Petrology, geochemistry, and Sr, Nd isotopes of mantle xenolith in Nghia Dan alkaline basalt (West Nghe An): implications for lithospheric mantle characteristics beneath the region

pinel is darkish red, dark brown, irregularly shaped, distributed in the interstices between, or included in, large oli- vine crystals (Figure 4). ← Figure 3. Classification diagram of mantle perido- tites, showing mantle xenoliths in alkaline basalts in Vietnam; Nghia Dan mantle xenoliths filled, red squares Five representative samples were chosen among the collected samples in the Nghia My and Nghia Loi communes. The samples were processed to measure for major ele- ments of the rock-forming minerals using a JEOL 8800 Electron Probe Microanalyzer (EPMA) at the Geological Survey of Japan (Tsukuba, Japan). The accuracy of the analy- sis was estimated between ±2 and ±3% based on the repeated measurements of JEOL standards using natural minerals such as jadeite (Na), albite (K), kyanite (Al), wollas- tonite (Ca, Si), forsterite (Mg), rutile (Ti) and manganese ferrite (Fe) etc. Data for coexist- ing pyroxenes were used to calculate for crystallization temperatures using two py- roxene geothermometer by Wells (1977), Brey and Kohler (1990), Putirka et al. (2003), and Putirka (2008, 2017). The data are shown in Table 1. Figure 4. Photomicrograph of a spinel-lherzolite xenolith with xenogranular texture recovered at Mt. Ke Lui (Nghia My) showing a mineral assemblage comprising olivine, orthopyroxene, clinopyroxene and spinel: cross polarized (left), plain polarized (right) Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 212 Table 1. Major element concentrations (wt.%) of rock-forming minerals of Nghia Dan mantle xenoliths B040313-16Cpx/1 (n=3) B040313-16Cpx/2 (n=5) OL SP CPX OPX OL SP CPX OPX SiO2 40.60 0.05 52.32 54.89 40.58 0.03 52.32 54.88 TiO2 0.00 0.21 0.61 0.22 0.03 0.15 0.61 0.12 Al2O3 0.01 59.80 7.37 5.07 0.01 59.71 7.37 4.90 FeO 10.21 11.63 3.27 6.69 10.18 11.46 3.27 6.73 MnO 0.15 0.12 0.09 0.14 0.12 0.11 0.09 0.17 MgO 48.61 19.47 14.74 31.74 48.59 19.73 14.74 31.93 CaO 0.05 0.00 19.18 0.70 0.06 0.01 19.18 0.71 Na2O 0.00 0.00 1.69 0.09 0.01 0.00 1.69 0.12 K2O 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 Cr2O3 0.02 8.33 0.69 0.33 0.01 8.44 0.69 0.33 NiO 0.35 0.38 0.05 0.12 0.40 0.37 0.05 0.09 Total 100 100 100 100 100 100 100 100 Mg# 89.5 74.9 88.9 89.4 89.5 75.4 88.9 89.4 Cr# 8.5 5.9 4.2 8.7 5.9 4.4 Fs 6.0 10.4 6.0 10.4 En 48.5 88.2 48.5 88.2 Wo 45.4 1.4 45.4 1.4 B040313-16Cpx/3 (n=3) B040313-16Cpx/4 (n=3) OL SP CPX OPX OL SP CPX OPX SiO2 40.70 0.03 52.36 55.16 40.49 0.07 51.90 54.78 TiO2 0.00 0.17 0.66 0.13 0.02 0.19 0.68 0.12 Al2O3 0.02 60.05 7.28 4.78 0.04 59.55 7.31 4.79 FeO 10.33 11.43 3.01 6.44 10.37 11.56 3.29 6.63 MnO 0.11 0.07 0.10 0.14 0.15 0.13 0.11 0.16 MgO 48.43 19.57 14.82 32.16 48.47 19.81 15.00 32.35 CaO 0.06 0.03 19.39 0.69 0.08 0.01 19.26 0.66 Na2O 0.00 0.01 1.62 0.10 0.02 0.01 1.74 0.10 K2O 0.00 0.00 0.01 0.00 0.00 0.00 0.01 0.00 Cr2O3 0.00 8.29 0.74 0.28 0.00 8.28 0.67 0.27 NiO 0.35 0.35 0.02 0.12 0.36 0.40 0.04 0.11 Total 100 100 100 100 100 100 100 100 Mg# 89.3 75.3 89.8 89.9 89.3 75.3 89.0 89.7 Cr# 8.5 6.4 3.7 8.5 5.8 3.7 Fs 5.5 10.0 6.0 10.2 En 48.7 88.7 48.9 88.5 Wo 45.8 1.4 45.1 1.3 B040313-16Cpx/5 (n=5) A040313-7Cpx/1 (n= 3) OL SP CPX OPX OL SP CPX OPX SiO2 40.90 0.06 51.84 55.22 40.56 0.03 52.37 54.65 TiO2 0.00 0.15 0.62 0.20 0.00 0.17 0.61 0.15 Al2O3 0.04 59.66 7.49 4.76 0.01 60.05 7.11 4.96 FeO 10.17 11.51 3.11 6.40 10.41 11.43 3.26 6.82 MnO 0.14 0.12 0.08 0.07 0.13 0.07 0.08 0.12 MgO 48.32 19.62 14.86 32.15 48.50 19.57 14.61 32.09 CaO 0.06 0.01 19.62 0.67 0.08 0.03 19.53 0.63 Na2O 0.01 0.03 1.64 0.14 0.00 0.01 1.70 0.09 K2O 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.01 Cr2O3 0.01 8.46 0.65 0.27 0.00 8.29 0.69 0.36 NiO 0.34 0.38 0.08 0.12 0.31 0.35 0.03 0.12 Vietnam Journal of Earth Sciences, 40(3), 207-227 213 B040313-16Cpx/5 (n=5) A040313-7Cpx/1 (n= 3) OL SP CPX OPX OL SP CPX OPX Total 100 100 100 100 100 100 100 100 Mg# 89.4 75.2 89.5 90.0 89.3 75.3 88.9 89.3 Cr# 8.7 5.5 3.7 8.5 6.1 4.7 Fs 5.7 9.9 6.0 10.5 En 48.4 88.7 47.9 88.2 Wo 46.0 1.3 46.1 1.3 A040313-7Cpx/2 (n= 3) A040313-7Cpx/3 (n= 4) OL SP CPX OPX OL SP CPX