Investigation of in situ weathering of quartz diorite bedrock in the Rio Icacos basin, Luquillo Experimental Forest, Puerto Rico
Investigation of in situ weathering of quartz diorite bedrock in the Rio Icacos basin, Luquillo Experimental Forest, Puerto Rico
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DOI：
10.1016/j.chemgeo.2003.05.001
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发表时间：
2003-12
期刊：
影响因子：
3.9
通讯作者：
Benjamin F. Turner;R. Stallard;S. Brantley
中科院分区：
文献类型：
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作者：
Benjamin F. Turner;R. Stallard;S. Brantley
波多黎各的里奥伊卡科斯盆地是花岗闪长岩集水区化学溶质通量测量最高的地方；部分原因是年降雨量高(4300毫米)、平均气温高(23摄氏度)和中等起伏。这些通量的大部分是由腐泥岩下部分风化岩带(厚度0.5-1.5米)中的斜长石和角闪石的溶解贡献的。这些带的特征是洋葱皮“林德利特”系统(每个3-10厘米厚)，其中孔隙度的发育主要是斜长石到高岭石的风化作用。含铁铝硅酸盐矿物角闪石、辉石和黑云母在斜长石风化完成后仍留在风化岩石中，但角闪石和辉石消失在多孔的角闪石系统和腐泥岩之间。新的分水岭尺度的溶质质量平衡证实了早期的研究，并产生了与岩石(除石英和黑云母外)完全风化成高岭石和Fe3+氧化物非常相似的总体反应化学计量比。我们的发现表明，稳态假设在该地点的风化研究中是合理和有用的。据估计，3.24×10−9至3.6 0×10−9 kg m−2s−1的稳态泥沙产生量(基于净溶质通量和平均基岩成分)小于实测值，表明目前泥沙生成相对于稳态加速；然而，泥沙生成最近已更接近稳定状态。由于Rindlet系统内的风化发生在向上和向下移动的多个锋面上，因此发展了两个新的稳态概念模型来解释(1)多个平行风化锋面和(2)由于孔洞发育而导致的风化锋面不同的推进速率。在此基础上，计算了斜长石在2.7×10−15和5.0×10−15molNag−1s−1两个采样点的原位风化速率，与2.7×10−16到5.0×10−15molNam−2s−1的表面积归一化速率相当。
The Rio Icacos basin, in Puerto Rico, is the site of the highest measured chemical solute fluxes for a catchment on granodiorite; this is partly attributable to high annual rainfall (4300 mm), high average temperature (23 °C), and moderate relief. The bulk of these fluxes is contributed by dissolution of plagioclase and amphiboles in zones of partially weathered rock (0.5–1.5 m thick) underlying saprolite. These zones are characterized by systems of onion-skin “rindlets” (each 3–10 cm thick) in which porosity development is dominated by weathering of plagioclase to kaolinite. Fe-bearing aluminosilicate minerals hornblende, augite, and biotite persist in the weathered rock after plagioclase has weathered to completion, but hornblende and augite disappear between the porous rindlet systems and saprolite. A new watershed-scale solute mass balance corroborates earlier studies and yields an overall reaction stoichiometry that closely resembles complete weathering of the rock (except quartz and biotite) to kaolinite and Fe3+oxides. Our findings indicate that steady-state assumptions are reasonable and useful in studying weathering at this site. Estimated steady-state sediment yields (based on net solute fluxes and average bedrock composition) of 3.24×10−9to 3.60×10−9kg m−2s−1are smaller than measured values, indicating that sediment generation is currently accelerated with respect to steady-state; however, sediment generation has recently been closer to steady-state. Since weathering within the rindlet systems occurs on multiple fronts moving upward and downward, two new steady-state conceptual models are developed to account for (1) multiple parallel weathering fronts, and (2) differential rates of advance of weathering fronts due to porosity development. Based on these models, in situ weathering rates for plagioclase at two sample locations of 2.7×10−15and 5.0×10−15mol Na g−1s−1are calculated, values comparable to BET surface area normalized rates of 2.7×10−16to 5.0×10−15mol Na m−2s−1.