IODP Expedition 360
SW Indian Ridge Lower Crust and Moho
Daily Science Report for 29 January 2016
Location: Underway to Port Louis, Mauritius
Science Update: Scientists submitted finalized site reports. Expected time of arrival at the Port Louis, Mauritius, pilot station is 0930 h on 30 January.
Daily Science Report for 28 January 2016
Location: Underway to Port Louis, Mauritius
Science Update: Scientists worked on finalizing site reports. Expected time of arrival at the Port Louis, Mauritius, pilot station is 0700 h on 30 January.
Daily Science Report for 27 January 2016
Location:
Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Underway to Port Louis, Mauritius
Science Update: The Reverse Circulation Junk Basket (RCJB) was washed to the bottom of the hole in an attempt to fish for the lost sleeve retainer ring of the mechanical bit release (MBR) top connector lost during logging operations. A 30-barrel mud sweep was pumped and the string was worked up and down three times. At 0015 h, the steel ball was dropped down the drill pipe to trigger the reverse circulation. After circulating and rotating on bottom for 80 min, the top drive was set back and the drill string was retrieved, clearing the rig floor at 0545 h. The sleeve retainer ring was not recovered; however, its presence at the bottom of Hole U1473A was confirmed with the recovery of three rounded cobbles that have clear tool marks indicating milling on and with the sleeve retainer.
With no time left in the expedition, the remaining drill collars were laid out to the drill collar racks and the rig floor was cleaned up and secured for transit. Final Hole U1473A depth is 789.7 mbsf. At 0440 h the single acoustic positioning beacon was recovered. The thrusters and hydrophones were raised and the transit to Port Louis, Mauritius, began at 0830 h on 27 January.
Sampling for postcruise research was completed. Scientists worked on finalizing site reports.
Daily Science Report for 26 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: After reentering Hole U1473A with an RCB bit last night, the drill string was washed and reamed to the bottom of the hole at 789.2 mbsf, which included a 30-barrel high viscosity mud sweep and deployment and recovery of a wash barrel. A core barrel was dropped at 0315 h and an attempt was made at coring. Applying a maximum of 2–3 klb of weight and 350 A of torque, the driller noted erratic torque on bottom. The drill string was retrieved, clearing the rig floor at 1020 h. Upon inspection, the drill bit showed signs of contact with metal—likely the missing part of the mechanical bit release (MBR) that was lost during our last coring and logging run. The Reverse Circulation Junk Basket (RCJB) was made up with two boot type junk baskets and deployed in an attempt to retrieve the missing part. The bit reentered Hole U1473A at 1420 h. At 1745 h, when the top drive was picked up to lower the pipe to the bottom of the hole, power to the top drive was lost. The circulation head was installed so we could keep circulating and raising/lowering the drill string while the top drive electrical problem was being repaired. At 2300 h the top drive was picked up again and the drill string was washed towards the bottom of the hole to complete the fishing attempt.
Scientists completed core observations and laboratory measurements, and worked on site reports. Research plan discussions were held and sampling for postcruise research is ongoing.
Daily Science Report for 25 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: The third wireline logging run with the Ultrasonic Borehole Imager (UBI) was completed from 784 mbsf (near bottom of hole) to 29 mbsf (the bottom of the pipe) and the tool was back on the rig floor at 0245 h. The drill string was retrieved and cleared the rig floor at 0555 h. Upon inspection of the mechanical bit release (MBR) top connector we found that the sleeve retainer ring was missing. The most likely situation was that the ring’s safety bolt sheared and the retainer dropped to the bottom of the hole; we had to retrieve it before we could resume coring. We made up and deployed a magnet fishing tool, reentered Hole U1473A at 1012 h, and reached the bottom of the hole at 789.2 mbsf. A 50-barrel mud sweep was pumped and the fishing magnet was worked up and down near the bottom of the hole according to protocol (1430–1530 h). The drill string was retrieved and cleared the rig floor at 1850 h. The sleeve retainer was not recovered. Because a small possibility existed that the retainer was dropped on the seafloor rather than in the hole, we decided to attempt a coring run. An RCB bit was made up and deployed, reentering Hole U1473A at 2229 h.
Scientists completed laboratory measurements and observation and worked on site reports. Research plan discussions were held and sampling for postcruise research is ongoing.
Daily Science Report for 24 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: The day was spent wireline logging in Hole U1473A, which currently has a total depth of 789.2 mbsf.
The first run with the triple combo tool string, which measures bulk density, bulk resistivity, spectral natural gamma radiation, and magnetic susceptibility, was deployed at 0045 h and reached 788 mbsf. The run included a 150 m upward calibration pass from the bottom of the hole and the main upward logging pass from 788 mbsf. The tool string was back at the rig floor at 0530 h.
The second run with the FMS-sonic tool string, which measures microresistivity to generate a partial borehole wall image, and propagation of compressional and shear waves, was deployed at 0815 h and also reached 788 mbsf. The run included two full logging passes from 788 mbsf up to the end of drill string at ~40 mbsf. The tool string was back at the rig floor at 1530 h.
The third run with the Ultrasonic Borehole Imager (UBI) tool string, which generates a borehole wall image using ultrasound, was deployed at 1800 h and reached 788 mbsf. Deployment of the UBI experienced some difficulties and by midnight the upward pass had reached ~300 mbsf.
