High water content in primitive continental flood basalts

As the main constituent of large igneous provinces, the generation of continental flood basalts (CFB) that are characterized by huge eruption volume (>105 km3) within short time span (<1–3 Ma) is in principle caused by an abnormally high temperature, extended decompression, a certain amount of mafic source rocks (e.g., pyroxenite), or an elevated H2O content in the mantle source. These four factors are not mutually exclusive. There are growing evidences for high temperature, decompression and mafic source rocks, albeit with hot debate. However, there is currently no convincing evidence of high water content in the source of CFB. We retrieved the initial H2O content of the primitive CFB in the early Permian Tarim large igneous province (NW China), using the H2O content of ten early-formed clinopyroxene (cpx) crystals that recorded the composition of the primitive Tarim basaltic melts and the partition coefficient of H2O between cpx and basaltic melt. The arc-like H2O content (4.82 ± 1.00 wt.%) provides the first clear evidence that H2O plays an important role in the generation of CFB.

0.13 to 0.49 wt.% for the Ontong Java and from 0.24 to 0.69 wt.% for the Kerguelen oceanic plateaus and is only slightly higher than that of MORB. However, the low Mg# (= 100 Mg/(Mg + Fe) mol.) of 40-60 indicated that the basaltic glasses they analysed are evolved melts, again arguing against the representativeness of the initial melts. Overall, there is currently no unarguable evidence to show whether the generation of CFB is related to the high H 2 O content of the mantle source.
Here, we calculate the initial H 2 O content of the early Permian Tarim CFB in NW China (>200,000 km 2 flood basalts) 9,36 using the H 2 O content of clinopyroxene (cpx) macrocrysts crystallized from the primitive Tarim flood basalts and the H 2 O partition coefficient between cpx and basaltic melt. The inferred high H 2 O content in the initial basaltic melt provides the first firm evidence that H 2 O plays an important role in the generation of CFB.

Samples and previous study
Many cpx macrocrysts (1-15 mm of grain size) were hosted by one basaltic dyke that crosscuts to the Early Permian (~280 Ma) Xiaohaizi wehrlite intrusion in the Tarim large igneous province, NW China (Fig. 1a,b). They are fresh and usually prismatic and sub-to euhedral shapes (Fig. 1c), and they commonly have a high-Mg (Mg# = 80-89) core and a thin low-Mg rim (Mg# down to 70) that is resulted from the interaction with the host basalt 37 . Wei et al. 37 carried out a detailed geochemical analysis on these macrocrysts. These cpx generally have low TiO 2 (0.26-1.09 wt.%), Al 2 O 3 (1.15-3.10 wt.%) and Na 2 O (0.16-0.37 wt.%) compared to the cpx in mantle peridotites (0.31-2.50 wt.% TiO 2 , 1.32-12.55 wt.% Al 2 O 3 and 0.2-1.90 wt.% Na 2 O), so they are not likely to be xenocrysts from mantle peridotites. The macrocrysts have strong resorption textures and are not in chemical equilibrium with the host basaltic dyke, arguing against a phenocryst genesis. In addition, the cpx macrocrysts define a coherent compositional trend (e.g., negative correlations between Mg# and Ti, Al, Na, La, Nd, Yb) 37 with the cpx from the wehrlites crosscut by the basaltic dyke hosting the cpx macrocrysts, and these cpx have identical trace element distribution patterns, demonstrating a comagmatic origin. Accordingly, these macrocrysts have been ascribed to be antecrysts that crystallized from the earlier and more primitive melts and have been reincorporated into the host basalt dyke before intrusion. High-Mg values indicate that the cpx macrocrysts were formed from a nearly primary basaltic melt. Although an assimilation and fractional crystallization process may operate during the formation of the Xiaohaizi intrusion that was evidenced by higher 87 Sr/ 86 Sr i (0.7038-0.7041) and lower ε Nd i (1.0-1.9), the preservation of the high-Mg feature and depleted Sr-Nd isotope compositions ( 87 Sr/ 86 Sr i = 0.7035-0.7037, and ε Nd i = 4.5-4.8) suggests that the cores of these cpx macrocrysts may have recorded the composition of the primitive Tarim basaltic melts, with little crustal contamination 37 . The Cpx macrocrysts in this paper were from the same dyke studied by Wei et al. 37 .

