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Major elements

Type 3 to 8 basalts and metabasalts show similar variations in the major elements, such as Mg# versus SiO2, TiO2, and P2O5 (Fig. F3). Higher grade Type 7 and 8 metabasalts have slightly lower K2O and Mg# than lower grade Type 3 and 4 basalts. Type 3 and 4 samples (312-1256D-176R-1, 21–24 cm, at 1276.29 mbsf and 186R-1, 33–36 cm, at 1319.82 mbsf) have slightly higher LOI values (2.36 and 1.18 wt%, respectively) and notably high K2O content (0.16 and 0.19 wt%, respectively) (Fig. F3B). This indicates that some elements such as K2O and Na2O had been affected by alteration. It is noted that K2O and LOI are well correlated for those with LOI <0.8 wt%, whereas K2O is scattered at higher LOIs. The plots characterized by very low LOI and K2O are exclusively highly recrystallized dikes such as Type 7 and 8 metabasalts. The high-LOI (>0.8 wt%) samples had no clear correlation between K2O, Na2O, and LOI. Irrespective to rock types, a strong negative correlation is apparent between Mg# and TiO2 (Fig. F3C). Gabbro 1 shows wider spectra of SiO2 and Mg# (SiO2 = 48.4–51.4 wt%, Mg# = 56–72) than Gabbro 2 (SiO2 = 49.9–51 wt%, Mg# = 61–63). The highest SiO2 value (51.4 wt%) occurs at 1440.21 mbsf in the lower middle of the Gabbro 1 subunit. This sample is a slightly altered orthopyroxene-bearing gabbro (Sample 312-1256D-221R-1, 61–64 cm; 1440.21 mbsf) with notably high P2O5 (0.17 wt%) and TiO2 (1.56 wt%) contents. Type 8 metabasalts from the lower dike screen in the lowermost part of Hole 1256D show very low P2O5 contents that plot away from the main trend of Mg# versus P2O5 variation (Fig. F3D), although they plot on the main trend in the TiO2-Mg# diagram. Downhole variations of major and trace elements are plotted in Figure F4. The SiO2, Mg#, TiO2, and P2O5 contents of the dike screen Type 8 metabasalts plot within the range of other metabasalts. Gabbro 1 shows a decreasing trend of TiO2 and P2O5 downhole, except for the sample with the highest P2O5 content.

Trace elements

Both Zr and Zr/Y show similar downhole variations to those of P2O5 (Fig. F4E, F4F). Type 3 to 7 metabasalts show N-MORB-like patterns for rare earth elements (REE), Sr, P, Zr, Hf, and Y (Fig. F5). Concentrations of these elements are higher in Type 6 and 7 basalts than in Type 3 and 4 basalts (Fig. F5A). U, Th, and Ba values of Type 3 to 7 basalts and metabasalts are slightly higher than the N-MORB values (N-MORB–normalized U = 0.8–2.08, Th = 1.09–2.54, and Ba = 0.99–5.67). The two low-grade metabasalts (Samples 312-1256D-176R-1, 21–24 cm, at 1276.29 mbsf and 186R-1, 33–36 cm, at 1319.82 mbsf) with high LOI contents are enriched in K, Ba, and Rb. The enrichment of these elements may be ascribed to alteration. The patterns of upper dike screen Type 8 metabasalts are in accordance with the patterns of Type 3 to 7 basalt and metabasalts. On the other hand, lower dike screen metabasalts have very low concentrations of REE, Zr, Hf, P, U, and Th (Fig. F5B). The downhole variation of the N-MORB–normalized La/Sm ratio (La[N]/Sm[N]) shows that the lower dike screen has notably low La[N]/Sm[N] ratios (0.51–0.57) (Fig. F4G). Gabbros 1 and 2 are characterized by Zr, Hf, and P depletion, except for the high-P2O5 sample (312-1256D-221R-1, 61–64 cm; 1440.21 mbsf) with a Zr-, Hf-, U-, and Th-enriched pattern (Fig. F5C). Gabbro 1 has higher La[N]/Sm[N] ratios (0.97–1.42) than Gabbro 2 (0.76–0.90). The Gabbro 1 La[N]/Sm[N] ratio is inversely correlated with REE, Zr, Hf, and P concentrations (Fig. F6).

