An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer
An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer
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DOI：
10.1007/s00410-016-1298-9
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发表时间：
2016-09
影响因子：
3.5
通讯作者：
Euan Mutch;Euan Mutch;Jon D. Blundy;B. Tattitch;Frances J. Cooper;Richard A. Brooker
中科院分区：
文献类型：
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作者：
Euan Mutch;Euan Mutch;Jon D. Blundy;B. Tattitch;Frances J. Cooper;Richard A. Brooker
我们报告了在近固相温度和0.8-10 kbar压力下，由各种花岗岩（sensu lato）块状成分合成的岩浆角闪孔组成的新实验数据。合成钙质角闪石的总铝含量（Altot）随压力(P)有系统的变化，但在低压（<2.5 kbar）时呈非线性关系。在较高的压力下，这种关系类似于其他实验研究，这表明altoto和P之间的一般关系对体成分相对不敏感。我们开发了一种新的铝角闪石地球气压计，适用于低变异矿物组合的花岗岩：角闪石+斜长石(An15-80) +黑云母+石英+碱长石+钛铁矿/钛铁矿+磁铁矿+磷灰石。角闪洞分析应该从与斜长石接触的颗粒边缘开始，并在接近单长花岗岩固体的温度（725±75℃）下与其他矿物组合处于明显的结构平衡状态，这是由角闪石-斜长石测温确定的。符合这些标准的平均角闪孔边缘成分可用于根据表达式从Altot（每个公式单位的原子数，apfu）计算ep（以kbar为单位）：$${\textit{P }}\left( {\text{kbar}} \right) = 0.5 + 0.331\left( 8 \right) \times {\text{Al}}^{\text{tot}} + 0.995\left( 4 \right) \times \left( {{\text{Al}}^{\text{tot}} } \right)^{2}$$该表达式恢复了我们的校准数据集的平衡压力，包括新的和已发表的实验和自然数据，在±16范围内% relative uncertainty. An uncertainty of 10 % relative for a typical Altotvalue of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.
We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Altot) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Altotand P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An15–80) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculateP(in kbar) from Altot(in atoms per formula unit, apfu) according to the expression: $${\textit{P }}\left( {\text{kbar}} \right) = 0.5 + 0.331\left( 8 \right) \times {\text{Al}}^{\text{tot}} + 0.995\left( 4 \right) \times \left( {{\text{Al}}^{\text{tot}} } \right)^{2}$$ This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Altotvalue of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.