OPX SiO2 40.35 0.07 52.17 54.65 40.62 0.06 52.27 54.85 TiO2 0.02 0.19 0.68 0.15 0.00 0.15 0.63 0.14 Al2O3 0.03 59.55 7.23 4.96 0.06 59.66 7.41 4.86 FeO 10.25 11.56 3.30 6.82 10.26 11.51 2.89 6.65 MnO 0.17 0.13 0.08 0.12 0.17 0.12 0.07 0.14 MgO 48.75 19.81 14.79 32.09 48.44 19.62 14.77 32.21 CaO 0.06 0.01 19.34 0.63 0.05 0.01 19.47 0.68 Na2O 0.04 0.01 1.64 0.09 0.01 0.03 1.74 0.10 K2O 0.02 0.00 0.00 0.01 0.01 0.00 0.01 0.00 Cr2O3 0.00 8.28 0.68 0.36 0.01 8.46 0.69 0.28 NiO 0.31 0.40 0.07 0.12 0.37 0.38 0.05 0.11 Total 100 100 100 100 100 100 100 100 Mg# 89.5 75.3 88.9 89.3 89.4 75.2 90.1 89.6 Cr# 8.5 6.0 4.7 8.7 5.9 3.7 Fs 6.1 10.5 5.3 10.2 En 48.4 88.2 48.6 88.4 Wo 45.5 1.3 46.1 1.3 A040313-7Cpx/4 (n= 5) A040313-7Cpx/5 (n= 3) OL SP CPX OPX OL SP CPX OPX SiO2 40.87 0.03 52.51 54.78 40.63 0.05 52.17 55.22 TiO2 0.03 0.17 0.61 0.12 0.02 0.21 0.68 0.20 Al2O3 0.00 60.05 7.40 4.79 0.00 59.80 7.23 4.76 FeO 10.17 11.43 2.82 6.63 10.22 11.63 3.30 6.40 MnO 0.15 0.07 0.07 0.16 0.13 0.12 0.08 0.07 MgO 48.39 19.57 14.85 32.35 48.54 19.47 14.79 32.15 CaO 0.06 0.03 19.33 0.66 0.04 0.00 19.34 0.67 Na2O 0.00 0.01 1.70 0.10 0.01 0.00 1.64 0.14 K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Cr2O3 0.00 8.29 0.69 0.27 0.03 8.33 0.68 0.27 NiO 0.33 0.35 0.02 0.11 0.39 0.38 0.07 0.12 Total 100 100 100 100 100 100 100 100 Mg# 89.5 75.3 90.4 89.7 89.4 74.9 88.9 90.0 Cr# 8.5 5.9 3.7 8.5 6.0 3.7 Fs 5.2 10.2 6.1 9.9 En 48.9 88.5 48.4 88.7 Wo 45.8 1.3 45.5 1.3 Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 214 The trace element and Sr-Nd isotopic com- positions were acquired on separated clinopy- roxenes from two presentative large (5× 10×10cm) spinel-lherzolite samples collected from Mt. Ke Rui and Nghia Loi (Figure 2). The clinopyroxene separation was treated as fol- lows. About 50g of the mantle xenoliths were crushed and ground, followed by 1mm sized sieving. The sieved particles were cleaned ul- trasonically for about 30 minutes, followed by multiple rinse with clean water. The cleaned samples were dried in an oven at 90°C for about 1 hour. The samples were left to cool. Clinopy- roxenes were then hand-picked under a stereo binocular microscope. The clinopyroxene sepa- rates were again roughly ground, ultrasonically cleaned, dried and hand-picked to eliminate any other minerals than clinopyroxene. About 100mg of clinopyroxene separates were weighed in 15ml Teflon beakers, ready for dissolution. For sample dissolution, a mix- ture of 1ml and 2ml, respectively, of concen- trated HNO3 and HF was added to the beak- ers, capped and left on a hotplate at about 140°C for two days, followed by complete evaporation. The dried samples were added with 3ml of 7M HNO3, capped and left on a hotplate at about 80°C for an overnight to en- sure the samples were dissolved completely. The samples were evaporated, weighed, then diluted with 6ml of 0.3M HNO3. An aliquot of about 3ml (ca. 50mg) of the sample was taken for trace element concentration determination using a Neptune Elemental quadrupole induc- tively coupled plasma mass spectrometer (Q- ICP-MS) at the Department of Physics and Earth Sciences, University of the Ryukyus, Okinawa, Japan. The trace element composi- tions are shown in Table 2. The remaining sample solutions were treat- ed for chromatographic work to extract Sr and Nd elements. The 87Sr/86Sr, 143Nd/144Nd isotop- ic ratios were acquired using a VG Sector 54 Thermo- Ionization Mass Spectrometer (TIMS) at the Geological Survey of Japan, Tsukuba, Japan. The data are shown in Table 2. Table 2. Trace element and Sm-Nd-Sr isotopic compositions of clinopyroxene separates from Nghia Dan spinel- lherzolite xenoliths Sample B040313-16Cpx A040313-7Cpx A040213-16A A040313-7B Location Nghia Dan Nghia Dan Nghia Dan Nghia Dan Latitude N19°25'0.1" N19°21'26" N19°25'00.1" N19°21'26" Longitude E105°26'07.9" E105°31'37" E105°26'07.9" E105°31'37" Sm (ppm) 0.349 0.328 11.24 11.33 Nd (ppm) 0.709 0.518 64.3 64.74 147Sm/144Nd 0.304 0.391 0.121 0.108 143Nd/144Nd 0.513225 0.513292 0.512776 0.512773 T (DM) 1.27E+08 1.22E+08 εNd 11.45 12.76 2.69 2.65 87Sr/86Sr 0.702493 0.702694 0.704576 0.704564 In ppm Rb 0.56 0.49 54.3 89.65 Sr 25.83 23.66 1779 1898.75 Y 15.77 16.12 29.5 28.80 Zr 15.86 