Scientists continued working on the core material. Core 89R consists mostly of coarse-grained subophitic olivine gabbro, with subordinate coarse-grained granular disseminated-oxide olivine gabbro and few felsic veins. In Cores 87R to 89R, alteration intensity ranges from slight to moderate. More intense alteration is characterized by milky white plagioclase replacement. Several coarse-grained intervals show slight alteration. Cores 88R and 89R are characterized by patchy to planar grain size layering and the planar layering is mostly inclined. Magmatic fabrics, which commonly occur together with grain size layering, are inclined, feeble to moderate in intensity, and defined by elongated plagioclase and pyroxene. The few felsic veins observed include a nascent magmatic breccia and an en-echelon pair of veins. Crystal-plastic fabrics are restricted to embryonic, sub-horizontal, leucocratic, millimeter-thick bands. Alteration veins, including one amphibole vein, are rare.
The microbiologists measured ATP activity in subsamples of the latest 17 samples. Over the interval of Cores 68R–86R, veins were primarily felsic rather than carbonate, as in the shallower part of Hole U1473A. Of the samples containing felsic veins, many had green inclusions (perhaps chlorite). Interestingly, we recorded positive ATP readings from 13 samples in this interval, with the top three values 5.34, 2.89, and 1.43 pg/ml. These are some of the highest ATP concentrations in Hole U1473A to date.
Daily Science Report for 23 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We retrieved Cores U1473A-87R through 89R (760.1–789.2 mbsf) and recovered 28.8 m (99%), pumping 30-barrel high viscosity mud sweeps every 5 m and a 50-barrel mud sweep at the end.
At 1445 h we began our wireline logging program for Hole U1473, which will include three tool string deployments. First, the triple combo tool string will measure bulk density, bulk resistivity, spectral natural gamma radiation, and magnetic susceptibility. Second, the FMS-sonic tool string will measure resistivity using four multi-probe array pads to generate a partial borehole wall image, as well as propagation and waveforms of compressional and shear waves. Third, the Ultrasonic Borehole Imager (UBI) will generate a complete borehole wall image using ultrasound. All three tool strings will measure total natural gamma radiation and borehole fluid temperature.
With the bit near the bottom of the hole, we displaced the hole with 290 barrels of drill water (fresh water) to enhance the salinity contrast between a relatively small amount of formation pore fluid and a relatively large amount of borehole fluid. We raised the drill string to a bit depth of 205 mbsf and displaced the upper part of the hole with another 49 barrels of drill water. Next we pulled out of the hole, clearing the seafloor at 1830 h, and offset the ship 30 m east. Using two wireline runs with the rotary shifting tool (RST), we dropped the drill bit to the seafloor at 1930 h. Next we repositioned the ship and reentered Hole U1473A at 2145 h, lowering the end of the pipe to 45.5 mbsf. The subsea camera system was recovered and the rig floor was prepared for logging. At 2330 h we began to rig up the logging tools for the triple combo run.
Scientists continued to describe cores and corresponding thin sections. Cores 87R to 88R consist mostly of coarse-grained subophitic olivine gabbro, with subordinate coarse-grained granular disseminated-oxide olivine gabbro. In Cores 83R to 86R alteration intensity varies from moderate to extensive. Primary mineral phases in rocks with intense alteration are extensively replaced by greenschist assemblages. Cores 85R–87R have a few discrete mm to cm thick shear zones, which are typically incipient, leucocratic, subhorizontal, and have a reverse sense of shear. They are often associated with local melt accumulation. In some intervals, the shear zones are Fe-Ti oxide-rich. Magmatic fabrics are weak and are defined by elongated plagioclase and pyroxene. Rare felsic veins range from inclined to horizontal and are up to 4 cm thick. Alteration and fractures are limited to Core 85R.
Daily Science Report for 22 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We retrieved Cores U1473A-83R through 86R (731.0–760.1 mbsf) and recovered 27.5 m (94%), pumping 30-barrel high viscosity mud sweeps every 5 m.
Cores 82R to 86R consist mostly of coarse-grained subophitic olivine gabbro, with subordinate medium- to coarse-grained granular and subophitic gabbros containing oxide, rare medium- to coarse-grained granular olivine-bearing gabbro intervals, and felsic veins. The interval has m thick shear zones consisting of cm thick subhorizontal to inclined Fe-Ti oxide-rich bands that are either discrete or part of a thicker shear zone. One 1.5 m wide shear zone that overprints primary layering with a porphyroclastic fabric is crosscut by a subhorizontal, Fe-Ti oxide-rich mylonite near its base. Undeformed Fe-Ti oxide-rich zones occur in discrete, inclined bands commonly associated with coarse-grained pyroxene. Rare alteration veins are inclined and associated with m thick zones of alteration. Alteration intensity is mostly moderate. The replacement of plagioclase is dominated by milky white plagioclase.
Daily Science Report for 21 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: At 0200 h, after retrieving Core U1473A-81R (712.8–721.3 mbsf) recovering 7.6 m (90%), drill bit usage had reached 40 h and it was time for a pipe trip to replace the bit. The drill string was retrieved, clearing the rig floor at 0635 h. We made up another RCB coring bit, this time with a mechanical bit release, which will allow us at the end of this coring run to pull out of the hole, drop the bit on the seafloor, reenter the hole, and conduct wireline logging without the need for a full pipe trip. The drill string was redeployed for Run 21 and reentered Hole U1473A for the twentieth time at 1038 h. We reached the bottom of the hole without drag or excessive torque and, after a 30-barrel mud sweep, resumed coring at 1515 h. By the end of the day, Core 82R (721.3–731.0 mbsf) had recovered 9.8 m (101%).
