大屯火山群土壤之石英來源

Abstract

大屯火山群位於台北盆地西北方，岩性以安山岩為主，其岩石之組成礦物不包含石英顆粒，然而在深度近1公尺風化土壤剖面中之組成礦物，除了有與母岩性質相同的礦物外，也存在白色與透明的礦物，這些礦物經由能量頻譜分析儀以及X光粉末繞射儀分析為低溫石英；然而土壤是岩石風化的最終產物，那麼這些石英究竟來自何處？ 本研究藉由觀察粒徑大於125μm的石英，並針對其外型特徵與鍶同位素值，追溯大屯火山群土壤石英之來源。由掃描式電子顯微鏡影像顯示此地區石英分成兩大類：單晶質和多晶質。單晶質石英外形保有新鮮的貝殼狀斷口、尖銳的外形與晶型，表面無撞擊坑洞與刮痕；多晶質石英則保有較完整低溫石英之晶型。就外形與表面構造而言，大屯火山群之石英顆粒與風成石英之特徵不符。此外，大於125 μm之石英是無法藉由風力做遠距離搬運，而比對臺灣海峽碎屑沉積物之鍶同位素值均比大屯火山群基質之鍶同位素值高，因此推論大屯火山群土壤中之石英並非來自於臺灣海峽之沉積顆粒，也非風成堆積物。 鍶同位素分析結果顯示，大於125 μm之石英與土壤基質（粒徑小於10 μm）的鍶同位素值一致，因此顯示其為同源物質。然而XRD結果顯示土壤基質之組成礦物除了含有石英之外，亦包含普通輝石、直輝石、赤鐵礦、低溫方矽石，部分含有明礬石、鉀明礬石、高嶺石、鱗石英，其中控制土壤基質鍶同位素值之礦物，可能以普通輝石和高嶺石為主。 於大屯火山群土壤基質之鍶同位素主要分成四群，其鍶同位素值均大於0.709，與地層之鍶同位素值一致，唯獨七星山南麓地區之鍶同位素值最低，與大屯火山群安山岩和溫泉水之鍶同位素值相同，可能是受到大屯火山群岩石與溫泉水之影響。由於過去在馬槽地區曾經發生過熱水蒸氣爆發的事件，加上本研究的證據，推論大屯火山群土壤石英來源是經由熱水蒸氣爆發或者是蒸氣爆發，將大屯火山群底部沉積岩層之石英帶至地表並堆積。

The main rock type of Tatun Volcano Group (TVG) is andesite, which predominantly consists of plagioclase, hypersthene, augite, and hornblende with minor magnetite but without quartz. However, a lot of quartz grains have been found in the top of 1 m thick soils which the host rocks are the andesitic rocks. In order to find out where the quartz are from, we focus on studying the grain size being larger than 125μm quartz, and are analyzed by Energy Dispersive Spectrum (EDS), X-ray Powder Diffraction (XRD), Scanning Electron Microscope (SEM) and TIMS in this study. Meanwhile, we also analyze quartz from the sedimentary strata underneath the TVG and soils matrix with grain size being smaller than 10μm. The results show the soil matrix and quartz having the similar strontium isotopic ratio, that means they are from the same source. These white and transparent minerals are quartz (grain size > 125 μm ), which are identified by SEM-EDS and XRD. Mineral compositions for soil matrix is composed of α-quartz, augite, hematite, α-cristobalite, alunite, k-alunite, kaolinite and tridymite, which the strontium is predominantly distributed in augite and clay minerals. Therefore, the ratio of 87Sr/86Sr in soil matrix is predominantly controlled by augite and clay mineral contents. SEM images show that the shape of quartz grains (>125 μm) is sub-rounded to sub-angular with fresh conchoidal fracture and crystalline faces. Meanwhile, there are no eolian structures on grain surfaces, e.g. dish-shaped or V-shaped and scratches can be found. In general, wind cannot transport large particles, except some special conditions which the grains of dust larger than 75μm can be transported to 10,000 km (Pitty, 1968; Betzer et al., 1988; Middleton et al., 2001), but their mechanism was still unknown. So, the soil particles deposited on the peak of TVG may be not generated by wind. Furthermore, the 87Sr/86Sr value of soils from TVG can be divided into four groups. Most of them are higher than 0.709 and almost are the same as quartz of sedimentary strata, except the sample from the Chishingshan southern piedmont with the Sr isotopic ratio being similar to the andesite and hot spring of TVG. Based on those lines of evidence, the soil quartz deposited on the peak of TVG may be originated from the underneath by phreatic or steam eruption.