16.45 366 320.85 Nb 0.43 0.36 137.2 110.29 Cs 0.004 0.004 0.58 1.23 Ba 3.27 2.89 723 711.58 Vietnam Journal of Earth Sciences, 40(3), 207-227 215 Sample B040313-16Cpx A040313-7Cpx A040213-16A A040313-7B La 0.40 0.34 75.9 77.48 Ce 1.61 1.35 152.6 149.91 Pr 0.34 0.33 17 16.99 Nd 0.709 0.518 64.3 64.74 Sm 0.349 0.328 11.24 11.33 Eu 0.52 0.51 3.4 3.50 Gd 1.61 1.67 10.97 10.31 Tb 0.37 0.39 1.246 1.30 Dy 2.55 2.32 5.73 6.19 Ho 0.58 0.56 0.998 1.09 Er 1.69 1.65 2.42 2.98 Tm 0.24 0.23 0.319 0.37 Yb 1.55 1.52 1.98 2.30 Lu 0.22 0.22 0.264 0.32 Hf 0.63 0.65 6.55 6.22 Ta 0.05 0.05 6.83 6.06 Pb 0.11 0.08 4.35 3.34 Th 0.04 0.04 8.46 9.80 U 0.03 0.03 2.27 2.49 V 229.8 236.6 206 200 Cr 4,014 3,898 294 165 Ni 411.7 392.5 244 159 Remark: 147Sm/144Nd = [(Sm (ppm)/Nd (ppm)×(15×143.91)/ (23.8×146.9148); T(DM) = ln{[(0.51315-143Nd/144Nd)/ (0.2135-147Sm/144Nd)] +1}×1/ (6.54×10-12); εNd = [(143Nd/144Nd/ 0.512638)-1] ×10000 3. Analytical results 3.1. Geochemistry of the mantle xenoliths Note that most of the mantle xenoliths are spinel lherzolites. Geochemical compositions of their rock-forming minerals are relatively uniform regardless of being collected from two sites about 10km apart. Geochemical sim- ilarity is also seen in alkaline basalts sampled from the two sites (Table 2). 3.1.1. Spinel and olivine relationship Geochemically compositional characteris- tics of mantle xenoliths are reflected by the geochemistry of their rock-forming minerals (Figure 4). The olivine content in the Nghia Dan mantle xenoliths varies from (about) 70 to 80 vol.%, with forsterite composition [100× (Mg/(Mg+Fe2+)] ranging from 89 to 90.5 (Ta- ble 1). The spinel content varies from (about) 0.5 to 1.5 vol.% having relatively low Cr- number (Cr/Cr+Al), about 8.5 (Table 1). Hav- ing low Cr-number (e.g. high Al2O3 content) and relatively high forsterite component, Nghia Dan mantle xenoliths plot in the field of (intraplate) oceanic hotspot peridotites be- lieved to be fertile and enriched for experienc- ing low degrees of partial melting as com- pared to peridotite residues in the ocean ridges, arc settings or ancient cratons (Figure 5, after Arai, 1994). 3.1.2. Orthopyroxene The orthopyroxene content in Nghia Dan mantle xenoliths ranges from 19 to 25 vol.% (Figure 3). The mineral is an enstatite show- ing a compositional range of En88.2-88.7Fs5.5- 6.4Wo1.3-1.4. The Mg-number varies in a narrow range, from 89.3 to 90, about the range of co- existing olivine. The Cr2O3 contents are low, from 0.27 to 0.33 wt.%. The Cr2O3 increase Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 216 with increasing Al2O3, from 4.68 to 5.1wt.%. The variation of Cr2O3, Al2O3 and TiO2 con- centrations (from 0.12 to 0.22 wt.%) may be small to notice the change in their correspond- ing Mg-number (Table 1). Figure 5. Olivine - Spinel Mantle Array (OSMA) show- ing distribution fields of peridotites from various tecton- ic settings. Residual peridotites produced by high de- grees of partial melting are refractory (and depleted), having high Cr- numbers and forsterite contents. Re- drawn after Arai (1994), data for oceanic ridges and in- traplate oceanic hotspots are from Choi et al. (2005), Workman and Hart (2005) and Warren (2016) 3.1.3. Clinopyroxene The clinopyroxene is a diopside which constitutes 3 to 5 vol.%, in some rare cases up to 8-10 vol.% among the spinel lherzolite- forming minerals, showing a range of chemi- cal compositions of En47.9-48.9Fs5.2-6.1Wo45.1-46.1. The Mg-number increases with increasing en- statite component, from 88.9 to 90.4. The TiO2 and Cr2O3 contents are moderate, from 0.6 to 0.7 wt.%, and from 0.65 to 0.74 wt.%, respectively. The Al2O3 concentrations vary from 7.1 to 7.5 wt.