Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff
Paleointensity estimates from ignimbrites: An evaluation of the Bishop Tuff
复制标题
凝灰岩的古强度估计：主教凝灰岩的评估
DOI：
10.1029/2009gc002834
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发表时间：
2010
期刊：
影响因子：
3.7
通讯作者：
J. Bowles
中科院分区：
文献类型：
--
作者：
J. Gee;Yongjae Yu;J. Bowles
灰流凝灰岩或凝灰岩通常含有细粒磁铁矿，其尺寸范围涵盖超顺磁到单域尺寸范围，适合估计地磁场强度。然而，由于复杂的磁性矿物学以及热和蚀变历史的变化，熔结岩可能具有热和化学起源的剩磁。我们检查了~0.76 Ma Bishop Tuff 的三个地层剖面，其中可以获得有关后冷却和蚀变的独立信息，作为古强度研究中熔结岩的适用性测试。热磁曲线表明，低 Ti 钛磁铁矿 (Tc = 560°C–580°C) 是主要相，较高 Tc 相的贡献较小。在 580°C 以上时，剩磁显着解锁，表明磁赤铁矿和/或（钛）磁赤铁矿是大多数样品中剩磁的重要贡献者。我们通过 440°C 至 580°C 之间未堵塞的剩磁，对 89 个样本中的 46 个样本（总共三个地点中的两个地点的 15 个地点）进行了成功的古田估计。这些样本代表了一系列焊接程度，并具有可变的蚀变历史，但提供了 43.0 μT (±3.2) 的一致古田估计值，相当于 7.8 × 1022 Am2 的 VADM。凝灰岩最密集的焊接部分的就位温度推断高达~660°C，这表明剩磁可能主要是热源性的，尽管不能排除热化学剩磁的贡献。这些结果表明，凝灰岩可能构成可靠的古强度测定的可行材料。
Ash flow tuffs, or ignimbrites, typically contain fine‐grained magnetite, spanning the superparamagnetic to single‐domain size range that should be suitable for estimating geomagnetic field intensity. However, ignimbrites may have a remanence of thermal and chemical origin as a result of the complex magnetic mineralogy and variations in the thermal and alteration history. We examined three stratigraphic sections through the ∼0.76 Ma Bishop Tuff, where independent information on postemplacement cooling and alteration is available, as a test of the suitability of ignimbrites for paleointensity studies. Thermomagnetic curves suggest that low‐Ti titanomagnetite (Tc = 560°C–580°C) is the dominant phase, with a minor contribution from a higher Tc phase(s). Significant remanence unblocking above 580°C suggests that maghemite and/or (titano)maghemite is an important contributor to the remanence in most samples. We obtained successful paleofield estimates from remanence unblocked between 440°C and 580°C for 46 of 89 specimens (15 sites at two of three total localities). These specimens represent a range of degrees of welding and have variable alteration histories and yet provide a consistent paleofield estimate of 43.0 μT (±3.2), equivalent to a VADM of 7.8 × 1022 Am2. The most densely welded sections of the tuff have emplacement temperatures inferred to be as high as ∼660°C, suggesting that the remanence may be primarily thermal in origin, though a contribution from thermochemical remanence cannot be excluded. These results suggest that ignimbrites may constitute a viable material for reliable paleointensity determinations.