing. Small subsamples were chiseled from each clast and examined by electron microprobe to confirm their volcanic nature prior to dating. Compositionally, volcanic clasts ranged from basanite to peralkaline trachyte. Two clasts and the pumice were found to contain anorthoclase, a K-rich volcanic feldspar ideally suited for 40Ar/39Ar analysis. HF etching and hand picking were used to separate the anorthoclase from these samples. Clasts of lava lacking anorthoclase were prepared by washing in dilute HCl and, for a few phenocryst-rich clasts, by crushing and hand picking to remove K-poor phenocryst phases. INTRODUCTION The CRP-1 core was drilled near Cape Roberts, Antarctica, as part of an international effort to sample marine sediments in McMurdo Sound, with the goal of reconstructing Cenozoic and possibly Cretaceous paleoenvironment and paleoclimate of Antarctica. Because of sea-ice break out, CRP-1 penetrated only to 147.9 mbsf, less than 20% of the planned depth. Accurate interpretations of the paleoenvironmental record sampled by the CRP-1 drillcore require an accurate chronology for the core. Initial studies of diatom stratigraphy (Cape Roberts Science Team, 1998; Harwood et al., this volume) identified the upper interval of CRP-1 (19-43 mbsf) as Quaternary and the majority of the underlying interval (43-147.9 m) as lower Miocene. A number of other dating methods, including paleomagnetic analysis (Roberts et al., this volume) and Sr dating of shell material (Lavelle, this volume) have subsequently been applied to the question of CRP-1 chronology. This paper reports the results of 40Ar/39Ar dating of volcanic pumice and lava clasts recovered from the core at 33 mbsf and 62 to 117 mbsf. 40Ar/39Ar analysis is a well-established method of determining the eruption age of volcanic minerals and groundmass (McDougall &Harrison, 1988). VOLCANIC CLASTS IN CRP-1 Most of the volcanic clasts observed in CRP-1 are fragments of basaltic to trachytic lava and scoria, commonly rounded, generally 1 to 10 mm in diameter, but in a few cases as large as 2-3 cm in diameter (Smellie, this volume). Because these lava clasts were transported and deposited West Antarctic Rift System WAIS CRP-1 60 S EAST ANTARCTIC ICE SHEET © Terra Antartica Publication