%, which are slightly high- er as compared with those in Jeju mantle spi- nel- lherzolite and elsewhere in East Asia (Choi et al., 2001, 2008). Despite minor dif- ferences in geochemical composition, the Nghia Dan clinopyroxene is very much simi- lar to clinopyroxene in mantle xenoliths in Cenozoic alkaline basalts in east and northeast China (Tatsumoto et al., 1992, Qi et al., 1995), the Japan Sea (Choi et al., 2001, 2008), and elsewhere in the Western Highlands, south-Central Vietnam, and southeastern con- tinental shelf of East Vietnam Sea (Gorshkov, 1981; Fedorov and Koloskov, 2005; Mali- novsky and Rashidov, 2015). The Nghia Dan mantle xenolith separated clinopyroxenes, however, have slightly higher wollastonite (Ca2Si2O6) and (clino)- ferrosilite (Fe2Si2O6) components, shifting more toward the heden- bergite field as compared to clinopyroxenes separated from mantle xenoliths in the (mid-) ocean ridges (after Johnson et al., 1990; Workman and Hart, 2005; Warren, 2016). Al2O3 and TiO2 concentrations in Nghia Dan orthopyroxene and clinopyroxene are higher as compared with mid-ocean ridge mantle xenolith separated clinopyroxene (e.g. Work- man and Hart, 2005; Warren, 2016). Experi- mental data suggested that the higher Al2O3 content in pyroxenes the higher melting tem- perature and pressure of the minerals (Ku- shiro, 1996). 3.2. Elemental geochemistry of Nghia Dan separated clinopyroxene The trace element compositions of Nghia Dan clinopyroxene are given in Table 2. The data are normalized to the primitive mantle value (after Sun and McDonough, 1989), il- lustrated in Figure 6. Data of mantle xenolith separated clinopyroxenes from 1 Ma alkaline basalt in the Dat Do district (Ba Ria-Vung Tau) (Hoang unpublished data) are plotted for reference. The trace element distribution pat- tern of Nghia Dan clinopyroxene is relatively smooth, showing a gradual decrease from heavy, less mobile elements to lighter, highly incompatible elements. The observation sug- gests that the Nghia Dan mantle xenoliths may have experienced small melting degrees, and that, other than the effect of melting and Vietnam Journal of Earth Sciences, 40(3), 207-227 217 crystallization, Nghia Dan mantle peridotite residues may not undergo any significant post-melting process, such as melt addition or removal (as compared to, for example, the mantle xenolith in Dat Do alkaline basalt). In summary, the trace element distribution pat- tern of Nghia Dan clinopyroxene reflects typi- cal geochemical characteristics of clinopyrox- enes in alkaline basalt-borne mantle xenoliths (e.g. Embey-Isztin et al., 2001). Figure 6. Trace element primitive mantle normalization of Nghia Dan clinopyroxene separate showing relatively smooth trend from moderately immobile to highly mobile elements (normalizing data are after Sun and McDonough, 1989). Shown are data for the host alkaline basalt and mantle xenolith- separated clinopyroxenes from Dat Do (Ba Ria - Vung Tau) for comparison. See text for details 3.3. Isotope geochemistry of Nghia Dan mantle xenolith separated clinopyroxene The 87Sr/86Sr ratios vary from 0.70249 to 0.70269, and 143Nd/144Nd range from 0.51322 0.51329 (Table 2). They plot in the depleted mantle field (DMM) along with several other Vietnamese separated clinopyroxenes, alt- hough most of the latter are more enriched than the Nghia Dan clinopyroxenes and plot- ting in the field of oceanic island basalts (Fig- ure 7). In general, most (if not all) of the man- tle xenolith separated clinopyroxenes in Vietnam are isotopically depleted and plot in the upper corner of the depleted quadrant dif- ferentiating from their host-basalts, suggesting that they are not genetically related. However, the isotope trending between depleted (e.g. N- MORB: mid-ocean ridge basalt) and enriched mantle (EM1, EM2) may suggest possible in- teraction of depleted mantle-derived melts with enriched sources in the lithospheric man- tle (Figure 7). The isotopic characteristics of Nghia Dan mantle xenolith separated clinopy- roxenes (and elsewhere in Vietnam) are most- ly similar to clinopyroxenes separated from mantle xenoliths in alkaline basalts in north- east China (Tatsumoto et al., 1992) and Jeju island (Choi et al., 2005, 2008). In summary, the elemental and radiogenic isotopic characteristics of the mantle xenolith and their host alkaline basalt are vastly differ- ent, implying, most certainly, that they are not genetically related. Moreover, thermal con- tacts between host basalt and mantle xenolith are commonly observed, suggesting that the mantle xenolith is brought to the surface by alkaline basaltic melt formed at deeper levels. 