Li-Hui Chen

Magmatic recharge in continental flood basalts: Insights from the Chifeng igneous province in Inner Mongolia

Eruptive sequences can be used as windows into the thermal and chemical evolution of magma chambers. We examined a continuous vertical section of the Baichahe basalt flow associated with the late Cenozoic Chifeng flood basalt in Inner Mongolia, North China. From oldest to youngest, MgO increases, K2O, light rare earths and other incompatible elements decrease, and Nb/La and radiogenic Pb isotopic ratios increase, all of which indicate increasing primitiveness and decreasing contribution of crustal contamination with time. The variable Pb isotope and incompatible element ratios require a component of crustal contamination, most likely of the lower crust (unradiogenic Pb, and low Ce/Pb), in the earliest lavas. Fractional crystallization can explain some of the elemental systematics, but alone cannot explain variable incompatible element ratios and Pb isotopes, nor the temporal trend to more primitive compositions. Crustal assimilation with or without fractional crystallization also cannot explain all the elemental systematics. We find instead that recharge by a primitive magma, in combination with fractional crystallization and decreasing rates of crustal assimilation, is needed to explain the observed geochemical systematics. Our observations suggest that the delivery of fresh basalt to the magma chamber must increase at rates faster than the crust can be assimilated or that the rates of crustal assimilation must decrease. However, progressive addition of primitive magma should heat up the crust and lead to more crustal assimilation. We suggest that during the initial stages of forming a magma chamber, the magma cools and develops an outer crystalline rind of mafic to ultramafic cumulates. This results in a thickening nonconvecting chemical boundary layer, which serves to insulate the magma chamber from further assimilation of crust and cooling, the latter resulting in the reduction of crystallization rates and the buffering of magma compositions at more primitive compositions. We show that certain segments of other large igneous provinces also display an evolution toward more primitive magmas with time, indicating that magmatic recharge may be a common feature of basaltic magma chambers.

Lithospheric thickness controlled compositional variations in potassic basalts of Northeast China by melt‐rock interactions

Melt-rock interaction is a common mantle process; however, it remains unclear how this process affects the composition of potassic basalt. Here we present a case study to highlight the link between compositional variations in the potassic basalts and melt-rock interaction in cold lithosphere. Cenozoic potassic basalts in Northeast China are strongly enriched in incompatible elements and show EM1-type Sr–Nd–Pb isotopes, suggesting an enriched mantle source. These rocks show good correlations between 87Sr/86Sr and K2O/Na2O and Rb/Nb. Notably, these ratios decrease with increasing lithospheric thickness, which may reflect melt-lithosphere interaction. Phlogopite precipitated when potassic melts passed through the lithospheric mantle, and K and Rb contents of the residual melts decreased over time. The thicker the lithosphere, the greater the loss of K and Rb from the magma. Therefore, the compositions of potassic basalts were controlled by both their enriched sources and reactions with lithospheric mantle.

Growing magma chambers control the distribution of small-scale flood basalts.

Small-scale continental flood basalts are a global phenomenon characterized by regular spatio-temporal distributions. However, no genetic mechanism has been proposed to explain the visible but overlooked distribution patterns of these continental basaltic volcanism. Here we present a case study from eastern China, combining major and trace element analyses with Ar–Ar and K–Ar dating to show that the spatio-temporal distribution of small-scale flood basalts is controlled by the growth of long-lived magma chambers. Evolved basalts (SiO2 > 47.5 wt.%) from Xinchang–Shengzhou, a small-scale Cenozoic flood basalt field in Zhejiang province, eastern China, show a northward younging trend over the period 9.4–3.0 Ma. With northward migration, the magmas evolved only slightly ((Na2O + K2O)/MgO = 0.40–0.66; TiO2/MgO = 0.23–0.35) during about 6 Myr (9.4–3.3 Ma). When the flood basalts reached the northern end of the province, the magmas evolved rapidly (3.3–3.0 Ma) through a broad range of compositions ((Na2O + K2O)/MgO = 0.60–1.28; TiO2/MgO = 0.30–0.57). The distribution and two-stage compositional evolution of the migrating flood basalts record continuous magma replenishment that buffered against magmatic evolution and induced magma chamber growth. Our results demonstrate that the magma replenishment–magma chamber growth model explains the spatio-temporal distribution of small-scale flood basalts.