Cores 80R to 81R consist mostly of subophitic coarse-grained olivine gabbro with subordinate coarse-grained granular gabbro containing oxide. A shear zone in Section 81R-6 is comprised of inclined, dm thick, leucocratic porphyroclastic horizons culminating in a 5 cm thick Fe-Ti oxide-rich mylonite and ultramylonite horizon near the base of the shear zone. Several Fe-Ti oxide and sulfide-rich horizons were observed both within and outside of the shear zone. Discrete felsic veins range from subhorizontal to vertical and mm to dm thick. Section 80R-3 has an inclined pull apart felsic vein with a reverse-sense of shear.
In Cores 78R to 80R, alteration intensity is mostly moderate. Milky white plagioclase was observed throughout the cores and was particularly conspicuous in mylonitic zones.
We conducted three more contamination test for microbiology work, one using the PFMD tracer and two using the PFMC tracer for comparison.
Daily Science Report for 20 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We retrieved Cores U1473A-77R through 80R (680.7–712.8 mbsf) and recovered 31 m (97%). We pumped 30-barrel high-viscosity mud sweeps every 5 m.
Cores 76R to 79R consist mostly of subophitic or granular fine- to coarse-grained olivine gabbro with subordinate oxide-bearing olivine gabbro and rare felsic veins. The cores are characterized by the continuation of an 11 m thick porphyroclastic to ultramylonitic shear zone. Inclined fine- and coarse-grained domains become progressively sheared and develop into coarse-grained, shallowly to moderately dipping mylonite. The culmination of the shear zone is an inclined, 5 cm thick, normal sense, Fe-Ti oxide-rich ultramylonite surrounded on either side by 5 cm thick leucocratic mylonite. The remainder of the cores have several cm thick intervals of Fe-Ti oxide-rich mylonite, irregular grain size patches, weak and inclined magmatic fabrics, several magmatic veins, and actinolite veins. Magmatic veins have a variety of thicknesses and dips; one forms along a mylonitic contact between coarse- and fine-grained gabbro. Alteration veins range from subhorizontal to subvertical and often have a leucocratic halo.
In Cores 74R to 77R alteration intensity ranges from moderate to substantial. XRD results show the green minerals associated with felsic veins are chlorite or amphibole, likely actinolite/tremolite.
Daily Science Report for 19 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We continued to deploy the RCB coring assembly and resumed coring at 0300 h. We retrieved Cores U1473A-73R through 76R (651.9–680.7 mbsf) and recovered 27 m (94%).
Cores 73R to 75R consist mostly of subophitic or granular medium- to coarse-grained olivine gabbro; most are coarse-grained. Zones containing oxides are rare. Sulfide is common in this section and occasionally associated with oxide-pyroxene patches. Igneous layering, indicated by grain size and sometimes by modal variations, is still present in Core 73R but seems to disappear in the deeper part of this section. Alteration intensity in Cores 71R to 73R ranges from moderate to substantial. Milky white plagioclase is conspicuous throughout these cores.
Cores 73R–75R are characterized by few felsic veins, minor microfracturing in plagioclase and pyroxene, and limited crystal-plastic deformation or magmatic fabrics. In Section 75R-4, a 20 cm thick mylonitic to ultramylonitic zone occurs that grades from shallowly dipping mylonite into moderately to steeply dipping porphyroclastic shear zone, then into shallowly dipping mylonite to steeply dipping oxide-rich ultramylonite. Several 1 cm thick oxide-rich mylonites also crosscut the cores. Alteration veins are rare and, if present, are inclined and filled with actinolite. Brittle deformation occurs as minor plagioclase microfracturing and, in some cases (e.g., Section 74R-2), as fractures in coarse-grained pyroxene crystals.
The Microbiology team ran one more tracer contamination test. However, no new samples have been taken in a few days because of the type of rock material recovered.
Daily Science Report for 18 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We continued to retrieve the drill string for a bit change, thus making good use of the pause in coring due to excessive ship heave, clearing the rig floor at 0240 h. A new RCB C-7 drill bit was installed and lowered toward the seafloor, and we waited 0.5 h for the swell to subside with the bit half way down to the seafloor. We then lowered the bit the rest of the way to the seafloor and reentered Hole U1473A at 0653 h. RCB coring resumed at 1030 h and Core 72R was advanced from 648.4 to 651.9 mbsf with varying torque and mud sweeps every few meters. At 1215 h the drill pipe became stuck with the bit at 651.9 mbsf, with loss of rotation despite applying torque up to 800 A and overpull up to 175 klb. Eventually the pipe was freed at 1400 h by working the bit up to 648 mbsf where rotation and circulation was reestablished. As we pulled up to 555 mbsf the torque gradually normalized. We think that a significant piece of rock fell from higher up in the hole onto the tapered drill collar and the drill string became free when this was gradually broken up. Core U1473A-72R (648.4–651.9 mbsf) was retrieved at 1615 h, recovering 3.6 m (103%). We decided to retrieve and inspect the drill bit to ensure there was no damage to the bit and minimize the overall risk to the hole. The bit cleared the rig floor at 2015 h. The bit looked as new and was redeployed, reentering Hole U1473A at 2350 h.