0.001 0.01 0.1 1 10 100 Rb Ba Th U Nb Ta La Ce Pb Nd Sr Sm Hf Zr Ti Eu Y Yb Lu Pr im itiv e M an tle N or ma liz ed Nghia Dan_Cpx Nghia Dan basalt Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 218 The basaltic melt must move up at a speed that is fast enough to tear off the mantle xeno- lith from its bed in the lithospheric mantle to bring to the surface (see Nixon (Editor), 1987; Qi et al., 1995; Choi et al., 2001; Embey- Isztin et al., 2001). Figure 7. Plots of 87Sr/86Sr versus εNd [= (143Nd/144Ndm/0.512638)-1) × 10000] of Vietnam alkaline basalt-borne man- tle xenolith separated clinopyroxenes (empty circles; Hoang unpublished data) including Nghia Dan (red filled dia- mond) (Table 2) and host basalts (cross). Shown are fields of Depleted Mid-Ocean Ridge Basalt Mantle (N-MORB), Enriched Mantle type 1 and 2 (EM1 and EM2) (after Zindler and Hart, 1986); field of continental crust (CC) (after Taylor and McLennan, 1981) and Oceanic Island basalt (OIB Hawaii, after Norman and Garcia, 1999) for reference 4. Discussion 4.1. Mantle peridotite melting Compiled data of mantle peridotites from oceanic ridges and other tectonic settings worldwide showed that an upper mantle peri- dotite is averagely composed of (in vol.%) ol- ivine (Ol: 57), orthopyroxene (Opx: 28), cli- nopyroxene (Cpx: 13) and spinel (Sp: 2). Par- tial melting of a peridotite having the above mentioned mineral assemblage to produce ba- saltic melt, according to a number of recent experimental studies (e.g. Takahashi and Ku- shiro, 1983; Takahashi, 1986; Johnson et al., 1990; Hirose and Kushiro, 1993; Kushiro, 1996, 1998), would occur in the following proportion (in vol.%) Ol: 10, Opx: 20, Cpx: 68, and Sp: 2. Moreover, the melting process is fractional rather than batch melting (John- son et al., 1990). Basic fractional melting equation to show the change in concentration of an element in clinopyroxene with melting was developed by Gast (1968) and Shaw (1970) as follows: 𝐶𝐶𝑠𝑠 𝑖𝑖 𝐶𝐶0 𝑖𝑖= ( 1(1−𝐹𝐹))(1 − 𝑃𝑃𝐹𝐹𝐷𝐷0𝑖𝑖 )1/𝑝𝑝 (1) Where Csi is the concentration of element i in the residue (s) as a function of partial melt- ing degree (F). C0i is initial concentration of element i and D0i is bulk solid partition coeffi- cient of element i. P weighted partition coeffi- cient of liquid: P = Σ𝑝𝑝𝑎𝑎𝐷𝐷𝑎𝑎 𝑖𝑖 = 𝑝𝑝𝑜𝑜𝑜𝑜𝑖𝑖𝑜𝑜𝐷𝐷𝑜𝑜𝑜𝑜𝑖𝑖𝑜𝑜𝑖𝑖 + 𝑝𝑝𝑜𝑜𝑝𝑝𝑜𝑜𝐷𝐷𝑜𝑜𝑝𝑝𝑜𝑜𝑖𝑖 + 𝑝𝑝𝑐𝑐𝑝𝑝𝑜𝑜𝐷𝐷𝑐𝑐𝑝𝑝𝑜𝑜 𝑖𝑖 +𝑝𝑝𝑠𝑠𝑝𝑝𝑖𝑖𝑠𝑠𝐷𝐷𝑠𝑠𝑝𝑝𝑖𝑖𝑠𝑠𝑖𝑖 (2) Vietnam Journal of Earth Sciences, 40(3), 207-227 219 Dαi: partition coefficient of element i in α mineral phase; pα: proportion of mineral phase entering liquid. Because the bulk trace element abundance of upper mantle peridotite is mainly incorpo- rated in clinopyroxene, Equation (1) therefore may be changed to (3) (e.g. Johnson et al., 1990): (3) To apply equation (4) it needs to change bulk rock variable to clinopyroxene as fol- lows: (4) (5) Where xα is weight fraction of mineral phase. Using equations 4 and 5, the trace element abundance of a peridotite can be interpolated from the trace element concentrations in cli- nopyroxene separated from the peridotite. Applying equation 5 to the trace element concentration of Nghia Dan separated clino- pyroxene (Table 2), trace element abundance of parental mantle peridotites can be deter- mined. Depending on the bulk partition coef- ficient of elements in individual rock-forming minerals (Table 3), their constituents in a giv- en peridotite and proportion of mineral enter- ing the liquid (Table 4), and the melting de- grees, the interpolated trace element abun- dances may be differently acquired. The com- puted data are shown in Table 4 and illustrat- ed in Figures 8a, b. Table 3. Elemental liquid/solid partition coefficients (𝐷𝐷𝑠𝑠𝑜𝑜𝑜𝑜𝑖𝑖𝑠𝑠 𝑜𝑜𝑖𝑖𝑙𝑙𝑙𝑙𝑖𝑖𝑠𝑠) for rock-forming minerals of mantle peridotite Ol Opx Cpx Sp La 0.0002 0.026 1.414 0.016 Ce 0.0005 0.137 3.930 0.027 Pr 0.0002 0.034 0.744 0.003 Nd 0.002 0.194 4.044 0.013 Sm 0.004 0.109 1.585 0.003 Eu 0.002 0.056 0.611 0.001 Gd 0.007 0.239 2.209 0.004 Tb 0.003 0.051 0.415 0.001 Dy 0.026 0.396 2.916 0.010 Ho 0.010 0.096 0.633 0.003 Er 0.036 0.363 1.735 0.012 Yb 0.086 0.376 1.615 0.017 Lu 0.016 0.065 0.235 0.003 Table 4. Mineral modal constituents of Nghia Dan alkaline basalt-bearing mantle xenoliths and rare earth element concentrations estimated for related mantle xenolith whole rock Sample ID ND-MX-1 