Fe isotopes and the contrasting petrogenesis of A-, I- and S-type granite
Fe isotopes and the contrasting petrogenesis of A-, I- and S-type granite
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DOI：
10.1016/j.lithos.2014.10.015
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发表时间：
2015
期刊：
影响因子：
3.5
通讯作者：
J. Foden;P. Sossi;Christine M. Wawryk
中科院分区：
文献类型：
--
作者：
J. Foden;P. Sossi;Christine M. Wawryk
我们提出了新的铁同位素数据的42 S，I和A型（亚铁）花岗岩的寒武纪Delamerian造山带在南澳，古生代Lachlan褶皱带和美国西部。对这些数据的解释和模拟表明，岩浆作用确实导致了相当复杂的铁同位素分异趋势，并可能导致δ 57 Fe> 0.35‰的同位素重铁花岗岩。相比之下，大洋中脊玄武岩（MORB）的δ 57 Fe = 0.15‰（Teng等人，2013年）。这些变化类似于先前报道的那些（Poitrasson和Freydier，2005; Heimann等人，2008; Telus等人，2012），但是，与一些解释相反（Beard和约翰逊，2006; Heimann等人，2008年），重价值不一定是后期热液流体损失的产物，尽管这一过程在某些情况下无疑也是一个重要因素。A型花岗岩的δ ~（57）Fe值很高（0.4-0.5‰），而I型花岗岩的δ ~（57）Fe值较低（0.2‰）。S型花岗岩的δ ~（57）Fe值介于两者之间，但也倾向于重同位素（δ ~（57）Fe = 0.2-0.4‰）。结果表明，铁同位素值及其变化趋势是反映花岗岩形成过程的标志.使用Rhyolite-MELTS软件的建模表明，对比的轨迹和终点在Fe同位素演化到花岗岩取决于：不断发展的岩浆的氧化状态，以及系统是否是氧气缓冲。铁同位素演化支持亚铁A型花岗岩的起源，从长期的，封闭的岩浆房分馏适度减少镁铁质岩浆。在这些系统中，磁铁矿饱和延迟，三价铁预算有限。I型系统起源于地幔楔向上板块地壳提供相对氧化、含水、与俯冲有关的岩浆。然后，这些经历氧气缓冲开放系统AFC过程中的下地壳热区。S型岩浆是地壳熔体，在硫化物或石墨沉积原岩最初施加于源的还原条件下结晶。由此产生的熔体的组合物反映了部分熔融的优势，其中的条件，因此缓冲（开放系统），随后的后期，封闭系统分馏这些提取，减少岩浆。
We present new Fe isotope data of 42 S-, I- and A-type (ferroan) granites from the Cambrian Delamerian orogen in South Australia, the Palaeozoic Lachlan Fold Belt and Western USA. Interpretation of these data, together with modelling suggests that magmatic processes do result in quite complex Fe-isotopic differentiation trends and can lead to granites with isotopically heavy iron with δ57Fe > 0.35‰. By comparison Mid-Ocean Ridge Basalts (MORBs) have δ57Fe = 0.15‰ (Teng et al., 2013). These variations are similar to those previously reported (Poitrasson and Freydier, 2005; Heimann et al., 2008; Telus et al., 2012), but, contrary to some interpretations (Beard and Johnson, 2006; Heimann et al., 2008), heavy values are not necessarily the product of late-stage hydrothermal fluid loss, though this process is undoubtedly also an important factor in some circumstances. A-type (ferroan) granites reach very heavy δ57Fe values (0.4–0.5‰) whereas I-types are systematically lighter (δ57Fe = 0.2‰). S-type granites show a range of intermediate values, but also tend to be isotopically heavy (δ57Fe ≈ 0.2–0.4‰). Our results show that the iron isotopic values and trends are signatures that reflect granite generationprocesses. A modelling using the Rhyolite-MELTS software suggests that contrasting trajectories and end-points in Fe isotope evolution towards granite depend on: oxidation state of the evolving magma and, whether or not the system is oxygen-buffered. Iron isotopic evolution supports an origin of ferroan A-type granite from protracted, closed magma chamber fractionation of moderately reduced mafic magmas. In these systems magnetite saturation is delayed and the ferric iron budget is finite. I-type systems originate with the supply of relatively oxidised, hydrous, subduction-related magmas from the mantle wedge to the upper plate crust. These then experience oxygen-buffered open-system AFC processes in lower crustal hot-zones. S-type magmas are crustal melts that crystallise under reduced conditions initially imposed at source by sulphidic or graphitic sedimentary protoliths. The composition of the resulting melts reflects the domination of partial melting where conditions are hence buffered (open system) followed by subsequent late-stage, closed system fractionation of these extracted, reduced magmas.