Cores 70R to 72R (635.7–651.9 mbsf) consist mostly of subophitic fine- to coarse-grained olivine gabbro; most are coarse-grained. When gabbros were affected by crystal-plastic deformation, the primary texture was replaced by a granular texture. Zones containing oxides are very rarely observed. Igneous layering is common and indicated by grain size and sometimes by modal variations. Felsic veins are rare. In Cores 68R to 70R alteration intensity varied from slight to extensive. In a mylonitic zone in Core 69R, plagioclase (which shows more intense alteration than olivine and pyroxene) was replaced by secondary plagioclase.
Cores 70R–72R are characterized by irregular grain size variations and layering, limited crystal-plastic deformation, and discrete zones of alteration and veining. The grain size variations and layering range from fine- to coarse-grained and have subhorizontal to moderate dips. Several of the layers have weak to moderate magmatic fabrics defined by the shape preferred orientation of plagioclase and pyroxene and are subparallel to the layering. Only one 2 cm thick crystal-plastic fabric interval was observed, which is moderately to shallowly dipping and porphyroclastic. Sections 70R-3 and 4 have background greenschist grade alteration with intervening veins that have steep dips. In thin section, the steep veins are often sheared. Brittle deformation is limited with very few discrete fractures and limited microfracturing in plagioclase.
Daily Science Report for 17 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We retrieved Cores U1473A-67R through 71R (606.6–648.4 mbsf) with a recovery of 41.3 m (99%). At 2215 h we had to suspend coring and pull out of the hole because the ship’s heave of up to 5.7 m didn’t allow safe coring anymore. Given the time spent on the bit already we began retrieving the drill string all the way for a bit change rather than just suspending it near the seafloor while waiting on weather.
Cores 67R to 69R consist mostly of subophitic medium- to coarse-grained olivine gabbro; most are coarse-grained. When gabbros were affected by crystal-plastic deformation, the primary texture was replaced by a granular texture. Zones containing oxides are very rarely observed and are associated with deformation. Igneous layering is common and indicated by grain size and modal variations. Several felsic veins also occur. Cores 66R and 67R are rather fresh and exhibit only slight alteration. Alteration is more pronounced near felsic intrusions. Alteration phases commonly observed were chlorite, amphibole, talc, and secondary plagioclase.
Core 67R is characterized by several shallowly dipping, discrete ultramylonite zones, 1 cm in thickness, crosscutting the unfoliated gabbro. In Core 69R the deformation intensifies. Section 69R-1 shows a 1.5 m thick, normal-sense shear zone centered by two ultramylonites juxtaposed against an isotropic gabbro. The sharp contact against the undeformed gabbro indicates that ultramylonite formed near or at the brittle-ductile transition. In Section 69R-5, the undeformed gabbro transitions again into a 5 m wide mylonitic deformation zone with a 3 cm thick oxide-rich ultramylonite core. Three deformation stages are identified in Cores 67R to 69R by the following macroscopic observations: (i) crosscutting relationship between two crystal-plastic fabrics in Core 69R-5, (ii) mylonite clasts within a subhorizontal felsic vein in Section 68R-1, and (iii) a felsic vein in Section 68R-4 cut by a narrow shear band. The latter two observations are indicative of a continuous progression of crystal-plastic deformation and felsic melt infiltration. Brittle deformation is characterized by discrete, moderately dipping fractures and horizontal fractures filled with clay minerals.
The Petrophysics team resumed measurements on seven discrete samples from Cores 58R and 59R. In these samples, sonic velocity (Vp) ranges from 6580 to 7000 m/s and is 6840 m/s on average. Vp anisotropy is very low (0.3% to 2.4%). Grain density and porosity range from 2.96 to 2.99 g/cm3 and from 0.1% to 0.4%, respectively.
Daily Science Report for 16 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We retrieved Cores U1473A-65R and 66R (587.2–606.6 mbsf) with a recovery of 18.1 m (93%), pumping 30-barrel high-viscosity mud sweeps every 5 m. At 0700 h we retrieved the drill string for another bit change, clearing the rig floor at 1055 h. A new C-7 RCB bit was made up and, following some rig servicing tasks, we reentered Hole U1473A at 1424 h. Coring resumed at 1830 h.
Cores 64R to 66R consist mostly of granular and subophitic fine- to mostly coarse-grained olivine gabbro. Subordinate subophitic coarse-grained olivine-bearing gabbro is also present. Zones containing oxides are very rare. Igneous layering is common and highlighted by grain size and modal variations. Some of the fine to coarse grain size variations are discordant and patchy and the fine grained intervals may be intrusive units. Several felsic veins are moderately to steeply dipping and include diffuse, tapered, and branched types. Some of the veins formed in pull-apart structures related to shear zones, for example, in Section 66R-5. Crystal-plastic deformation is minor, restricted to mm to cm thick subhorizontal bands that crosscut the layering. Some of the shear bands are oxide-rich, for example in Section 65R-7. Alteration veins are absent in this interval. Several discrete fractures occur, two with slickenlines that have steep rakes. Sections 61R-1 to 64R-1 show typical greenschist alteration with talc, amphibole, and chlorite present. Static background alteration intensities in Sections 64R-2 to 65R-7 are fresh to slight.
The Petrophysics team continued measurements on whole-round and section half data loggers. Magnetic susceptibility remains relatively low (<500 × 10−5 SI on average), indicating an overall relatively low content of Fe-Ti oxides. Natural gamma radiation remains very low (≤1 counts/s), except for a few peaks corresponding to felsic veins and patches.
Daily Science Report for 15 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: Today we cut Cores U1473-61R through 65R (548.4–587.2 mbsf) and recovered 25.3 m (65%). Eight 30-barrel high-viscosity mud sweeps were pumped, one every 5 m cored.
