Hydrous komatiites from Commondale, South Africa: An experimental study
Hydrous komatiites from Commondale, South Africa: An experimental study
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南非 Commondale 的含水科马提岩：一项实验研究
DOI：
10.1016/j.epsl.2009.04.029
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发表时间：
2009
影响因子：
5.3
通讯作者：
A. Wilson
中科院分区：
文献类型：
--
作者：
J. A. Barr;T. Grove;A. Wilson
这项研究考察了南非3.33Ga Commondale超镁铁质岩套中科马提岩的侵位条件。康蒙代尔的科马提岩在物理结构和化学性质上都不同于任何其他科马提岩。Komatiite单元冷却边缘保存了原始的Komatiite液体成分，其Mg#为0.91，MgO=31.9wt.%，Al2O3wt.%/TiO2=80(wt.%)，SiO2含量为49.7wt.%。整个科马提岩序列的一个共同特征是存在斜方辉石尖晶石，在那里原始的斜方辉石晶体仍被保存下来。保存在这些旋辉岩带中最原始的天然辉石中的成分信息(Mg#=0.92)，提供了对科马提岩原始侵位条件的洞察。本研究采用无水和有水平衡实验，以及非平衡降温速率实验，量化了Commondale科马提石的结晶条件。在1550℃下发现了无水的1-atm液相线，初始晶相为Fo97橄榄石，然后是尖晶石，然后是原顽火辉石，在1335℃时为Mg#0.95。在200℃下，水饱和条件下也研究了相关系。~4wt.%H2O的加入使初始辉石的出现温度降低了210℃，从而产生了镁#接近于科马提岩中保存最原始的斜方辉石的斜方辉石。此外，动态降温速率实验表明，天然辉石保留了指示充气流动复合体中结晶和冷却的化学特征。据估计，Commondale科马提岩喷发前的H2O含量在~2到4.3wt%H2O之间。这一范围与巴伯顿山脉3.5兆镓科马提岩的估算范围相似，可能表明这两套岩石都是在相似的构造环境中形成的。
This study examines the emplacement conditions of komatiites in the 3.33 Ga Commondale Ultramafic Suite in South Africa. The komatiites of Commondale are unlike any other komatiites in both their physical structure and chemical nature. Komatiite unit chill margins preserve original komatiite liquid compositions with an Mg# of 0.91, MgO=31.9 wt.%, Al2O3wt.%/TiO2=80 (wt.%), and SiO2content of 49.7 wt.%. A common feature throughout the komatiite sequence is the presence of orthopyroxene spinifex, where original orthopyroxene crystals are still preserved. The compositional information preserved in the most primitive of the natural pyroxenes present in these spinifex zones (Mg#=0.92), provides insight into the original emplacement conditions of the komatiites. This study used anhydrous and hydrous equilibrium experiments, along with disequilibrium cooling-rate experiments, to quantify the crystallization conditions of the Commondale komatiites. The anhydrous, 1-atm liquidus was found at 1550 °C, with Fo97 olivine being the initial crystallizing phase, followed by spinel and then by protoenstatite, Mg# 0.95, at 1335 °C. The phase relations were also examined at 200 MPa under H2O saturated conditions. The addition of ~4 wt.% H2O lowers the appearance temperature of the initial pyroxene by 210 °C, thereby producing orthopyroxene with a Mg# closer to that of the most primitive preserved orthopyroxenes found in the komatiites. Additionally, dynamic cooling-rate experiments show that the natural pyroxenes preserve a chemical signature indicative of crystallization and cooling within an inflated flow complex. Estimates of the pre-eruptive H2O content for the Commondale komatiites are between ~2 and 4.3 wt.% H2O in the liquid. This range is similar to that estimated for 3.5 Ga komatiites of the Barberton Mountainland and may indicate formation of both suites in similar tectonic environments.
影响因子：
64.8
作者：
Berry, Andrew J.;Danyushevsky, Leonid V.;Sutton, Stephen R.
通讯作者：
Sutton, Stephen R.