ND-MX-2 ND-MX-3 ND-MX-4 ND-MX-5 OL 77.40 74.70 70.94 71.06 69.60 CPX 2.96 4.27 3.31 5.51 4.32 OPX 18.90 20.10 25.37 22.72 24.50 SP 0.73 1.00 0.37 0.71 1.66 La (ppm) 0.055 0.078 0.062 0.099 0.079 Ce 0.097 0.133 0.113 0.169 0.139 Pr 0.133 0.185 0.152 0.235 0.191 Nd 0.203 0.279 0.234 0.354 0.290 Sm 0.343 0.460 0.400 0.577 0.483 Eu 0.394 0.522 0.463 0.652 0.552 Gd 0.401 0.524 0.474 0.649 0.556 Tb 0.504 0.647 0.596 0.795 0.691 Dy 0.574 0.724 0.681 0.882 0.776 Ho 0.627 0.777 0.744 0.937 0.836 Er 0.714 0.869 0.845 1.038 0.938 Tm 0.675 0.821 0.798 0.980 0.887 Yb 0.787 0.928 0.923 1.085 1.002 Lu 0.803 0.934 0.937 1.082 1.007 Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 220 Figure 8a, b. Chondrite normalized computed rare earth element abundance produced by fractional melting (Eq. 1); A%M and A%R: rare earth element composition of melt (M) or residue (R) produced by A melting degree (8a, left); rare earth element chondrite normalized distribution pattern of Nghia Dan clinopyroxene (thick, red line) and corre- sponding residual parental peridotite (thin, red lines) according to different mineral constituents (8b, right). Shown for comparison are clinopyroxene separate and their residual peridotite from Dat Do (Ba Ria- Vung Tau), and clino- pyroxene (DM_Cpx) separated from the representative depleted mantle peridotite (Computing template is after Workman and Hart, 2005; Warren, 2016). Normalizing data are after Anders and Grevesse (1989) By interpolation, the trace element con- tents of Nghia Dan clinopyroxene may be ob- tained by melting degree between 8 to 12% (Figures 8a, b) of a mantle peridotite which is relatively depleted (and refractory). The Nghia Dan clinopyroxene is more depleted in light rare earth element concentrations as compared to an average mid-ocean ridge peri- dotite separated clinopyroxene (DM-Cpx) (Figure 8b), suggesting that the Nghia Dan mantle xenolith may have experienced multi- ple melting events (Takahashi, 1986; Hirose and Kushiro, 1993). Note that clinopyroxenes separated from Dat Do mantle xenoliths (Ba Ria - Vung Tau) show strong geochemical heterogeneity, suggesting mantle peridotites in the lithospheric mantle may undergo vari- ous melting events and/or melt addition or removal (Carlson and Irving, 1994) (Figure 8b). 4.2. Thermal state of the lithospheric mantle under Nghia Dan A number of geothermometers for applica- ble mineral assemblages of mantle peridotite xenoliths have been introduced over the years. Equilibrium temperatures may be estimated using geothermometers based on (1) enstatite component of coexisting two pyroxenes (Wells, 1977; Brey and Kohler, 1990), (2) Al- solubility in orthopyroxene coexisting with olivine and spinel (Sachtleben and Seck, 1981; Webb and Wood, 1986); (3) Mg-Fe2+ exchange between olivine and spinel (Ball- haus et al., 1991); (4) concentration of Group II elements (Cr, Al, Sc, Ca and Na) in mantle peridotite olivine (De Hoog et al., 2010). Brey and Köhler (1990), following many testing combinations of geothermobarometers, sug- gested that the geobarothermometer of Köhler and Brey (1990) may provide a reasonable T- P estimate for spinel peridotite. Experimentally, Putirka et al. (1996), fol- lowed by Putirka et al. (2003) developed equations (1) and (2) two-pyroxene pressure and temperature estimates. Equation (2) has been improved from a previous equation by Putirka (2008, 2017). 𝑃𝑃(𝑘𝑘𝑘𝑘𝑘𝑘𝑘𝑘) = − 88.3 + 2.82 𝑥𝑥10−3𝑇𝑇(𝐾𝐾) 𝑙𝑙𝑙𝑙[�𝐽𝐽𝐽𝐽𝑐𝑐𝑝𝑝𝑜𝑜]/ Vietnam Journal of Earth Sciences, 40(3), 207-227 221 [𝑁𝑁𝑘𝑘𝑜𝑜𝑖𝑖𝑙𝑙𝐴𝐴𝑙𝑙𝑜𝑜𝑖𝑖𝑙𝑙(𝑆𝑆𝑆𝑆𝑜𝑜𝑖𝑖𝑙𝑙)2� + 2.19 𝑥𝑥 10−2𝑇𝑇(°𝐾𝐾) −25.1 𝑙𝑙𝑙𝑙�𝐶𝐶𝑘𝑘𝑜𝑜𝑖𝑖𝑙𝑙𝑆𝑆𝑆𝑆𝑜𝑜𝑖𝑖𝑙𝑙� + 7.03[𝑀𝑀𝑀𝑀′𝑜𝑜𝑖𝑖𝑙𝑙]] +12.41 𝑙𝑙𝑙𝑙[𝐶𝐶𝑘𝑘𝑜𝑜𝑖𝑖𝑙𝑙] (1) 𝑃𝑃(𝑘𝑘𝑘𝑘𝑘𝑘𝑘𝑘) = −26.3 +39.2 𝑥𝑥 10−4𝑇𝑇(𝐾𝐾) � 𝑋𝑋𝐽𝐽𝐽𝐽𝑐𝑐𝑐𝑐𝑐𝑐(𝑋𝑋𝑁𝑁𝑁𝑁𝑙𝑙𝑖𝑖𝑙𝑙)(𝑋𝑋𝐴𝐴𝑙𝑙𝐴𝐴1.5𝑙𝑙𝑖𝑖𝑙𝑙 )(𝑋𝑋𝑆𝑆𝑖𝑖𝐴𝐴2𝑙𝑙𝑖𝑖𝑙𝑙 )2� −4.22𝑙𝑙𝑙𝑙�𝑋𝑋𝐷𝐷𝑖𝑖𝐷𝐷𝑠𝑠𝑐𝑐𝑝𝑝𝑜𝑜 � + 78.4�𝑋𝑋𝐴𝐴𝑜𝑜𝐴𝐴1.5𝑜𝑜𝑖𝑖𝑙𝑙 � +394[𝑋𝑋𝑁𝑁𝑎𝑎𝐴𝐴0.5𝑜𝑜𝑖𝑖𝑙𝑙 + 𝑋𝑋𝐾𝐾𝐴𝐴0.5𝑜𝑜𝑖𝑖𝑙𝑙 ]2 (2) The temperature is in Kelvin, pressure is in kbar, Jdcpx is molecule component of jadeite in clinopyroxene, number of cations in pyroxene are calculated based on 6 oxygen atoms. DiHdcpx is molecule component of diopside and hedenbergite in clinopyroxene. Alliq is AlO1.5 cation content in melts, MgO’liq is cation content of MgOliq/ (MgOliq + Feliq). See