Cores from 60R to 63R consist mostly of subophitic to granular olivine gabbro with subordinate olivine-bearing gabbro. Grain size is variable and three classes are recognized: very coarse-grained (mostly subophitic), coarse-grained (mostly subophitic), and medium-grained (mostly granular). Contacts between these gabbros are sheared and original relationships cannot be determined. Most granular gabbros are deformed, and the primary magmatic texture cannot be determined. Some rare coarse-grained oxide-bearing gabbro and disseminated-oxide olivine gabbro are also present and strongly deformed. Several felsic veins and patches also occur.
The gabbros are overprinted by an alternating weak to moderate crystal-plastic deformation and crosscut by later mylonitic to ultramylonitic shear bands. The weak crystal-plastic fabric is moderately to shallowly dipping and is more evident in finer grained intervals. The mylonitic to ultramylonitic shear bands range from mm to dm thick zones with both normal and reverse sense of shear. In Section 61R-4 (~550 mbsf), a steep leucocratic 7 cm thick paired ultramylonite and coarse-grained mylonite crosscut a medium-grained porphyroclastic crystal-plastic fabric. One greenschist grade shear zone was observed in Section 63R-2. Magmatic veins range from discrete with sharp boundaries to sheared or brecciated; a magmatic breccia is present in Section 62R-1. The majority of alteration veins are filled by amphibole and are moderately to steeply dipping, some with haloes, others are sheared. Few carbonate veins are are observed in intervals characterized by extensive alteration. Nine slickenlines were observed with moderate to steep rakes.
The Microbiology team processed five more samples for analyses. Another tracer contamination test was run by sampling drill fluids, sample exterior prior to cleaning, sample exterior after cleaning, and sample interior (triplicate samples for all). Analyses of those samples are underway. ATP was measured in four samples from Sections 46R-1 through 51R-1, and detected in two samples from Sections 47R-1 and 48R-2 (0.1 and 0.4 pg/cm3, respectively).
Daily Science Report for 14 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: After completing the slip and cut maintenance on the drilling line, we reentered Hole U1473 with a new RCB bit at 0210 h, reamed down from 510.6 to 519.2 mbsf, and circulated a 30-barrel mud sweep. When the wash barrel was recovered at 0730 h, it contained 0.4 m of core, which was registered as Core U1473A-57R (519.2–519.6 mbsf). We resumed RCB coring and Cores 58R–60R penetrated from 519.6 to 548.4 mbsf and recovered 21.1 m (73%).
Cores 57R–59R consist of olivine gabbro and olivine-bearing gabbro. The section is deformed with alternating bands of distributed porphyroclastic fabrics and weakly deformed to undeformed fine- to medium-grained gabbro. The fine-grained intervals have a more pronounced crystal-plastic fabric, even though in thin sections it is evident that the coarser grained intervals exhibit similar intensity of deformation. The section also has up to 20 cm thick mylonitic zones, and all fabrics are crosscut by moderate ~0.5 cm thick mylonitic shear bands. In Section 58R-8 the igneous contact between coarse and fine grained gabbro is marked by an ultramylonite. Several moderately dipping magmatic veins are in some cases crosscut by mm thick mylonitic bands. Alteration veins are rare and filled with low-temperature clay. Carbonate veins are noticeably absent compared to cores further upsection. Fractures are abundant and many have vein fill forming slickenlines. The slickenlines are commonly defined by amphibole and range from shallow (6°) to steep (90°).
Daily Science Report for 13 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We continued to drill ahead with a 9.875 inch tricone bit from 483.0–519.2 mbsf. Drilling conditions were reasonably good; however, the rate of advance barely matched, and certainly did not exceed, the rates we had achieved with coring earlier in the hole. Not seeing a significantly faster penetration that would justify drilling over coring, we decided in the afternoon to switch back to coring. The drill string was retrieved and cleared the rig floor at 2010 h. A new 9.785 inch RCB C-7 bit was made up and deployed to near the seafloor, when deployment was suspended to allow the rig crew to slip and cut 115 ft of drilling line as part of their routine rig maintenance.
Daily Science Report for 12 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We continued to deploy of the fishing magnet in an attempt to recover the metal junk at the bottom of the hole. We reentered Hole U1473A at 0321 h and tagged the bottom of the hole at 0615 h without encountering any fill. Two 30-barrel mud sweeps were pumped while working the magnet and its milling shoe according to protocol. The drill string was retrieved, clearing the rig floor at 1105 h, and we found that the fishing magnet had successfully recovered the roller cone we suspected to have plugged the RCB bit and prevented advancement and recovery of core the previous day. Some gravel was recovered as well in the boot type junk baskets, curated as Run15-FMM.
We decided to drill ahead with a tricone bit (without coring) for an interval not to exceed 100 m. The tricone is a more robust bit than the RCB coring bit and therefore more suitable to mitigate potential issues near the bottom of the hole, such as reaming a slightly tight hole. We also wanted to get an idea of how much faster drilling would deepen the hole compared to coring, and we wanted to do that before reaching the modeled depth interval (~600 mbsf or deeper) where we expect to penetrate the magnetic polarity transition from reversed to normal.
We started to make up a 9.875 inch tricone drilling bit at 1200 h, reentered Hole U1473A at 1545 h, washed and reamed down to the bottom of the hole (481.7 m), began to drill the new hole at 2300 h, and had deepened the hole to 483.0 mbsf by midnight.
