2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Trace element partitioning in minerals from the agpaitic syenite and phonolite of the Khibina massif

L. Arzamastseva,  A. Arzamastsev

Geological Institute of the Kola Science Center RAS, Apatity 

arzamas@geoksc.apatity.ru

The aim of this study was to measure the LILE, HFSE, and REE contents in minerals from major rocks of the Khibiny massif and determine the chemical features of minerals formed at early and late magmatic stages. Using local LA-ICP-MS analysis, we determined contents of 38 elements (Cs, Li, Rb, Ba, Th, U, Ta, Nb, Sr, Hf, Zr, Pb, Be, Sc, V, Cr, Ni, Co, Cu, Zn, Ga, Mo, Sn, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) in feldspars, nepheline, clinopyroxene, amphibole, eudialyte, lamprophyllite, and titanite from agpaitic nepheline syenites, potassium nepheline syenites, urtites and related pegmatoids of the Khibina massif. Minerals were analyzed in polished sections on an Excimer Sopra SEL 510. Ultraviolet laser provides the minimum thermal effect. During ablation, the pit ranging from 1 mm to 10 µm across and from 30 to 40 µm deep is formed depending on beam focusing. Mass spectrometric analysis combined with inductively coupled plasma was performed on a Perkin Elmer Sciex ELAN 6000.

Clinopyroxene. Three pyroxene generations can be distinguished. Early generation is represented by diopside Wo₄₆En₃₄Fs₂₀ with <0.2 Na p.f.u., which composes pyroxene cores in potassium nepheline syenites, urtites, and ijolites. Rims (II generation) have higher alkalinity and correspond to aegirine-augite with aegirine content from 20 to 80%. Nearly pure aegirines of III generation occur in the pegmatoid rocks. Agpaitic nepheline syenites (khibinites) from peripheral zone contain zoned clinopyroxenes varying from aegirine to aegirine-augite within Di_{7 - 26}Aeg_{41 - 77}Hed_{5 - 30}. Our measurements showed that early clinopyroxene from ijolites, urtites, and potassium nepheline syenites contains the highest Sr content. By contrast, late pyroxenes from pegmatoid rocks have significantly lower Sr content. The similar Sr distribution is typical of pyroxenes from khibinites. Such regularity is also observed in V, Zr, and Hf distribution, with gradual decrease of V content in pyroxenes in a series of potassium nepheline syenite - massive urtite - pegmatoid urtite. Early pyroxenes in ijolites, potassium nepheline syenites, and urtites are characterized by relative LREE enrichment ((La/Yb)_N = 3.9 - 7.3). However, more alkaline pyroxenes from agpaitic nepheline syenites, like late pyroxene rims around early magmatic aegirine-augite in potassium nepheline syenites, are depleted in LREE and MREE ((La/Yb)_N = 0.8 - 1.4). The REE distribution in pyroxenes from massive urtites and associated pegmatoids shows the same tendency: late generation are strongly depleted in all REE, with the exception of Yb and Lu.

Amphibole. Amphibole occurs in two generations. According to petrographic observations, early generation, Na - Ca amphibole (richterite), postdates aegirine-diopside and partially or completely replaces it. Late amphibole, arfvedsonite, occurs in the matrix of nepheline syenites and foidolites, as well as in pegmatoids. It develops after both clinopyroxene and early Na - Ca amphibole. Trace element contents in amphiboles from ijolites and khibinites are represented in Table 5. It is seen that amphiboles I and clinopyroxenes from ijolites are geochemically similar: diopside and richterite have similar shapes of REE patterns, with the lower total REE content in the latter. On the other hand, amphibole II (arfvedsonite) is geochemically similar to late aegirine, which is characterized by strong decrease in all REE, except for Yb and Lu.

Nepheline. Nepheline in potassium nepheline syenite are characterized by the extremely low contents of all trace elements (no more than 10 ppm). The typomorphic Li, Rb, Cs, and Ga occur in very low amounts. The REE contents in nepheline are generally higher than those in the coexisting feldspars. The (La/Yb)_N ratio varies within 45.6 - 104.9. Nepheline from ijolites is characterized by strongly pronounced negative Eu anomaly (Eu/Eu* = 0.13), whereas nepheline from potassium nepheline syenites shows low Eu anomaly (Eu/Eu* = 1.1). Similar Eu anomaly was found in nepheline from ijolites of the Kovdor massif.

