How was the Bushveld complex assembled? Pulsed intrusions, crystallization conditions and the origin of layering in the Main Zone

该更新项目的目的是与BGR汉诺威合作，开发自动化光谱岩心扫描的新方法。这些可以提供比正在进行的项目中预期的更多数量和质量的矿物学信息，并使我们能够更全面地了解岩浆是如何被添加到主带岩浆室的，以及它们是如何在其中演化的。正在进行的研究的工作假设是，主带是由来自较深岩浆房的岩浆反复注入而形成的，这一点在该复合体的北叶中得到了物理性质（密度变化、矿物丰度和成分）的周期性垂直变化的证明。我们对马鲁拉东叶主带岩心的研究将检验这一假设，并通过以下方式提供岩浆结晶和分化的记录：(1)记录岩石密度和矿物化学变化；(2)测量反映冷却速率和与残余熔体再平衡程度的主要成分矿物的微结构特征；(3)利用微量元素和稳定同位素测温法计算结晶温度。正在进行的项目成功地收集了272个样本的密度测量值，这些样本沿主要区域的1500米剖面有规律地分布。密度剖面和初步的矿物化学数据表明，与北叶相比存在重大差异。我们发现了三个不同的亚带，而不是规则的周期变化，这些亚带指导了正在进行的地球化学和微结构研究的样本选择。与汉诺威BGR合作的新研究将涉及使用激光诱导击穿光谱（LIBS）和微xrf扫描主区样品。如果校准得当，这些方法可以提供定量的矿物比例、二维矿物分布图和多元素浓度数据。重要的是，扫描是快速和非破坏性的，因此可以研究剖面的所有样本，而不是最初计划的一小部分。微xrf的空间分辨率为20微米，将在单个样品中提供许多主要矿物相（斜长石，正长石和斜辉石）的分析。统计分析将揭示每个阶段的化学非均质性程度，这可能与玄武岩的相关系和结晶模型与岩浆组成和/或温度的变化有关。microXRF的二维元素图将揭示矿物结构、分带以及晚期间隙相的分布和特征。我们还将使用LIBS光谱来获得选定矿物中的微量元素浓度，这些矿物可以跟踪母岩浆的化学演化。

The aim of this renewal project is to exploit new methods of automated spectroscopic drill core scanning in cooperation with the BGR Hannover. These can provide a much greater amount and quality of mineralogic information than anticipated in the ongoing project and will allow us to reach a more complete understanding of how magmas were added to the Main Zone magma chamber and how they evolved within it. The working hypothesis of the ongoing study was that the Main Zone was assembled from repeated magma injections from a deeper magma chamber, as evidenced in the northern lobe of the complex by cyclic vertical variations in physical properties (density variations, mineral abundances and compositions). Our studies of Main Zone drill cores in the eastern lobe, Marula site will test this hypothesis and provide a record of magma crystallization and differentiation by: (1) documenting rock density and mineral-chemical variations (2) measuring microtextural features of the main constituent minerals that reflect cooling rates and the extent of re-equilibration with residual melt; (3) calculating crystallization temperatures from trace-element and stable isotope geothermometry. The ongoing project succeeded in collecting density measurements on 272 samples regularly spaced along a 1500 meter section of the Main Zone. The density profile and preliminary mineral-chemical data suggest important differences compared with the northern lobe. Instead of regular, cyclic variations we found three contrasting subzones that guide sample selection for geochemical and microtextural studies in progress. The new studies in collaboration with the BGR in Hannover will involve scanning of the Main Zone samples using laser-induced breakdown spectroscopy (LIBS) and micro-XRF. If properly calibrated, these methods offer quantitative mineral proportions, 2D mineral distribution maps and multi-element concentration data. Importantly, the scanning is rapid and non-destructive, so all samples of the profile can be studied instead of a small subset originally planned. The spatial resolution of 20 microns for the microXRF will provide many analyses of the major mineral phases (plagioclase, ortho- and clinopyroxene) in a single sample. Statistical analysis will reveal the degree of chemical heterogeneity of each phase, which may be related via phase relations and crystallization models in basalt to changes in magma composition and/or temperature. The 2D element maps from microXRF will reveal mineral textures, zoning and the distribution and identity of late interstitial phases. We will also use of LIBS spectra to derive trace element concentrations in selected minerals that can track the chemical evolution of parental magmas.