Daily Science Report for 11 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We continued to wash the RCB assembly to the bottom of Hole U1473A, which included a 50-barrel mud sweep after the bit encountered 0.5 m of fill at the bottom of the hole. Our attempted to cut Core 53R (469.6–470.6 mbsf) from 0015–0400 h advanced only 1 m and achieved no recovery. Erratic torque was indicated and repeated mud sweeps were pumped. Then we attempted to cut Core 54R (470.6–480.3 mbsf), advancing the full 9.7 m but not recovering any core. Based on the assumption that rock had jammed the bit throat, we deployed a deplugger (1045–1130 h) in an attempt to dislodge the obstacle. This was followed with another coring attempt (Core 55R, 480.3–481.7 mbsf, 1.4 m advance, no recovery). Another deplugger was run, and another core barrel was dropped; however, no penetration could be achieved. At 1745 h we decided to recover the drill string and the bit cleared the rig floor at 2150 h. Upon inspection, the bit had lost most of the carbide buttons on the roller cones, particularly towards the inside of the bit, and was missing one of the four core guides. The bit had obviously encountered metal at the bottom of the hole, at a minimum its own missing core guide, and possibly one of the missing cones lost in the hole a few days ago. At 2215 h we began to make up and deploy a fishing magnet with mill tooth guide and two boot type junk baskets in an attempt to recover the metal junk at the bottom of Hole U1473A.
Daily Science Report for 10 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: The reverse circulation junk basket (RCJB) deployed last night (Run 12 of the RCJB) was worked up and down near the bottom of Hole U1473A, and reverse circulation was activated by dropping the steel ball down the hole. After some more working on the bottom, the drill string was recovered, clearing the rig floor at 0500 h. Some gravel was recovered but not the missing roller cone. The RCJB was made up with a new mill tooth cutting shoe and catcher assembly and deployed once more (Run 13), reentering Hole U1473A at 0846 h. After a 30-barrel mud sweep the RCJB was worked and activated again according to protocol. The drill string was retrieved, clearing the rig floor at 1605 h. This time the recovery consisted of a large cobble and the lost roller cone. An RCB assembly was made up and deployed, reentering Hole U1473A at 2037 h. By midnight the bit was near the bottom of the hole.
The material recovered during RCJB Runs 12 and 13 consist mostly of coarse-grained granular olivine-bearing gabbro with subordinated oxide gabbro. Although always coarse-grained, grain size is variable. Run 12 material has minor fracturing in plagioclase within oxide gabbro and a fragment of a felsic vein that has a partial coat of a calcite vein. Run 13 material has several pieces of a matrix-rich fault breccia and a cobble with a weak crystal plastic fabric.
XRD analysis of carbonate-rich zones sampled from Core 48R show that these materials are predominantly made up of calcite, with plagioclase, pyroxene, and amphibole making up the clasts. XRD results also show that the conspicuous brass-colored pyroxene grains are made up of relic pyroxene altered into amphibole, chlorite, and talc.
The average composition of Hole U1473 down to 469.6 mbsf is as follows: 75% olivine gabbro, 2% gabbro, 13% disseminated-oxide gabbro, 2% oxide-bearing gabbro, and 6% oxide gabbro. The average modal composition of the hole is 8% olivine, 55% plagioclase, 36% clinopyroxene, and 1% oxide.
Daily Science Report for 9 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: Today we recovered Cores U1473A-50R to 52R (449.6–469.6 mbsf) with a total recovery of 9.7 m (48%). Repeated high-viscosity mud sweeps and reaming were required to sustain coring. While cutting Core 52R, erratic high torque made us stop coring and retrieve the drill string, which cleared the rig floor at 1545 h. We discovered that the RCB C-7 bit was missing one of the four roller cones. A reverse circulation junk basket (RCJB) assembly was made up and deployed in an attempt to recover the cone. The bit reentered Hole U1473A at 2007 h and was washed to the bottom of the hole by midnight.
Cores 49R to 52R consist mostly of coarse-grained subophitic olivine gabbro, crosscut by dikes that are commonly associated with felsic veins. The rocks show variable alteration intensity, ranging from slight to extensive. Carbonate veining and reddish clay replacement after olivine are conspicuous. Most intervals in these cores are undeformed and so preserve primary magmatic textures and fabrics, including a 7 cm wide fine grained zone within coarse grained gabbro that may represent magmatic layering. Crystal plastic deformation only occurs as a 40 cm thick interval of near vertical porphyroclastically textured gabbro in Section 50R-3 and as a pair of 5 mm thick, en-echelon, normal-sense, subvertical shear bands with intermittent leucocratic patches. Section 51R-1 has a 25 cm thick leucocratic, amphibole-rich felsic vein and magmatic breccia associated with a late, 3 cm wide diabase dike. Felsic veins are common in Section 50R-2. Moderate fracturing in plagioclase and associated alteration occurs within up to 60 cm thick intervals. The occurrence of several steeply dipping carbonate veins and three carbonate-rich breccias in cores of relatively low recovery may indicate that this part of Hole U1473A cuts through a high angle fault system. The carbonate-rich breccia in Section 51R-2 has an undeformed carbonate vein core, indicating at least two generations of carbonate vein formation.
