Barometers Behaving Badly I: Assessing the Influence of Analytical and Experimental Uncertainty on Clinopyroxene Thermobarometry Calculations at Crustal Conditions
Barometers Behaving Badly I: Assessing the Influence of Analytical and Experimental Uncertainty on Clinopyroxene Thermobarometry Calculations at Crustal Conditions
复制标题
气压计表现不佳 I：评估分析和实验不确定性对地壳条件下单斜辉石温压计算的影响
DOI：
10.1093/petrology/egac126
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发表时间：
2023
影响因子：
3.9
通讯作者：
Krawczynski, Michael J
中科院分区：
文献类型：
--
作者：
Wieser, Penny E;Kent, Adam J;Till, Christy B;Donovan, John;Neave, David A;Blatter, Dawnika L;Krawczynski, Michael J
在火成岩体系中，单斜辉石和单斜辉石-液体(CPx-Liq)对的组成经常被用来计算结晶/平衡压力。虽然典型的不确定度通常被分配给基于对校准或测试数据集的拟合而计算出的压力，但这些不确定度的来源(以及减少它们的方法)并没有得到严格的评估。我们发现，计算压力的相当大的不确定性来自与CPX的电子探针微分析器(EPMA)测量相关的分析误差。具体地说，由于计数次数不足和/或低束流，在分析浓度为<；1wt%的元素时，X射线计数过低会导致测量高度不精确(对于Na2O，1σ误差为10-40%)。低分析精度会扩展到对压敏矿物成分(如翡翠)的计算中。利用蒙特卡罗方法，我们证明了由分析精度引起的元素变化对于单个CPX产生了~4千巴(~15千米)的压力，对于单个CPX-Liq对产生了~6千巴的压力。此外，矿物成分的分析不确定性产生了压力和温度之间的高度相关的阵列，以前被认为是跨地壳岩浆储存。在引用这样的地质学解释之前，必须排除来自分析不精确的更普通的来源。最重要的是，分析精度低不仅影响气压计在自然系统中的应用，还影响了用于校准和测试气压计的实验产品中CPX的特性。精密度差对每一次测量的影响往往被实验装药内的少量测量放大，这意味着分析精度低和矿物成分的真实变异性没有通过平均多个EPMA分析得到充分的调和。我们汇编了用于校准现有气压计的INN= 307个实验和n= 490个新实验的CPx测量次数，发现约45%的实验电荷是由≤5单个CPx分析表征的。对实验相真实组成的表征不充分可能是所有基于CPX的气压计在使用全球实验数据集进行测试时都表现出较大误差(±3千巴)的原因。我们建议对分析和实验方案进行具体修改，例如在实验和自然样品中测量相对束流阻力较小的CPX中的低浓度元素时，增加计数次数和/或更高的束流。我们还主张增加每个实验费用的分析次数，解决实验室间的分析偏差，并改进数据报告。实施这些更改对于产生更强大的数据集以校准和测试下一代更精确和更精确的基于CPX的气压计至关重要。反过来，这将使更严格地研究各种构造环境中的岩浆储存几何图形(例如，区分真正的跨地壳储存与离散储集层中的储存)。
The composition of clinopyroxene and clinopyroxene-liquid (Cpx-Liq) pairs are frequently used to calculate crystallization/equilibration pressures in igneous systems. While canonical uncertainties are often assigned to calculated pressures based on fits to calibration or test datasets, the sources of these uncertainties (and thus ways to reduce them) have not been rigorously assessed. We show that considerable uncertainties in calculated pressures arise from analytical error associated with Electron Probe Microanalyser (EPMA) measurements of Cpx. Specifically, low X-ray counts during analysis of elements with concentrations <1 wt% resulting from insufficient count times and/or low beam currents yield highly imprecise measurements (1σ errors of 10–40% for Na2O).Low analytical precision propagates into the calculation of pressure-sensitive mineral components such as jadeite. Using Monte Carlo approaches, we demonstrate that elemental variation resulting from analytical precision alone generates pressures spanning ~4 kbar (~15 km) for a single Cpx and ~6 kbar for a single Cpx-Liq pair using popular barometry expressions. In addition, analytical uncertainties in mineral compositions produce highly correlated arrays between pressure and temperature that have been previously attributed to transcrustal magma storage. Before invoking such geological interpretations, a more mundane origin from analytical imprecision must be ruled out. Most importantly, low analytical precision does not just affect the application of barometers to natural systems; it has also affected characterization of Cpx in experimental products used to calibrate and test barometers. The impact of poor precision on each individual measurement is often magnified by the small number of measurements made within experimental charges, meaning that low analytical precision and true variability in mineral compositions have not been sufficiently mediated by averaging multiple EPMA analyses. We compile the number of Cpx measurements performed inN= 307 experiments used to calibrate existing barometers, andN= 490 new experiments, finding ~45% of experiment charges were characterized by ≤5 individual Cpx analyses. Insufficient characterization of the true composition of experimental phases likely accounts for the fact that all Cpx-based barometers exhibit large errors (± 3 kbar) when tested using global experimental datasets.We suggest specific changes to analytical and experimental protocols, such as increased count times and/or higher beam currents when measuring low concentration elements in relatively beam resistant Cpx in experiments and natural samples. We also advocate for increasing the number of analyses per experimental charge, resolving interlaboratory analytical offsets and improving data reporting. Implementing these changes is essential to produce a more robust dataset to calibrate and test the next generation of more precise and accurate Cpx-based barometers. In turn, this will enable more rigorous investigation of magma storage geometries in a variety of tectonic settings (e.g. distinguishing true transcrustal storage vs. storage in discrete reservoirs).