亏损上地幔中的富钾熔体和碳酸盐交代作用:来自CCSD预先导孔橄榄岩的地球化学证据

K-Rich and Carbonatic Melt Metasomatism in Depleted Upper Mantle: Geochemical Evidences from Peridotites in Pre-Pilot Hole of Chinese Continental Scientific Drilling Project

中国大陆科学钻探工程预先导孔(CCSD-PP1)打在苏鲁超高压变质带芝麻坊超镁铁岩体上,钻孔穿透超镁铁岩体115m.超镁铁岩体由二辉橄榄岩、方辉橄榄岩和少量单辉橄榄岩和纯橄岩组成,与上下围岩接触的橄榄岩被强烈蛇纹石化.多数橄榄岩含有石榴石或其假象,普遍含有金云母和菱镁矿,少量样品中有钛斜硅镁石.在化学成分上,橄榄岩的Mg#指数变化于90.3~92.6之间,MgO含量(36.61%~49.15%,平均45.17%)与Na2O(0.01%~0.25%)、Al2O3(0.07%~3.71%,多数<2.0%,平均1.46%)和CaO(0.12%~2.53%,一个高达3.30%,平均1.00%)呈负相关关系.与主量易熔元素相对亏损的特点相反,橄榄岩中显示了稀土元素富集、分馏和配分曲线显示近于平行和相似的特点,(La/Lu)N比值为3.18~33.05;此外,多数样品具有高Ba(最高比原始地幔高100多倍)含量,在蛛网图上显示Rb、Nb、Ta、Zr、Hf和Sr的负异常,Ti/Eu比值均低于1300.岩相学特征和难熔主量元素与不相容元素之间的无相关性表明橄榄岩至少受到了形成金云母和菱镁矿的2次交代作用.富含金云母的橄榄岩(例如C25-143-61,C32-149-71)具有富钾趋势,并且显示K2O与Rb、Ba和Th等大离子亲石元素的正相关关系,未见K2O和稀土元素、Sr和Ca之间的相关关系.这些特征表明橄榄岩被含水、硅铝质碱性熔体交代,之后又被高Ba低Rb和高场强元素的镁质(菱镁矿)碳酸岩熔体交代,并且强烈地改变了Ba的丰度和显示了特定地幔碳酸盐的稀土元素配分型式.全岩具有不均一的高放射性Sr(87Sr/86Sr=0.7084~0.7201)和低放射性Nd(εNd(t)=-1.14^-8.55)组成,结合已有的氧同位素研究资料,表明预先导孔PP1中的橄榄岩所代表的地幔可能在早期就遭受了来自深部的介质的交代作用.

The Chinese pre-pilot hole （PP1） is located at Zhimafang Village, Donghai County in the Sulu UHP terrane, east China. Ultrahigh-pressure peridotites of 115 m thick within gneiss recovered from the PP1 are composed of abundant lherzolite, harzburgite, and minor wehrlite and dunite. Peridotite near to the contacts with gneiss is strongly serpentinized. More than 90 vol% peridotites contain garnet and phlogopite; some contain magnesite and Ti-clinohumite. All peridotites contain lower ＂fertile elements＂ compared with primitive mantle, their Mg^# numbers range from 90. 3 to 92. 6, and MgO content （36. 61%-49. 15%, averagely 45. 17%） has negative correlation with Na2O（0. 01%-0. 25%）, Al2O3 （0. 07%-3. 71%, most 〈2. 0%, averagely 1.46%） and CaO（0. 12%-2. 53%, one up to 3. 30%, averagely 1.00%） contents. In contrast to major element depletion feature, the PP1 peridotites show light rare earth element-enriched and slight to moderately fractionated REE pattern of nearly parallel curves and roughly identical pattern with （La/Lu）N ratios of 3. 18-33.05. Most of the peridotites contain high Ba （higher than 92 times of primitive mantle） and LREE and low HFSE, and are characterized by negative Rb, Nb, Ta, Zr, Hf and Sr anomalies （e. g. , C39-157-81） in spidergrams. Ti/Eu ratios are lower than ca. 1 300. The lack of correlation between refractory degree and enrichment of incompatible elements documents effect by metasomatism after mantle melting. Petrographic characteristics show multiple metasomatism of phlogopite and magnesite. Phl-rich peridotites （such as samples C25-143-61, C32-149-71） have the K2O enrichment trend and good correlations between K2Oand some LILE such as Rb, Ba and Th. No positive correlations between K2O and REE and between Sr and Ca are seen. These signatures show that peridotites were metasomatised by hydrous, silicic, aluminous and alkaline melts containing some LILE, and then severely overprinted by metasomatism of magnesite melt containing high Ba and low Rb and HFSE which modified Ba content drastically and endued REE patterns of mantle carbonatite melt. Whole rock has heterogeneously high radiogenic Sr（87 Sr/86 Sr=0. 708 4-0. 720 1） and low radiogenic Nd（εNd （t）=- 1.14 to -8. 55）, which indicates the peridotites from PP1 hole was probably derived from long-term enriched mantle by agents from depth, especially combined with oxygen isotope compositions of anhydrous and hydrous minerals reported by previous studies.