Daily Science Report for 8 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: The RCB coring assembly was lowered into Hole U1473A and a wash core barrel was deployed to facilitate the cleaning out of debris before resumption of coring. The last meter of the hole had to be reamed because the reverse circulation junk basked (RCJB) had created a slightly undersized hole the previous day. We finally pumped 50 barrels, then 30 barrels of high viscosity mud, and retrieved the wash core barrel at 0515 h. Another core barrel was deployed and Cores 46R–49R (410.8–449.6 mbsf) were recovered with a total recovery of 10.3 m (27%). The coring operation included another five 30-barrel mud sweeps. Due to excessive torque in the lowermost part of the hole, a wiper trip was conducted to condition the hole (1715–2145 h) before coring resumed.
Cores U1473A-45J to 49R consist mostly of coarse-grained subophitic olivine gabbro and coarse-grained granular disseminated-oxide olivine gabbro, with rare occurrence of coarse-grained granular oxide olivine gabbro. Several felsic veins occur in the deepest part of the described interval. The more altered parts of these cores are associated with carbonate veining. At least three generations of crystal plastic deformation were observed that vary both in strength and degree of localization. The thicker decimeter shear zones are characterized by medium to coarse grain sizes and show protomylonitic deformation. These are intruded by finer grained rocks, which sometimes show a mylonitic fabric that crosscuts the earlier fabric, thus recording a second phase of deformation. The third generation of high temperature deformation is shown by thin, mm-sized shear bands that crosscut all fabric types. In one case, Section 48R-1, a subvertical shear band crosscuts a leucocratic magmatic vein. These relationships indicate a down temperature interplay of magmatic intrusion and deformation with increasing strain localization. Cores 46R–49R include six breccias or cataclasites, more than any other interval of similar length cored so far. The breccias are characterized by clasts of pyroxene and carbonate vein networks, with the latter also filling or partially filling void space created by brecciation. The breccias are crosscut by later fractures filled with carbonate, indicating two generations of carbonation. These veins and beccias are likely part of a young high angle brittle fault system.
Daily Science Report for 7 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: The Bowen reverse circulating junk basket (RCJB) assembly deployed yesterday was retrieved and arrived at the rig floor at 0110 h. It recovered gravel, including a few boulders, of gabbroic and fault rock material, but no signs of the missing roller cone parts. The RCJB was made up once more and reentered in Hole U1473 at 0555 h. With the bit near the bottom of the hole, two 30-barrel and one 50-barrel high-viscosity mud sweeps were pumped before the bit was worked up and down from 0845–1030 h. Then the reverse circulation was activated and the bit rotated for nearly an hour before the drill string was recovered, clearing the rig floor at 1315 h. To everyone’s big surprise, we had recovered an unprecedented 0.5 m long, 18 cm diameter core from a cored interval of 0.6 m (83% recovery).
Given the tight fit of the RCJB assembly in the hole, it was extremely unlikely that any roller cone bits could have remained at the bottom of the hole. We speculated that the cones may have been lifted out of the hole with one or more of the previous three runs with two fishing magnets and the RCJB (Run07-FM, Run08-FMM, Run09-RCJB) and dropped on the seafloor before they could reach the rig floor. We therefore decided to deploy the RCB coring assembly and at 2320 h reentered Hole U1473 for the tenth time.
Scientists continued detailed thin section observations and submitted revised methods descriptions as well as revised reports for the Site 1105 legacy core descriptions.
Daily Science Report for 6 January 2016
Location: Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: After reentry into Hole U1473A last night, the Bowen 9 inch fishing magnet with milling guide was lowered to the bottom of the hole for the second attempt to retrieve three roller cones. We pumped 30 barrels of high viscosity mud and worked the magnet up and down near the bottom of the hole for one hour (0130–0230 h). The drill string was retrieved and the Bowen magnet cleared the rig floor at 0800 h, without any roller cones. Next a Bowen reverse circulating junk basket (RCJB) and two standard junk baskets were made up to the bottom-hole assembly (BHA) and deployed, reentering Hole U1473 at 1330 h. After tagging the bottom of the hole at 1830 h, the hole was swept with 20 barrels of mud. The RCJB assembly was worked up and down near the bottom of the hole and reverse circulation was activated by pumping the flow-deviating steel ball down the drill pipe (1915–2015 h). We began retrieving the drill string, which cleared the seafloor just before midnight.
Gravel recovered from the BHA with the fishing magnet with milling guide (FMM) was curated as U1473A-Run08-FMM and is mostly chlorite-rich fault breccia.
Scientists continued detailed thin section observations and report preparations.
Daily Science Report for 5 January 2016
Location:
Underway from medical evacuation transfer near Mauritius back to Site U1473
Hole U1473A (32°42.3622′S, 57°16.6880′E, 710.2 mbsl)
Science Update: We arrived at Site U1473 at 1030 h, completing the last 110 nmi of the 662 nmi transit back to Site U1473. A bottom-hole assembly was made up with new drill collars and a Bowen 9 inch fishing magnet with milling guide for the second attempt to retrieve three roller cones left at the bottom of Hole U1473A. Hole U1473A was reentered at 2146 h and by midnight the bit was near the bottom of the hole.
Technical support personnel completed the process of cutting and packing >1000 samples for postcruise research from the first 410 m interval cored at Site U1473. Scientists continued detailed thin section observations and worked on reports.
Daily Science Report for 4 January 2016
Location: Underway from medical evacuation transfer near Mauritius back to Site U1473.
Science Update: We completed 240 nmi of the 660 nmi transit back to Site U1473, with ~100 nmi remaining. We expect to arrive at Site U1473 on 5 January at 1030 h. Upon arrival, we will continue fishing operations for the three roller cone bits lost at the bottom of Hole U1473A.