The role of melt‐rock interaction in the formation of Quaternary high‐MgO potassic basalt from the Greater Khingan Range, northeast China

Melt-rock interaction between ascending melt and peridotite results in mantle metasomatism and also leads to compositional modification of the primary melt. While this process is known to occur, it is less well understood how the reactions and the composition of the resulting magma temporally evolve. Here whole-rock major and trace element, Sr-Nd-Pb-Hf isotopes, and olivine major element composition of Quaternary Nuominhe basalts in the Greater Khingan Range of northeast China are presented to unravel how melt-rock interaction modified the composition of the high-MgO potassic basalts as time progressed. The Nuominhe basalts are predominantly basanite with high MgO (8.1–16.8 wt %) and high total alkali content (K2O + Na2O = 6.0–9.2 wt %). They have high K2O/Na2O ratios (K2O/Na2O = 0.77–1.24) and low SiO2 and Al2O3 content (SiO2 = 44.4–48.7 wt %, Al2O3 = 10.5–13.2 wt %). They are characterized by enrichment in strongly incompatible elements, positive Ba, K, and Sr and negative Th, U, Zr, Hf, and Ti anomalies, similar to the composition of enriched mantle (EM1)-type oceanic island basalts (OIBs). Their isotopic composition also compares to that of EM1-type OIBs (i.e., with 87Sr/86Sr = 0.70467–0.70483, eNd = −4.1 to −1.5, eHf = −0.3 to 2.3, 206Pb/204Pb = 17.03–17.36). These elemental and isotopic characteristics are consistent with the interpretation that the potassium-rich melts were derived from recycled crustal materials with EM1 signature. Phlogopite-bearing mantle xenoliths and zoned olivine xenocrysts with high Fo89–92 and low CaO ( 0.1 wt %) rim composition record interaction between the ascending melt and mantle peridotite. Basalts erupted during late stages (Late Pleistocene and Holocene) of activity at the Nuominhe volcanic field show notably higher SiO2 content, Rb/Nb, Ba/Nb, K/La, and Ba/La, and lower MgO content than early-stage basalts (Early and Middle Pleistocene), which we infer to reflect a temporally decreasing extent of melt-rock interaction. During early stages of melt ascent, a reaction zone between melt channels and unreacted peridotite formed; at later stages this reaction zone effectively sealed the ascending melt from further reaction, resulting in increasing Rb/Nb, Ba/Nb, K/La, and Ba/La signatures of the erupted lavas.

Geodynamics of paleo‐Pacific plate subduction constrained by the source lithologies of Late Mesozoic basalts in southeastern China

Widespread late Mesozoic volcanic magmatism in southeastern China is generally thought to represent products in response to the subduction of paleo-Pacific plate; however, it remains unclear when this process began to affect the mantle and the related magmatism. Here we present a systematic study on the source lithology of late Mesozoic basalts in this area to highlight a link between lithological variations of mantle and subduction process of paleo-Pacific plate. Late Mesozoic basalts can be subdivided into four groups based on their erupted ages: 178 ~ 172 Ma, c.150 Ma, 137 ~ 123 Ma and 109 ~ 64 Ma. The primary source lithology of these rocks is pyroxenite rather than peridotite, and this mafic lithology can be formed by either ancient or young recycled crustal components. Notably, the source lithology of the c.150 Ma and 137 ~ 123 Ma basalts is SiO2-rich pyroxenite, and the former is carbonated. Because the carbonated eclogite cannot be preserved in the upper part of upper mantle based on the experimental solidus, the discovery of carbonated, SiO2-rich pyroxenite reflect the influence of a recently recycling event in the mantle. The subduction of paleo-Pacific plate is the most appropriate candidate, and can be responsible for the mantle-derived magmatism after c.150 Ma in southeastern China. Therefore, we suggest a paleo-Pacific slab rollback with increased dip angle as a possible model to control the lithological variations of late Mesozoic mantle beneath southeastern China.

