The Magmatic Evolution of the Whakamaru Supereruption, New Zealand, Constrained by a Microanalytical Study of Plagioclase and Quartz
The Magmatic Evolution of the Whakamaru Supereruption, New Zealand, Constrained by a Microanalytical Study of Plagioclase and Quartz
复制标题
新西兰华卡马鲁超级喷发的岩浆演化，受到斜长石和石英微观分析研究的约束
DOI：
--
复制
发表时间：
2010
期刊：
影响因子：
--
通讯作者：
R. Wysoczanski
中科院分区：
文献类型：
--
作者：
K. Saunders;D. Morgan;J. Baker;R. Wysoczanski
华卡马鲁火山喷发是已知源自新西兰高产陶波火山带的最大火山喷发。斜长石晶体的主要、次要和痕量元素浓度以及阴极发光图像用作石英晶体中钛浓度的代表，已用于探索其化学分区。鉴定出三个斜长石群体。第 1 族晶体的特征是继承了组成 An45^60、Ba 115^650 ppm 和 La 3^9 ppm 的核心，边缘为 c。 An30、Ba 450^800 ppm 和 La 7^10 ppm，并且在几个晶体核心上存在薄的过度生长边缘。第 2 族晶体是振荡分区斜长石，其组成为 An30^40、Ba 450^730 ppm 和 La 8·5^9·5 ppm。第 3 族斜长石晶体的核心为 An25^35，边缘为 An20^25，且 Sr 含量较低 (280^480 ppm)。根据这些斜长石晶体的化学成分，可以识别出四种物理化学性质不同的流纹岩熔体：（1）安山岩原始熔体，其中第 1 族晶体的核心结晶； (2) 杂砂岩熔体或杂砂岩原岩熔体导致第 1 族晶核上狭窄的过度生长边缘； (3)由结晶出3族斜长石晶体的成熟晶糊体再生得到的熔体； (4) 最终的流纹岩熔体，由不同比例的安山岩、杂砂岩衍生和再生熔体与随后的斜长石占主导地位的晶体组合的开放系统分步结晶合并而形成。石英晶体的阴极发光成像揭示了复杂的分区，这是潜在多基因来源的动态结晶历史的结果。对选定的石英晶体明亮核心^边缘界面的阴极发光图像的灰度强度（作为钛含量的代表）的扩散建模表明，边缘区域的石英重新生长发生在顶峰喷发之前5300年（峰值可能性为50^70年），并持续到顶峰喷发。这与根据第 1 族斜长石晶体的核心边缘界面确定的 5280 年的时间尺度一致，表明岩浆室是短暂的，源自喷发前不久来自多个来源的岩浆的混合。与积累前体岩浆和晶体所需的数十万年的时间尺度相比，这项研究增加了越来越多的证据，证明液态硅质岩浆体的短暂性和地质上的快速混合和流动导致了超级喷发。
TheWhakamaru eruption is the largest-volume eruption known to have originated from the hyper-productive Taupo Volcanic Zone, New Zealand. Major, minor and trace element concentrations of plagioclase crystals and cathodoluminescence images, used as a proxy for Ti concentrations in quartz crystals, have been used to explore their chemical zonation. Three plagioclase populations are identified. Group 1 crystals are characterized by inherited cores of composition An45^60, Ba 115^650 ppm and La 3^9 ppm, rims of c. An30, Ba 450^800 ppm and La 7^10 ppm and the presence of a thin overgrowth rim on several crystals cores. Group 2 crystals are oscillatory-zoned plagioclases of composition An30^40, Ba 450^730 ppm and La 8·5^9·5 ppm. Group 3 plagioclase crystals have cores of An25^35 and rims of An20^25 and low Sr contents (280^480 ppm). From the chemical composition of these plagioclase crystals, four physicochemically distinct rhyolitic melts are identified: (1) an andesitic progenitor melt in which the cores of Group 1 crystals crystallized; (2) a greywacke melt or greywacke protolith melt responsible for narrow overgrowth rims on Group 1 crystal cores; (3) melt derived from the rejuvenation of a mature crystal mush body from which Group 3 plagioclase crystals crystallized; (4) a final, rhyolitic melt created by the amalgamation of varying proportions of the andesitic, greywacke-derived and rejuvenated melts with subsequent, open-system fractional crystallization of a plagioclasedominant crystal assemblage. Cathodoluminescence imaging of quartz crystals reveals complex zonation, the result of a dynamic crystallization history from potentially polygenetic sources. Diffusion modelling of the greyscale intensity of cathodoluminescence images (as a proxy forTi content) for a selection of bright core^rim interfaces of quartz crystals suggests that renewed quartz growth at the rim zones occurred5300 years (peak likelihood 50^70 years) prior to and continued towards the climactic eruption. This is consistent with timescales of5280 years determined from core^rim interfaces of Group 1 plagioclase crystals, suggesting that the magma chamber was ephemeral, derived from mixing of magmas from multiple sources shortly prior to eruption. This study adds to a growing body of evidence for the ephemeral nature and geologically rapid mixing and mobilization of liquid silicic magma bodies leading to supereruptions, compared with the timescales of hundreds of thousands of years required to accumulate the precursor magma and crystals.