Mineral compositions


Clinopyroxene Mg# ranges from 64.1 to 86.6. The variation of clinopyroxene Mg# versus TiO2 shows two domains of high- and low-TiO2 contents (Fig. F7). Phenocryst, microlite, and microgranular clinopyroxene of basalts and metabasalts show a high-TiO2 trend, with TiO2 increasing with decreasing Mg# (Fig. F7A). Amphibole intergrowth–type and secondary-type clinopyroxene of the metabasalts are plotted in the low-TiO2 (<0.3 wt%) field (Fig. F7A, F7B). Secondary-type clinopyroxenes are found in Type 7 metabasalt, the bottom of Gabbro 1, the upper dike screen, the top of Gabbro 2, and the top of the lower dike screen (Fig. F8A). Igneous-type clinopyroxene of Gabbro 1 shows the high-TiO2 trend (Fig. F7B). Gabbro 2 igneous-type clinopyroxene also plots on the high-TiO2 trend, except some clinopyroxenes plot in the low-TiO2 field of amphibole intergrowth–type plots (Fig. F7B). The compositional range of the igneous-type clinopyroxenes in Gabbros 1 and 2 approximately equals the metabasalt phenocrysts and microlites. Gabbro 2 amphibole-type clinopyroxene has a lower TiO2 than the igneous-type clinopyroxene of Gabbros 1 and 2. Some amphibole-type clinopyroxenes overlap the secondary-type clinopyroxene. The low-TiO2 amphibole-type clinopyroxene does not preserve the primary compositions but is of metamorphic origin. Subophitic and coarse-grained network domains in Gabbro 1 clinopyroxenes have different composition. Clinopyroxene oikocryst of subophitic domains have constant Mg# (80.7–82.4). In contrast, coarse-grained network domains show more evolved and variable Mg# (66.3–76.3) than the subophitic domains. Gabbro 1 igneous-type clinopyroxene has negative correlation between TiO2 and Mg# and the lowest Mg# and the highest TiO2 occurs in a coarse-grained network domain at the top of the Gabbro 1 subunit (Figs. F7B, F8A, F8B). Gabbro 2 clinopyroxene is slightly evolved toward the top of the Gabbro 2 subunit (Fig. F8A).


Orthopyroxenes show a broad positive correlation between TiO2 and Mg#, except for some analyses with very low TiO2 (Fig. F9A). A disseminated oxide olivine gabbro (Sample 312-1256D-215R-2, 56–59 cm; 1417.69 mbsf) of Gabbro 1 includes these exceptionally low TiO2 orthopyroxenes, which occur as a cluster within an olivine pseudomorph. Gabbro 1 orthopyroxenes have a wider range of Mg# (56.2–75) than those from Gabbro 2 (62.2–73.2). Gabbro 1 orthopyroxene TiO2 decreases uphole through Gabbro 1 with decreasing Mg# (Fig. F8C). This is opposite behavior from the TiO2 versus Mg# plots of the clinopyroxene (Figs. F8C, F8D, F9A). These results strongly suggest that the orthopyroxene of Gabbro 1 is of metamorphic origin.

The lowest and highest Mg# orthopyroxenes occur at the top and bottom of Gabbro 2. In the lower part of Gabbro 2, TiO2 content increases uphole with decreasing Mg#. In contrast, TiO2 content in the upper part decreases uphole with decreasing Mg# (Figs. F8C, F8D). This suggests the possibility that the orthopyroxene in the lower part of Gabbro 2 is also of metamorphic origin.

Orthopyroxenes of Type 7 and 8 metabasalts have a similar composition to Gabbros 1 and 2 with respect to Mg# and TiO2. Type 8 metabasalts of the lower dike screen have a slightly higher Mg# than Type 7 metabasalts (Fig. F9A).


Olivine decreases in forsterite (Fo) contents with decreasing NiO. Gabbro 2 olivines have slightly lower NiO2 than those of Gabbro 1 at a given Fo. A sample from the upper part of Gabbro 1 (Sample 312-1256D-215R-1, 20–23 cm; 1415.92 mbsf) has olivine with a much higher Fo content (Fo = 76.6–80.2) than the other Gabbro 1 (Fo = 64.8–70.4) (Fig. F9B). Gabbro 1 olivines usually show normal zoning. In contrast, Gabbro 2 olivines are reversely zoned with Fo contents ranging from 67.3 to 74.5 (Fig. F10).


Plagioclase ranges in An content from 11 to 83, and most compositions fall within An40–80. Very low An content (<An40) is observed in some plagioclase rims of the upper part of Gabbro 1. These low-An plagioclases (<An40) are not primary magmatic products and may be ascribed to secondary albitization. Plagioclase forms two groups in the An-MgO variation diagram: high- and low-MgO groups. In the high-MgO group, MgO content increases with increasing An%, whereas in the low-MgO group, it does not vary with increasing An% (Fig. F9C, F9D). Plagioclases of the upper and lower dike screen have very low MgO. Most of the plagioclase does not exceed the detection limit of MgO (<0.02 wt%). The laths and phenocrysts of Type 3 basalts belong to the high-MgO group. The MgO content stays almost constant irrespective of An content, except the laths of one low-An sample (312-1256D-176R-1, 21–24 cm; 1276.29 mbsf) (Fig. F9C). The plagioclases of Type 6, 7, and 8 metabasalts belong to the low-MgO group, where the MgO content does not exceed 0.1 wt% over the entire range of An. Gabbro 1 and 2 plagioclases have uniformly low MgO contents over a wide range in An. These plagioclases generally show normal zoning (Fig. F8E).