Recycled ancient ghost carbonate in the Pitcairn mantle plume

Significance Lavas from Pitcairn Island are the best candidates for exploring the origin of the enigmatic EM1 component found in some mantle plumes because they show the most extreme isotopic compositions of Sr, Nd, Hf, and Pb that define the EM1 component. We find that these lavas have the lowest δ26Mg values so far recorded in oceanic basalts. Subducted late Archean dolomite-bearing sediments are the most plausible source of the low-δ26Mg feature of Pitcairn lavas. This requires that an ancient, originally sedimentary component has been emplaced near the core–mantle boundary to ultimately become part of the Pitcairn plume source. The extreme Sr, Nd, Hf, and Pb isotopic compositions found in Pitcairn Island basalts have been labeled enriched mantle 1 (EM1), characterizing them as one of the isotopic mantle end members. The EM1 origin has been vigorously debated for over 25 years, with interpretations ranging from delaminated subcontinental lithosphere, to recycled lower continental crust, to recycled oceanic crust carrying ancient pelagic sediments, all of which may potentially generate the requisite radiogenic isotopic composition. Here we find that δ26Mg ratios in Pitcairn EM1 basalts are significantly lower than in normal mantle and are the lowest values so far recorded in oceanic basalts. A global survey of Mg isotopic compositions of potentially recycled components shows that marine carbonates constitute the most common and typical reservoir invariably characterized by extremely low δ26Mg values. We therefore infer that the subnormal δ26Mg of the Pitcairn EM1 component originates from subducted marine carbonates. This, combined with previously published evidence showing exceptionally unradiogenic Pb as well as sulfur isotopes affected by mass-independent fractionation, suggests that the Pitcairn EM1 component is most likely derived from late Archean subducted carbonate-bearing sediments. However, the low Ca/Al ratios of Pitcairn lavas are inconsistent with experimental evidence showing high Ca/Al ratios in melts derived from carbonate-bearing mantle sources. We suggest that carbonate–silicate reactions in the late Archean subducted sediments exhausted the carbonates, but the isotopically light magnesium of the carbonate was incorporated in the silicates, which then entered the lower mantle and ultimately became the Pitcairn plume source.

Magma–magma interaction in the mantle recorded by megacrysts from Cenozoic basalts in eastern China

ABSTRACT Recently, besides magma–rock and rock–rock reaction, magma–magma interaction at mantle depth has been proposed as an alternative mechanism to produce diverse compositions of mantle. Clinopyroxene and garnet megacrysts can be formed at this condition since this process is suggested to trigger the high-pressure crystallization of these minerals. Studying on this type of megacrysts provides us important information on the genesis of intraplate basalts and the chemical heterogeneity of mantle, which has not been reported before. Here we present major, trace elements and Sr isotopes of clinopyroxene and garnet megacrysts hosted by Cenozoic basalts from Penglai, Shandong province of eastern China. The megacrysts are suggested to be formed by crystallization from magma because of their moderate Mg# (74.0–79.9 for clinopyroxene and 58.8–65.0 for garnet) and good correlations between Mg# and other elements (e.g. CaO, TiO2, Nd and Lu). The potential crystallized temperature and pressure are estimated to be ~1156°C at 2.6–3.2 GPa, which should occur at the top of asthenosphere or lithosphere–asthenosphere boundary based on the lithospheric thickness in this area (~60–70 km). Since the megacrysts show variable Sr isotopes, and their primary magmas show negative correlation between 87Sr/86Sr and Hf/Sm ratios, as well as positive correlation between Ba/Th and Nb/U for clinopyroxenes, it indicates a mixing origin. Cenozoic basalts from Shandong show a mixing trend, and high-pressure fractionation of clinopyroxene and garnet is suggested to occur during the mixing process because some basalts show significantly higher Sm/Yb and lower Ca/Al ratios than others, which again supports our interpretations. When compared to megacrysts and host basalts from other locations of eastern China, similar geochemical variations and a deviation trend relative to the mixing trend are also observed. It indicates that magma–magma interaction can be a common process for formation of intraplate basalts and basalt-borne megacrysts.