Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies
Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies
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DOI：
10.1007/s00410-002-0371-8
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发表时间：
2002-11-01
影响因子：
3.5
通讯作者：
Valley, JW
中科院分区：
文献类型：
--
作者：
Bindeman, IN;Valley, JW
大量火山碎屑喷发的产物具有数千米的喷发下降，提供了岩基规模的岩浆室的快照视图，并淬灭了矿物之间喷发前的同位素分馏（即温度）。我们报告了对单个石英斑晶以及来自火山口形成 0.76 Ma Bishop Tuff (BT)、前火山口玻璃山 (2.1-0.78 Ma) 和后火山口流纹岩 (0.65-0.04 Ma）来描述加利福尼亚州长谷火山口下方的长寿、基岩规模的岩浆房。 6180 的值显示出微妙的千分之一。从最古老的玻璃山熔岩减少到最年轻的火山口后流纹岩。较老的玻璃山熔岩表现出较大的（类似于千分之一）δ（18）O（石英）变化。玻璃山最好的圆顶与 BT 的 δ（18）O（石英）值相似，反映了 BT 岩浆房形成过程中的对流均质化，周围是极其不均匀的乡村岩石（范围从 2 到 +29%）。 BT 的氧同位素测温证实了 BT“晚期”（815 摄氏度）和“早期”（715 摄氏度）之间的温度梯度。 “早期”和“晚期”BT 的 6180（石英）值分别为 +8.33 和 8.21 份/千分率，与常数 delta(18)O（熔化）= 7.8 +/- 0.1 千分率和 100 摄氏度的温差一致。锆石熔体饱和平衡给出了类似的温度范围。 BT 不同地层单元和喷发前深度 6 至 11 公里范围内的浮石碎屑的 δ(18)O（石英）值（基于熔体包裹体），并记录 δ(18)O（熔体）的垂直和横向均匀性。在世界范围内，另外五个大型流纹岩（熔岩溪、下班德利尔、鱼峡谷、塞罗加兰山和托巴）在每次形成火山口的高潮喷发中，早期和晚期喷发部分都表现出相同的 delta(18)O（熔化）值。我们将 BT 和其他大型岩浆室的大尺度 delta(18)O 均质性解释为它们长寿（> 10(5) 年）和对流的证据。然而，一些石英斑晶中的剩余同位素分带、长石中的微量元素梯度以及石英和锆石晶体尺寸分布与更短的时间尺度（10（2）-10（4）年）更加一致。我们提出了一种侧壁结晶模型，该模型促进对流均质化、更多演化和停滞、富含挥发性液体的屋顶积聚，并在高潮前岩浆室中形成成分和温度梯度。晶体+熔体+气泡在可变δ(18)O的室壁附近糊状，随着新的更热的岩浆的重新填充，周期性地重新流动。在 0.76 Ma 破火山口塌陷之前 10(3)-10(4) 年，发生了一次由高 Ti、Sr、Ba、Zr 和挥发性较丰富的岩浆重新填充的事件，导致岩浆在底部混合，在顶部和墙壁附近融化，斑晶向下沉降到这种混合熔体中。
Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1-0.78 Ma), and post-caldera rhyolites (0.65-0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of 6180 show a subtle 1parts per thousand. decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (similar to1parts per thousand) variability of delta(18)O(quartz).,best domes of Glass Mountain are similar to The youngest domes of Glass Mountain are similar to BT in delta(18)O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29%.). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 degreesC) and "Early" (715 degreesC) BT. The 6180(quartz) values of "Early" and "Late" BT are +8.33 and 8.21parts per thousand, consistent with a constant delta(18)O(melt) = 7.8 +/- 0.1parts per thousand and 100degreesC temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of delta(18)O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of delta(18)O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal delta(18)O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale delta(18)O homogeneity of BT and other large magma chambers as evidence of their longevity (> 10(5) years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (10(2)-10(4) years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable delta(18)O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 10(3)-10(4) years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.