Song-e Yu

Effects of melt percolation on the Re-Os systematics of continental mantle lithosphere: A case study of spinel peridotite xenoliths from Heilongjiang, NE China

Os isotope ratios of mantle peridotites have been considered to be largely immune to recent melt-rock interaction. However, Os isotope ratios and PGE (Platinum group elements) concentrations of the Yong’an xenoliths have been significantly modified by melt percolation, and are not suitable for determining the formation age of lithosphere mantle in Yong’an. In this study, the Yong’an spinel peridotite xenoliths are divided into two groups: N-Type and E-Type. The N-Type group including cpx (clinopyroxene)-poor lherzolite and harzburgite, shows a large variation of Cr#(sp) (13.2–48) and sulfur contents (from 171 ppm to below detection limit), whereas the E-Type peridotites are mainly refractory harzburgites and are characterized by high Cr#(sp) (35.3–42.2) and overall low sulfur contents (below 51 ppm). Both types show similar major and REE (rare earth element) patterns. Furthermore, the N-Type peridotites display a restricted range of iridium-group PGE (IPGE), Os/Ir and Ru/Ir ratios (Os/Ir = 0.64−1.12, Ru/Ir = 1.52−1.79) and variable palladium-group PGE (PPGE) contents (3.4–14.9 ppb), whereas the E-Type peridotites show a large variation of Os/Ir and Ru/Ir ratios (Os/Ir = 0.33−0.84, Ru/Ir = 0.94−1.6), and a restricted range of PPGE (4.3–6.9 ppb). 187Os/188Os ratios of E-Type peridotites are higher than those of N-Type peridotites at comparable fertility levels. These results suggest that N-Type peridotites may have been overprinted by metasomatism via small melt fractions, in which the percolation of the volatile-rich, small melt fractions only resulted in LILE (large ion lithophile element) enrichment of clinopyroxene, and their whole rock PGE contents and Re-Os isotope values were little changed. Moreover, E-Type peridotites may have been modified by melt-rock reaction involving relatively large melt fractions, which may result in the formation of secondary cpx and olivine and the removal of IPGE-bearing minerals such as Ru-Os-(Ir) alloys or laurite, followed by precipitation of secondary sulfides from melt with radiogenic isotopic signature.

Controls on trace-element partitioning among co-crystallizing minerals: Evidence from the Panzhihua layered intrusion, SW China

Abstract The factors and processes that control trace-element partitioning among co-crystallizing cumulus minerals in layered intrusions have long been controversial. Here we address this issue using new laser ablation ICP-MS trace element data for magnetite, ilmenite, and clinopyroxene from the Panzhihua layered intrusion in the Emeishan large igneous province, SW China. The cumulus minerals display strong Ni, Co, and Cr depletions, indicative of parental magmas low in concentration of these elements probably due to prior sulfide removal and the fractionation of chromite or Cr-magnetite in a staging magma chamber at depth. Both magnetite and clinopyroxene show cyclical variations in some transition elements (e.g., Cr, V, and Ni) along the stratigraphic section. The average concentrations of these transition elements in magnetite are positively correlated with those in clinopyroxene, likely resulting from co-crystallization of magnetite and clinopyroxene. The incompatible element (e.g., Zr, Hf, and Nb) concentrations of the cumulus minerals from the Lower Zone are highly variable compared to those of the Middle and Upper Zones. These large variations in trace element compositions are attributed to a “trapped liquid shift” in the Lower Zone. Ilmenite crystals from the Panzhihua intrusion may have undergone extensive modification of transition elements during subsolidus re-equilibration with magnetite, leading to the decoupled variations of transition elements in ilmenite across the Lower Zone stratigraphy. Our study indicates that systematic trace element variations of the main cumulus mineral assemblage, rather than a single mineral, need to be considered to better constrain the magmatic differentiation and elemental fractionation of layered intrusions.

Platinum-group element geochemistry of the layered intrusions in the Emeishan large igneous province, SW China: Implications for the principal controls on magmatic sulfide immiscibility

It is widely accepted that the incorporation of external sulfur via crustal contamination is an important trigger for sulfide immiscibility that generates Ni-Cu-(PGE) sulfide mineralization, yet other controlling factors for sulfide immiscibility may also be present. The late Permian Panzhihua, Baima, Hongge, Xinjie and Taihe layered intrusions in the Emeishan Large Igneous Province (ELIP, SW China), are well-endowed with Fe-Ti oxide deposits, whereas their sulfide mineralization is mainly sub-economic. For example, the lower part of the Xinjie intrusion hosts a few thin PGE-rich ore layers, yet other ELIP layered intrusions do not contain any Ni-Cu sulfide mineralization and are PGE-depleted (0.01–1 ppb). Compared with the PGE-undepleted Emeishan high-Ti basalts that are genetically related to the intrusions, the extent of PGE depletion and elevated Cu/Pd ratios (up to 3.2×106) of the Panzhihua, Baima, Taihe and Hongge intrusions suggest PGE-depletion in their parental magmas due to early-stage sulfide removal. Sr-Nd isotopic compositions of the Panzhihua, Baima and Taihe intrusions suggest crustal contamination was insignificant and sulfide saturation produced mainly by crustal sulfur input was unlikely. MELTS modeling shows that extensive fractionation of chromite, olivine and clinopyroxene in deep-seated magma chambers may have induced early-stage sulfide saturation of the primary magmas. The relatively high sulfide contents in the Fe-Ti oxide layers at Panzhihua, Baima, Hongge and Taihe indicate a close relationship between the second-stage sulfide immiscibility and extensive Fe-Ti oxide crystallization. Positive correlations between sulfur and total Fe2O3, V and TiO2 suggest that Fe-Ti oxide (magnetite and ilmenite) crystallization may have triggered the second-stage sulfide saturation via sharply lowering the Fe concentration and oxygen fugacity of the magmas. Moderate degree of crustal contamination for the Xinjie Fe-Ti oxide-barren rocks may have induced sulfide saturation and accumulation at the lower part of the intrusion. Our calculations indicate that the Xinjie PGE-rich rocks have high R-factors (1000–10000), which are ascribed to PGE-upgrading of the sulfides via reaction with new replenishments of PGE-undepleted magmas. A few Panzhihua, Baima and Taihe samples that contain higher PGE concentrations suggest that the early-stage sulfide droplets at depths were entrained in later magma pulses delivered to shallower magma chambers. The very high R-factors determined by mass balance calculation, implies a good potential for discovering more PGE mineralization in the deep-seated intrusions of the magma plumbing system.