摘要
五大连池火山岩浆是一种浅部结晶(压力<91MPa)的碧玄质岩浆,含水2—4 wt％。在火山演化史中,岩浆基性度不断增高,碱度(K_2O)降低。从中更新世至全新世早期喷发,岩浆中氯、磷和氟的浓度(ppm)分别由1430、1500和2470降低到700、1100和200,演化到近代(1719—1721年)喷发,氯、磷和氟的浓度复又上升,分别增加到2800、7900和800。在火山岩浆结晶演化过程中,氟和磷趋向富集于残余岩浆中。而因氯易被水蒸汽携带逸失,使残余岩浆明显贫氯(100—300)。五大连池火山岩浆中硫、^(87)Sr/^(86)Sr和δ^(18)O值均低,表明它们源自上地幔,并未受陆壳物质的混染。
Fluorine, chlorine, sulfur and phosphorus were determined, using electron micr-oprobe, in magmatic inclusions trapped within minerals and glass mesostasis from Wu-dalianchi volcanic rocks. The initial volcanic magma from Wudalianchi corresponds to the basanitic magma crystallized near the surface (pressure < 91 Mpa). The potential H2O content of this magma is in the range 2-4 wt.%. The initial compositions of volcanic magmas vary regularly from early to late volcanic events. From the Middle Pleistocene to the recent eruptions (1719-1721 yr.), the basicity of volcanic magma tends to increase, as reflected by an increase in MgO and CaO contents and by a progressive decrease in SiO2 and K2O contents. Meanwhile, from early (Q2) to late (Q3) episodic eruptions of the Middle Pleistocene, the initial concentrations of chlorine in volcanic magmas range from 1430-1930 ppm to 1700 ppm and decrease to 700-970 ppm for the first episodic eruption during the Holocene (Q41). The chlorine concentrations of volcanic magmas of recent eruption (Q42) increased again to 2600-2870 ppm. A parallel evolution trend for phosphorus and chlorine concentrations in magmas has been certified: 1500 - 5970 ppm (Q2) →3500-4210 ppm (Q3)→ 1100-3500ppm.(Q41)→ 6800-7900ppm (Q42). The fluorine contents of volcanic magmas, from early to late volcanic events, show the same trend:770-2470 ppm → 200-700 ppm → 700-800ppm.During the crystallization-evolution of volcanic magmas, fluorine and phosphorus tend to be enriched in residual magmas as a result of crystal-melt differentiation, for example, the fluorine contents reach 5000-6800 ppm and the phosphorus contents, 2.93wt.% in esidual magmas. An appreciable amount of chlorine may be lost from waterrich volcanic magmas prior to eruption as a result of degassing. Apparently, water serves as a gas carrier for the chlorine. The chlorine contents of residual magmas may decrease to 100-300ppm.The volcanic magmas from Wudalianchi are poor in sulfur, normally ranging from 200 to 400ppm. On account of the behavior of sulfur in magmas and the strontium and oxygen isotopic analyses ((87Sr/86Sr)i = 0. 70503 -0. 70589 ;δ18O = + 5.50 -+6.89‰), it can be considered that the basanitic magmas in the Wudalianchi volcanic area came from the upper mantle and have not yet been contaminated probably by continental crust materials.
出处
《地球化学》
CAS
CSCD
北大核心
1990年第3期197-206,共10页
Geochimica
基金
国家自然科学基金