Putirka et al. (1996, 2003) and Putirka (2008) for more details. The computed pressure and temperature of mineral assemblages in Nghia Dan mantle peridotites basing on coexisting pyroxenes us- ing equations (1) and (2) (Putirka et al., 1996, 2003; Putirka, 2008, 2017) are shown in Table 5. The estimated data are compared with the temperatures calculated using approaches by Wells (1977) and Brey and Köhler (1990). The temperatures estimated for coexisting py- roxenes in Nghia Dan mantle xenoliths using equations of Putirka et al. (1996, 2003) and Putirka (2008, 2017) are about 30°C lower as compared to those using equations of Wells (1977), Brey and Kohler (1990) and De Hoog et al. (2009) (Figure 9). Table 5. Two-pyroxene temperature - pressure estimates and observed KD(Fe-Mg) values for Nghia Dan mantle xeno- liths (computed after Putirka, 2008). Shown for comparison are two-pyroxene crystallization temperatures by Brey and Kohler (1990) method Putirka (2008) (*) KD(Fe-Mg) Brey & Kohler (1990) T°C Sample ID T°(C) P (kbar) B040313-16Cpx/1 1023.5 13.1 1.053 1079.1 B040313-16Cpx/2 1029.3 13.9 1.053 1081.0 B040313-16Cpx/3 1011.3 13.4 0.948 1051.7 B040313-16Cpx/4 1023.9 13.6 1.014 1076.9 B040313-16Cpx/5 1034.4 14.1 1.070 1067.7 A040313-7Cpx/1 1016.9 14.1 0.927 1072.9 A040313-7Cpx/2 1037.1 13.6 1.051 1051.3 A040313-7Cpx/3 1009.0 13.8 1.050 1048.0 A040313-7Cpx/4 1043.6 14.1 1.121 1076.7 A040313-7Cpx/5 1027.2 14.2 1.050 1071.2 (*) www.minsocam.org/MSA/RIM/RiMG069/RiMG069_Ch03_two-pyroxene_P-T.xls ← Figure 9. Plots of temperature estimates for equilib- rium two-pyroxenes in Nghia Dan mantle xenoliths us- ing geothermometers of Wells (1977), Brey and Kohler (1990) versus Putirka (2008, 2017). Data for Pleiku (Western Highlands) and Tro Islet (Ile des Cendres, East Vietnam Sea) mantle xenoliths (Hoang unpublished da- ta) are shown for reference 4.3. Geotherm beneath Western Nghe An Cenozoic basaltic volcanism in the West- ern Highlands of Vietnam was driven by the regional thermally anomalous mantle fol- Tran Thi Huong and Nguyen Hoang/Vietnam Journal of Earth Sciences 40 (2018) 222 lowed by lithospheric extension (e.g. Garnier et al., 2005; Izokh et al., 2010, and references therein). Using primitive melt compositions computed for Cenozoic basalts from the Western Highlands, and elsewhere in the off- shore and coastal areas of Vietnam to interpo- late melting temperatures and pressures of the basalts by correlating with experimental man- tle peridotite melting data (e.g. Takahashi and Kushiro, 1983; Kushiro, 1990, 1996, 1998; Hirose and Kushiro, 1993) Hoang et al. (2014) reported that mantle beneath Vietnam is hotter than normal (e.g. 1280°C), reaching up to 1380°C. The thermally anomalous man- tle has been viewed as a consequence of the India- Eurasian collision that led to the Tethys closure, uplift of Himalaya and Tibet, subduc- tion initiation under the Himalaya (Tappon- nier et al., 1982, 1990), and collision-induced east-west mantle injection (Tamaki, 1995). The extrusion (injection) of deep, (thus) hot mantle flow to east and southeast Asia was the main cause of mantle perturbation and tem- perature upsurge, leading to mantle melting and wide-spread basaltic volcanism in the re- gion regardless of the regional lithospheric ex- tension factor is small (after Latin and White, 1990). The crystallization temperature was esti- mated for various mineral assemblages in al- kaline basalt-bearing spinel-lherzolites in the Western Highlands and Southeastern region of Vietnam using the geothermometer of De Hoog et al. (2009), showed a temperature range of 850-1150°C and pressure varying from 14 to 25 kbar (Hoang unpublished data). A geotherm constructed for Western High- lands, Southeastern region and Nghia Dan is shown in Figure 10, lying between two con- ductive geothermal lines in post- Phanerozoic continental lithosphere for heat flow of 80 to 90 mW/m2, respectively (after Pollack and Chapman, 1977). The Nghia Dan geothermal gradient (1020-1045°C corresponding to pres- sure range of 13-14.2 kbar, Table 1) is higher than that of conductive model and even higher as compared with that of Western Highlands and Southeastern region of Vietnam (Figure 10), suggesting a