Feldspar occur as K - Na perthite Or₅₀ - 45Ab_{50 - 55} in agpaitic nepheline syenites of peripheral zone of the massif (khibinites), orthoclase with minor albite component (Or_{99 - 85}Ab_{1 - 15}) in massive urtites, and microcline in pegmatoid urtites. Homogenous domains in feldspar from khibinite have higher contents of Sr, Rb, and Ba than feldspar from potassium nepheline syenites. Late microcline from pegmatoid urtites is characterized by the highest Ba contents and Cr presence, which was presumably incorporated in crystal lattice together with Fe³⁺. Feldspar from agpaitic nepheline syenites (khibinites) has the highest REE contents and weakly expressed positive Eu anomaly (Eu/Eu* = 1.30). Orthoclase from massive urtites and potassium nepheline syenites has similar shape of REE patterns ((La/Yb)_N = 7.3 - 11.7), with lower REE contents in the latters. The late Ba-bearing microcline from pegmatoid urtites has lower total REE contents than primary orthoclase and shows sharp positive Eu anomaly (Eu/Eu* = 6.3 - 51.1).

Eudialyte. We studied eudialyte from ijolites of the central arc of Khibiny, agpaitic nepheline syenites of peripheral zone of the massif (khibinites), as well as from their pegmatoid varieties. The data obtained indicate wide variations of Hf, Zr, U, Th, Sr, and Ba in eudialyte from these rocks. It should be noted that contents of the specified elements in magmatic eudialyte from ijolites and agpaitic syenites are significantly higher than those in late eudialyte from pegmatoids. The Zr/Hf ratio is 15.0 - 15.6 in eudialyte from ijolites, 13.3 - 20.1 in that from khibinites, and 21.9 - 30.7, in pegmatoids, which is related to the Hf content decrease. The total REE and Y contents in eudialyte from pegmatoids are higher than those in khibinite. The REE patterns in the early and late generations have similar shapes and negative Eu anomalies (Eu/Eu* = 0.5 - 0.8). Eudialytes from ijolites of the foidolite complex show only weakly pronounced Eu anomaly (Eu/Eu* = 0.9 - 1.0).

Lamprophyllite. Lamprophyllite in agpaitic nepheline syenites from peripheral zone, potassium nepheline syenites, and pegmatoid urtites significantly differs in contents of most trace elements and, primarily, Sr and Ba. Petrographic and microprobe data indicate that the earliest lamrophyllite in agpaitic syenites is represented by Sr-rich varieties with Sr/Ba = 3, whereas lamprophyllite in potassium nepheline syenite and pegmatoid urtite is Ba-lamprophyllite with Sr/Ba = 0.5 and 0.2, respectively. The Sr enrichment in the cores of zoned lamprophyllite was also found for late magmatic lamprophyllite from agpaitic syenites of the Niva Massif. Similar crystal structure was also found for lamprophyllite from eudialyte complex of the Lovozero Massif (Zaitsev et al., 2002), which consists of Sr-rich cores and Ba-rich rims. The trace element data show that the late Ba lamprophyllite from pegmatoid urtites accumulates Nb, Ta, Zr, Hf, Y, Zn, V, and Ni. By contrast, it shows decrease in total REE content. The Ba lamprophyllite from potassium nepheline syenites and pegmatoid urtites has MREE-depleted trough-shaped REE patterns and positive Eu anomaly, which is moderate in potassium nepheline syenites (Eu/Eu* = 3.7 - 13.0) and strongly pronounced (Eu/Eu* = 69.6-74.3) in Ba-lamprophyllite from pegmatoids. It is noteworthy that Sr-lamprophyllite in agpaitic syenites from peripheral zone of the massif shows weaker MREE depletion and does not contain Eu anomaly (Eu/Eu* = 0.9 - 1.1).

Titanite, common subordinate mineral of ijolites, urtites, and apatite - nepheline rocks, shows no significant differences in trace element composition. It should be noted that titanite in ijolites from differentiated foidolite complex is relatively enriched in Sr, Nb, Ta, and Hf. The REE patterns have identical shape with distinct LREE enrichment. Titanite from ijolites has the highest (La/Yb)_N ratio within 42.1 - 50.4, while titanite from massive urtites and apatite - nepheline rocks has (La/Yb)_N ratio of 19.5 and 26.0, respectively.

Apatite. REE is incorporated in apatite lattice following the schemes: Ca²⁺ + P⁵⁺ = REE³⁺ + Si⁴⁺ and 2Ca²⁺ = REE³⁺ + Na⁺. This is evidenced by the presence of SiO₂ and Na₂O in the Khibina apatite, which reach up to 0.7 wt % and 0.4 wt %, respectively, in the apatites highest in REE. Analyzed apatite in amphibole ijolites from differentiated complex is represented by Sr and REE-bearing variety (SrO = 3.26 wt %, total REE₂O₃ = 3.29 wt %), which is typical of accessory apatite from these rocks. Like the sample from the apatite-rich apatite - nepheline rock, apatite is characterized by high La/Yb ratio ((La/Yb)_N = 93.1 - 133.4) and absence of Eu anomaly (Eu/Eu* = 0.91 - 0.99).

Financial support from the Russian Foundation for Basic Research (Grant 09-05-00224).