Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada
Sulfur isotope behavior during metamorphism and anatexis of Archean sedimentary rocks: A case study from the Ghost Lake batholith, Ontario, Canada
复制标题
DOI：
10.1016/j.epsl.2020.116494
复制
发表时间：
2020-11
影响因子：
5.3
通讯作者：
C. Bucholz;J. Biasi;P. Beaudry;S. Ono
中科院分区：
文献类型：
--
作者：
C. Bucholz;J. Biasi;P. Beaudry;S. Ono
来自地表的硫再循环到地球深部可以赋予岩浆独特的硫同位素特征。然而，硫从沉积岩通过变质作用和挥发分和/或部分熔融作用转移到岩浆（最终是火成岩）的细节很难追踪。为了详细了解这一过程，我们研究了太古代（c。2685 Ma）鬼湖岩基（GLB）和其周围的主机变沉积岩的上级Quarterion（安大略省，加拿大）的高空间分辨率二次离子质谱，辅以高精度的气源同位素比质谱测量。GLB包括强过铝质黑云母+堇青石、黑云母+白云母、白云母+石榴石+电气石花岗岩到浅色花岗岩，它们被认为代表了周围变灰岩和变岩的部分熔融。变质沉积岩显示出一系列的变质等级，从距GLB约105公里的黑云母-钾长石（280-380° C）到紧邻岩基的硅线石-钾长石等级（10660 ° C），从而提供了一个自然实验来了解从低级到高级太古代沉积岩的硫同位素变化，以及代表其部分熔体的花岗岩。我们发现，变质沉积硫化物δ 34 S值随着变质作用的进行而增加，最多增加2-3‰（从-1 ‰增加到+1 ~+2 ‰）。磁黄铁矿δ 34 S值在变质作用中的增加可以用黄铁矿硫化反应中的瑞利分馏来解释。除一个样品外，所有花岗岩样品中的黄铁矿的δ 34 S值与高级变质沉积岩的δ 34 S值相似，表明部分熔融并未导致δ 34 S的显著分馏。唯一的例外是一个花岗岩样品，来自一个以丰富的变质沉积岩包体为特征的岩基的一部分。该样品含有不均匀的低δ 34 S值（低至− 16‰）的黄铁矿，这可能是由于花岗岩熔体和变质沉积岩包裹体之间硫的不完全均一化造成的。在变质沉积岩和花岗岩中均观察到小的（万分之几），大多为正的Δ 33 S。我们的研究结果表明，太古代强过铝花岗岩可以是一个高保真的档案量化的散装硫同位素组成的太古代硅质岩沉积物。此外，我们的研究结果表明，在太古代的还原含硫沉积物的俯冲δ 34 S等于或接近0‰，应导致释放的含硫流体在地幔楔具有类似的值（在几个permil）。S-MIF（如果最初存在于太古代表面物质中）可能在此过程中被保存下来。然而，火成岩中S-MIF的缺乏并不排除太古代沉积物的同化，因为S-MIF可能最初不存在于太古代沉积硫中和/或均质化或稀释可能会掩盖最初存在于同化的太古代沉积物中的任何S-MIF。
Recycling of surface-derived sulfur into the deep earth can impart distinct sulfur isotope signatures to magmas. The details of sulfur transfer from sedimentary rocks to magmas (and ultimately igneous rocks) through metamorphism and devolatilization and/or partial melting, however, is difficult to trace. To understand this process in detail we studied multiple-sulfur isotope compositions of sulfides in the Archean (c. 2685 Ma) Ghost Lake batholith (GLB) and its surrounding host metasedimentary rocks of the Superior Craton (Ontario, Canada) by high spatial resolution secondary ion mass spectrometry, complemented by high-precision gas source isotope ratio mass spectrometry measurements. The GLB comprises strongly peraluminous biotite+ cordierite, biotite+ muscovite, and muscovite+ garnet+ tourmaline granites to leucogranites, which are thought to represent the partial melts of surrounding metagreywackes and metapelites. The metasedimentary rocks display a range of metamorphic grades increasing from biotite-chlorite (280-380° C) at∼ 5 km away from the GLB to sillimanite-K-feldspar grade (∼ 660° C) immediately adjacent to the batholith, thus providing a natural experiment to understand sulfur isotope variations from low-to high-grade Archean sedimentary rocks, as well as granites representative of their partial melts. We find that metasedimentary sulfide δ 34 S values increase with progressive metamorphism at most 2-3‰(from− 1‰ up to+ 1 to+ 2‰). An increase in δ 34 S values in pyrrhotite during prograde metamorphism can be explained through Rayleigh fractionation during pyrite desulfidation reactions. Pyrite from all but one of the granite samples preserve δ 34 S values similar to that of the high-grade metasedimentary rocks, indicating that partial melting did not result in significant fractionation of δ 34 S. The exception to this is one granite sample from a part of the batholith characterized by abundant metasedimentary inclusions. This sample contains pyrite with heterogeneous and low δ 34 S values (down to− 16‰) which likely resulted from incomplete homogenization of sulfur between the granitic melt and metasedimentary inclusions. Small (several tenths of a permil), mostly positive Δ 33 S are observed in both the metasedimentary rocks and granites. Our results suggest that Archean strongly peraluminous granites could be a high-fidelity archive to quantify the bulk sulfur isotope composition of the Archean siliciclastic sediments. Further, our findings indicate that subduction of reduced sulfur-bearing sediments in the Archean with δ 34 S at or near 0‰ should result in release of sulfur-bearing fluids in the mantle wedge with similar values (within a few permil). S-MIF (if initially present in Archean surface material) may be preserved during this process. However, the absence of S-MIF in igneous rocks does not preclude assimilation of Archean sedimentary material as either S-MIF may not be originally present in the Archean sedimentary sulfur and/or homogenization or dilution could obscure any S-MIF originally present in assimilated Archean sediments.