perturbation of thermal structure in the lithospheric mantle under the relating region (e.g. Hoang et al., 2014). Figure 10. A geotherm for Nghia Dan determined by spinel lherzolite (thick dashed line) using two-pyroxene geothermobarometers by Putirka et al. (1996, 2003) and Putirka (2008, 2017). Thin solid lines are model conduc- tive geotherms of continental areas with surface heat flows from 30 to 90mW/m2 (Pollack and Chapman, 1977). Thin dashed line (tm) thermally normal mantle adiabatic line ca. 1280°C, Thick continuous line (TM) continuous line, elevated temperature adiabatic, ca. >1380°C (Hoang unpublished. Shown for comparison are mantle xenoliths from Pleiku: red filled diamonds, Tro islet (Ile des Cendres): white filled square (Mali- novsky and Rashidov, 2015), Western Highlands and Southeastern region of Vietnam: filled rectangle (Hoang unpublished data) 4.4. Lithospheric mantle dynamics under Western Nghe An Any process causing isotope disequilibri- um, for example, melt extraction, source mix- ing or crustal assimilation to the isotopic sys- tems such as Sm-Nd, Lu-Hf, Rb-Sr can be traceable and determined, especially for the systems that are durable to secondary altera- tion such as Sm-Nd and Lu-Hf (Carlson and Lugmair, 1979, 1981; DePaolo and Wasser- burg, 1976; DePaolo, 1981). Vietnam Journal of Earth Sciences, 40(3), 207-227 223 Determination of Sm-Nd model age for al- kaline basalt-borne mantle xenolith is one of the ways to understand geodynamic processes causing isotope disequilibrium to the isotopic system before the xenolith being brought to the surface. Suppose the lithospheric mantle was formed about 3 Ga following the mantle melting to form the crust. Any major geody- namic event having occurred in the lithospher- ic mantle after the 3 Ga time causing isotopic perturbation and changing the evolutional trend of the isotopic system may be deter- mined using basic isotopic parent-daughter relationship (see footnote in Table 2). The model age for Nghia Dan spinel lherzolites varies between 122 and 127 million years (Table 2). Assuming the computed the age could be significant, it would mean that there was a major geodynamic event having oc- curred in the lithospheric mantle in the mid- Early Cretaceous (Aptian). Figure 11. Plots of 143Sm/144Nd vs. 143Nd/144Nd of Nghia Dan spinel lherzolite separated clinopyroxene (red filled diamond); shown for reference are clinopy- roxene separated from mantle xenoliths collected in the Western Highlands (Pham Tich Xuan, personal commu- nication). Host basalts (cross) are shown for comparison 5. Conclusions From the above descriptions the following conclusions may be drawn: The mantle xenoliths in Pliocene alkaline basalts in Nghia Dan (West Nghe An) are ge- ochemically depleted spinel lherzolites. They are residual entities of mantle peridotite melt- ing from 8 to 12% that became basic compo- nents of the lithospheric mantle before being brought to the surface by basaltic melt. Temperature and pressure estimates for mineral assemblages in Nghia Dan mantle xenoliths by various geothermobarometers vary from 1020 to 1050°C and 13 to 14.2 kbar (ca. 40 to 43 km), having much higher geo- thermal gradient as compared to that of the conductive model. This observation is sup- ported by previous studies of mantle thermal state under Western Highland and elsewhere in Vietnam that the mantle in the region is anomalously higher than normal by 50 to 100°C. Sm-Nd model age calculated for Nghia Dan mantle xenolith separated clinopyroxene yielded 127 and 122 Ma (mid-Early Creta- ceous). Assuming the model age is meaning- ful there would be a major geodynamic event having occurred under Western Nghe An dur- ing this period, large enough to cause pertur- bation in the Sm-Nd isotopic system. Acknowledgments This report was conducted under thematic study entitled “Study of elemental and isotop- ic geochemistry of mantle xenolith in alkaline basalt in Nghia Dan (Nghe An) with implica- tions to lithospheric mantle characteristics un- der the region” funded by Vietnam Academy of Science and Technology to TTH for the year 2017. The support is gratefully acknowl- edged. 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