運用「水晶鐘」計算岩漿在火山爆發前儲存了多久

運用「水晶鐘」計算岩漿在火山爆發前儲存了多久

最近一項研究結果指出供給火山的熔岩可以在地殼中待上數千年之久，這有助於人們管理火山災害並且更準確地預測火山什麼時候會噴發。

冰島Holuhraun熔岩原在2014年8月噴發出岩漿。圖片來源：Bob White

英國劍橋大學的研究人員在發表於期刊《科學》(Science)的成果中，運用像是「水晶鐘」的火山礦物，計算出岩漿可以待在火山系統最深部多久。這是首度有研究成功估計出岩漿儲存在地殼和地函交界處(稱為莫荷面，Moho)附近的時間。

「這在地質學上就像是偵探辦案一樣。」研究第一作者，劍橋大學地球科學系的Euan Mutch博士表示。「我們可以經由觀察岩石重建火山噴發當時的情境，並了解岩漿儲存在地下時所處的環境，但是想知道火山系統最深處發生的事物卻相當困難。」

「找出岩漿能在地殼中儲存多久，可以幫助我們改良用來模擬火山爆發過程的模型。」共同作者，同樣來自劍橋大學地球科學系的John Maclennan博士表示。「岩漿的上升速度和儲存時間跟火山所在區域的化學條件與傳熱因子有密切關係，這兩者對於地熱發電和火山氣體如何釋放到大氣也相當重要。」

研究人員探討了冰島北部Theistareykir火山的Borgarhraun噴發事件。發生在大約10000年前的這起火山爆發事件，岩漿是從莫荷面直接湧上來。莫荷面在岩漿從地函產生，接著上升到地表的過程中具有非常重要的影響。為了計算岩漿在莫荷面儲存了多久，研究人員運用火山中一種稱為「尖晶石」的礦物，它的功能就像是微小的碼表或水晶鐘一樣。

研究人員透過研究這種水晶鐘，模擬了岩漿儲存在莫荷面的時候，尖晶石的成分如何隨著時間而改變。精確來說，他們關注的是尖晶石裡鋁和鉻的擴散速度，以及這些元素的分布情形。

「晶體裡的元素會為了和周遭環境達成化學平衡而發生擴散作用。」Maclennan表示。「如果我們知道這些元素的擴散速率，就能得出礦物在岩漿裡待了多久。」

研究人員探討晶體裡的鋁和鉻如何分布之後，發現分布模式和岩漿儲存時間有所關聯，這項新訊息令他們感到相當興奮。他們採用了之前其他研究在實驗室估計出來的擴散速率，接著結合了有限元素模型和貝氏巢狀抽樣的新方法，估算出岩漿儲存在莫荷面時的變化歷程。

「對於岩漿是從哪個深度上來的，我們現在已經可以估算得十分精準。」Mutch說。「但是岩漿在地殼深處待了多久，卻還沒有人可以得出這類資訊。」

研究人員透過算出岩漿儲存在地下的時間，也能得出岩漿運輸到地表的過程。一般的火山模型下方常常畫有一座巨大的岩漿庫，但研究人員表示其實火山下方更像是一套蔓延在地殼內部的「管路系統」，其中有許多小型「噴口」可以讓岩漿快速來到地表。

同一團隊近日發表在期刊《自然―地球科學》(Nature Geoscience)的另一篇論文中，發現岩漿上升速度跟釋放二氧化碳有所關聯，這項結果可以應用於火山監測。

研究團隊觀察到火山爆發數天之前會有許多二氧化碳從岩漿中變成氣體釋放出來，代表監測二氧化碳是相當有用的方式，可以用來當作冰島火山噴發的前兆。運用同樣從Borgarhraun事件找到的礦物晶體，研究人員發現岩漿可以在短短四天之內就從20公里深的岩漿庫來到地表。

這項研究的經費來自英國自然環境研究委員會。

 ‘Crystal clocks’ used to time magma storage before volcanic eruptions

The molten rock that feeds volcanoes can be stored in the Earth’s crust for as long as a thousand years, a result which may help with volcanic hazard management and better forecasting of when eruptions might occur.

Researchers from the University of Cambridge used volcanic minerals known as ‘crystal clocks’ to calculate how long magma can be stored in the deepest parts of volcanic systems. This is the first estimate of magma storage times near the boundary of the Earth’s crust and the mantle, called the Moho. The results are reported in the journal Science.

 “This is like geological detective work,” said Dr Euan Mutch from Cambridge’s Department of Earth Sciences, and the paper’s first author. “By studying what we see in the rocks to reconstruct what the eruption was like, we can also know what kind of conditions the magma is stored in, but it’s difficult to understand what’s happening in the deeper parts of volcanic systems.”

“Determining how long magma can be stored in the Earth’s crust can help improve models of the processes that trigger volcanic eruptions,” said co-author Dr John Maclennan, also from the Department of Earth Sciences. “The speed of magma rise and storage is tightly linked to the transfer of heat and chemicals in the crust of volcanic regions, which is important for geothermal power and the release of volcanic gases to the atmosphere.”

The researchers studied the Borgarhraun eruption of the Theistareykir volcano in northern Iceland, which occurred roughly 10,000 years ago, and was fed directly from the Moho. This boundary area plays an important role in the processing of melts as they travel from their source regions in the mantle towards the Earth’s surface. To calculate how long the magma was stored at this boundary area, the researchers used a volcanic mineral known as spinel like a tiny stopwatch or crystal clock.

Using the crystal clock method, the researchers were able to model how the composition of the spinel crystals changed over time while the magma was being stored. Specifically, they looked at the rates of diffusion of aluminium and chromium within the crystals and how these elements are ‘zoned’.

“Diffusion of elements works to get the crystal into chemical equilibrium with its surroundings,” said Maclennan. “If we know how fast they diffuse we can figure out how long the minerals were stored in the magma.”

The researchers looked at how aluminium and chromium were zoned in the crystals and realised that this pattern was telling them something exciting and new about magma storage time. The diffusion rates were estimated using the results of previous lab experiments. The researchers then used a new method, combining finite element modelling and Bayesian nested sampling to estimate the storage timescales.

“We now have really good estimates in terms of where the magma comes from in terms of depth,” said Mutch. “No one’s ever gotten this kind of timescale information from the deeper crust.”

Calculating the magma storage time also helped the researchers determine how magma can be transferred to the surface. Instead of the classical model of a volcano with a large magma chamber beneath, the researchers say that instead, it’s more like a volcanic ‘plumbing system’ extending through the crust with lots of small ‘spouts’ where magma can be quickly transferred to the surface.

A second paper by the same team, recently published in Nature Geoscience, found that that there is a link between the rate of ascent of the magma and the release of CO2, which has implications for volcano monitoring.

The researchers observed that enough CO₂ was transferred from the magma into gas over the days before eruption to indicate that CO₂ monitoring could be a useful way of spotting the precursors to eruptions in Iceland. Based on the same set of crystals from Borgarhraun, the researchers found that magma can rise from a chamber 20 kilometres deep to the surface in as little as four days.

The research was supported by the Natural Environment Research Council (NERC).

原始論文：Euan J. F. Mutch*, John Maclennan, Tim J. B. Holland, Iris Buisman. Millennial storage of near-Moho magma. Science, 2019, Vol. 365, Issue 6450, pp. 260-264 DOI: http://dx.doi.org/10.1126/science.aax4092

引用自：University of Cambridge. "'Crystal clocks' used to time magma storage before volcanic eruptions." 

原文網址：https://www.cam.ac.uk/research/news/crystal-clocks-used-to-time-magma-storage-before-volcanic-eruptions