Progreso y retos de la geocronología 40Ar/39Ar en Costa Ricay Nicaragua
Progreso y retos de la geocronología 40Ar/39Ar en Costa Ricay Nicaragua
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哥斯达黎加 尼加拉瓜
DOI：
10.15517/rgac.v0i45.1907
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发表时间：
2012
影响因子：
3.5
通讯作者：
B. Turrin
中科院分区：
文献类型：
--
作者：
I. Saginor;E. Gazel;M. J. Carr;C. Swisher;B. Turrin
为了更好地估计中美洲弧的喷出通量，从2002-2008年，我们从哥斯达黎加和尼加拉瓜获得了61个地理位置良好的熔岩和火山灰的高精度40Ar/39Ar年龄。在这里，我们用四个例子描述了在这项研究中遇到的一些观察结果，这些例子很好地证明了40Ar/39Ar年龄的精密度、准确性和总体可靠性。首先，低K2O值，特别是来自尼加拉瓜-瓜岛的样品，是1 My以下样品获得可靠日期的主要限制或尝试。其次，由于中美洲的热带气候，样品的广泛风化导致了不同程度的Ar损失，即使手部样品看起来没有变化。第三，我们的现场和地质年代学数据让我们得出结论，在过去的15到20年里，火山喷发的速度并不是恒定的，而是被长达数百万年的间隔所打断。我们试图通过几种方式解决时间差距。首先，使用地球化学分析来识别可能在一段时间内喷发的样本，而不是已知的火山活动。例如，中美洲活动弧区的U/Th值明显高于中新世Coyol群的U/Th值，但有4个样品具有中间值，以确定它们的年龄是否也是中等的。然而，所有这些样本都被发现来自已知的火山活动时期。其次，我们试图找到活动弧群最古老的部分和Coyol群最年轻的部分，以便更好地限制火山生产力明显差距的时间和持续时间。这种方法也未能找到没有已知火山活动的时期的样本。当这些方法被证明基本上不成功时，我们的重点转向了活动火山前锋和科约尔火山前锋之间以及科西吉纳和圣克里斯托巴尔之间的小型火山活动区，圣克里斯托巴尔是中美洲火山前锋没有活火山活动的最长地段。这项工作获得了1.1至3.6 Ma的样品年龄，从而大大缩小了尼加拉瓜的明显火山缺口。
To better estimate the extrusive flux of the Central American Arc, from 2002-2008, we obtained sixty one high precision 40Ar/39Ar ages on geographically well-situated lavas and tephra from Costa Rica and Nicaragua. Here, we describe a number of observations encountered during this study using four examples that well document the preci- sion, accuracy and general reliability of the 40Ar/39Ar ages. First, low K2O values, particularly in samples from Nicara- gua, is a major limitation in or attempts to obtain reliable dates on samples under 1 My. Second, extensive weathering of samples due to the tropical climate of Central America has resulted in various levels of argon loss even when the hand sample appeared unaltered. Third, our field and geochronological data lead us to conclude that eruptive rates have not been constant over the past 15 to 20 My, but rather appears punctuated by gaps of up to several million years. We attempted to address the temporal gaps in several ways. First, geochemical analyses were used to identify samples that may have erupted during time periods without known volcanism. For example, U/Th values in the active Central American arc are significantly higher than those obtained from the Miocene Coyol Group except for four samples with intermediate values that were dated to determine if their ages were intermediate as well. However, all of these samples were found to be from a period with known volcanism. Second, we sought to locate the oldest sections of the active arc and the youngest sections of the Coyol Group in order to better constrain the timing and duration of the apparent gap in volcanic productivity. This approach also failed to locate samples from periods without known volcanism. When these methods proved largely unsuccessful, our focus shifted to dating regions of minor volcanism between the active and Coyol volcanic fronts as well as between Cosiguina and San Cristobal, the longest stretch of the Central American Volcanic Front without active volcanism. This effort yielded ages on samples ranging from 1.1 to 3.6 Ma and, thus, substantially reduced the apparent volcanic gap in Nicaragua.