Results
The chemical composition and H 2 O content in 10 cpx grains were obtained by an electron probe micro-analyzer (EPMA) and a Fourier transform infrared spectrometer (FTIR), respectively (see Methods). Wei et al. 37 have shown that the rims of the Tarim cpx macrocrysts may have reacted with the host basalt, so only the clean core area of each cpx grain was measured here, in order to retrieve the information about the initial and primitive basaltic melts. 4-6 clean analysed spots in the core area were selected to run EPMA and FTIR for each grain (Fig. 1c), and in individual grains they show same chemical compositions and IR spectra (Fig. 1d). The average values of the analysed spots of each grain were, therefore, used to represent the element and H 2 O contents of that grain. Ten cpx grains have TiO 2 (0.39-0.66 wt.%), Al 2 O 3 (1.23-1.77 wt.%) and Na 2 O (0.15-0.26 wt.%) ( Table 1), which is within the range reported by Wei 37 . The cpx Mg# values are 85.2 to 87.8 (Table 1), corresponding to a Mg# of ~70 for the equilibrated basaltic melts using the experimental Mg-Fe partition coefficient (0.34 ± 0.04) 38 . This suggests that the analysed cpx grains were crystallized from a nearly primary basaltic source 39 , in agreement with the trace element and Sr-Nd isotope characteristics of the Tarim cpx macrocrysts 37 .

Discussion
The arc-like H 2 O contents in the early Permian Tarim primary basaltic melts indicate an addition of water from subduction-related processes. In the mid-Proterozoic, the Tarim was surrounded by subduction zones 49 . In addition, ophiolite mélanges and arc-like magmatic events along the northern margin of the Tarim were dated at 600-418 Ma and 422-363 Ma, respectively, suggesting an active convergent margin 50 . These subduction processes may have provided water to the source of the Tarim Early-Permian basalts. However, the Tarim basalts do not display arc-like geochemical signatures (i.e., LILEs-enrichment and HFSEs-depletion) 36,[51][52][53][54] . This suggests that the extra water in the source of the Tarim basalts was not from the released fluids from the subducting plates, but was instead from the dehydrated plates stagnated in the deep earth.  63 . If so, the classic core-mantle boundary-derived plume model 15,64 cannot be applied to the Tarim large igneous province.
In conclusion, the high water content in the primary early Permian Tarim basalts provides clear evidence that water, in addition to the temperature, pressure and source lithology, plays an important role in the generation of continental flood basalts. Furthermore, when high water content is considered, abnormally high temperature and extended decompression that are two critical factors in the widely accepted mantle plume model 65 are not always to be prerequisites in the generation of CFB (and LIPs).

Methods
The H 2 O content of cpx was determined with a Nicolet iso50 FTIR coupled with a Continuμ m microscope in School of Earth Sciences, Zhejiang University, following the unpolarized method described in Xia et al. 40 . For each cpx grain, several analysed spots (~50 μ m × 50 μ m) were set in the clean core area and they display almost same spectra, therefore the average spectrum was used to calculate the H 2 O content of that grain. The modified Beer-Lambert law [c = A/(I × t)] was used to calculate to H 2 O content, in which c is the content of water (H 2 O ppm wt.), A is the total integral absorption of OH bands (cm −2 ) that is 3 times of the integral area of unpolarized absorption 66 , I is the integral specific absorption coefficient (7.09 ppm −1 cm −2 ) 67 , t is the thickness (cm). The uncertainty of H 2 O content is less than 30% 40 .
The major element contents of cpx were analysed using a Shimadzu EPMA 1600 at University of Science and Technology of China. The 15 kV accelerating voltage, 20 nA beam current and 1 μ m beam diameter were used. Standards are natural minerals and synthetic oxides. Data correction was obtained by a program based on the ZAF procedure. The reproducibility is <1% for elements with concentration > 5% and < 3% for elements with concentration > 1%. The analysed points were set within the FTIR analysed area. Several points in each cpx grain have homogeneous element contents, and the average values were used ( Table 1).
The H 2 O content of the basaltic melts equilibrated with cpx is estimated by the H 2 O content of cpx and the H 2 O partition coefficients (Dcpx/melt) between cpx and melt. Dcpx/melt can be calculated by the equation 10 in O'Leary et al. 68 : D = exp(− 4.2 + 6.5 * X( iv Al)-X(Ca)), where X( iv Al) and X(Ca) are the concentration of octahedrally coordinated Al 3+ in tetrahedral site and Ca 2+ in cpx calculated on the basis of 6 oxygen. This equation was derived by compiling experimental results run at temperatures between 1025 °C and 1440 °C, pressures between 0.5-5.0 GPa, melt H 2 O contents between 1.09 wt.% and 24.9 wt.%, and cpx iv Al between 0.002 and 0.306. Considering the uncertainties from Dcpx/melt (~10%) 68 and H 2 O content in cpx (<30%), the total uncertainty of H 2 O contents in melts is estimated to be less than 40% [40][41][42] .