Scientists clarified a few last issues regarding their requested postcruise research samples in 183 working section halves from Cores U1473A-2R to 44R. Technical support personnel made progress cutting and processing the samples. All laboratory teams continued to finalize Methods sections and Site 1105 reports as well as draft Site U1473 reports. The laboratory teams also continued observations and measurements on remaining samples.
Thin sections of olivine gabbro show subophitic coarse-grained textures and small amounts of late magmatic amphibole. Most of the studied intervals show partial plastic deformation as indicated by plagioclase recrystallization to smaller grain size. Detailed observations on interactions between igneous and deformation processes focused on felsic intrusions (veins or pervasive injection within gabbros), which commonly contain large amounts of zircon and some titanite. Most felsic veins are leucocratic diorite in which primary plagioclase is intensely replaced by secondary plagioclase that is often filled with numerous inclusions of dusty material of submicroscopic size. In one felsic vein representing a former trondhjemite with a typical granophyric texture, quartz was completely replaced by secondary albite, implying the presence of intense hydrothermal flux within this vein. The majority of the felsic veins show limited crystal plastic deformation; however, in one example, the vein was clearly deformed, indicating that the felsic material injected syn-deformation or near the end of deformation.
The Petrophysics team completed measurements on the last of 94 discrete cube samples from Cores 2R to 44R. Compressional P-wave velocity values range from 5936 to 7152 m/s and average 6804 m/s. Vp apparent anisotropy is 2.4% on average. Grain density ranges from 2.87 to 3.13 g/cm3, and porosity ranges from 0.1% to 4.2%.
Daily Science Report for 3 January 2016
Location: Underway from medical evacuation transfer near Mauritius back to Site U1473.
Science Update: We completed 250 nmi of the 660 nmi transit back to Site U1473. We expect to arrive at Site U1473 on 5 January at 1030 h.
Scientists marked their requested postcruise research samples in 183 working section halves from Cores U1473A-2R to 44R, laid out across the shipboard laboratories. Technical support personnel began cutting samples, entering the information into the shipboard database, and packing the samples for shipping at the end of the cruise.
Igneous Petrologists analyzed two thin sections using the SEM and observed strong core-rim zonations in fine-grained plagioclase from mylonites (from Ca-richer in the core to Na-richer in the rim). They also used the opportunity to review all working section halves laid out for sampling to verify or modify the lithologic units previously defined for Hole U1473A. Thin section observations allowed the metamorphic petrology team to illustrate the high temperature replacement evolution recorded in Hole U1473A olivine gabbro. The earliest alteration stage is represented by the replacement of primary clinopyroxene with secondary clinopyroxene hosting minor amounts of plagioclase with parallel lamellae. This alteration stage was followed by the onset of crystal-plastic deformation, which produced recrystallization of plagioclase, olivine, and clinopyroxene. Both secondary and neoblastic clinopyroxenes are typically associated with minor amounts of red-brown amphibole. Microstructural characterization included fabrics ranging from magmatic to ultramylonitic, with the full continuum of deformations present in Section 23R-3. At the top of the section, weak deformation is exemplified by kinks in pyroxene and plagioclase. Further down the section the samples have a subhorizontal moderate crystal-plastic fabric defined by recrystallized plagioclase and elongate pyroxene grains. Even further down the section the subhorizontal crystal plastic fabric is cut by a subvertical, normal-sense mylonite. This continuum of deformation is the beginning of a 10 m thick zone of mylonite that extends through Core 24R.
All laboratory teams continued to finalize Methods sections and Site 1105 reports as well as draft Site U1473 reports.
Daily Science Report for 2 January 2016
Location: Underway from Site U1473 towards Mauritius for a medical evacuation, and back.
Science Update: After a 658 nmi transit, we arrived at the medical evacuation rendezvous point near Mauritius at 1634 h. The ship changed to dynamic positioning and waited for the helicopter, which landed at 1717 h. The helicopter departed with the patient at 1725 h and the ship reverted to transit mode, beginning the voyage back to Site U1473 at 1742 h. We expect to arrive back at Site U1473 early on 5 January.
The Sample Allocation Committee reviewed, adjusted, and approved all personal sample claims identified by tentative labels. The scientists subsequently began marking their postcruise research samples according to detailed instructions aimed at optimizing collaborative sampling and research, and preserving the remainder of the material for future sampling and study.
The core description teams also continued to describe thin sections arriving in daily batches. Focusing on the comparison between the different types of dikes encountered in Hole U1473A, the Igneous Petrology team identified late intergranular diabase, emplaced in a cooled environment, and dikes of granoblastic amphibolite that were emplaced in hot gabbro. The host gabbro was probably already hydrothermally altered and possibly suffered hydrous partial melting while being intruded. Another focus was on the comparison between magnetic susceptibility data and the amount of oxide in gabbro in order to calibrate the oxide modal composition from physical measurements.
Total static alteration intensity in Cores 25R to 33R, studied by the Metamorphic Petrology team, varies from slight to extensive. Most samples observed contain a variety of alteration minerals of secondary clinopyroxene, amphibole, chlorite, talc, clay minerals, and secondary plagioclase, which formed under a wide range of temperature conditions.
The Structural Geology team observed the presence of crosscutting shear bands, igneous contacts, and magmatic textures with weak crystal plastic overprint. The crosscutting relationships consist of subhorizontal crystal plastic fabrics cut by subvertical centimeter thick shear bands. This is in contrast to other crosscutting relationships where subvertical crystal plastic fabrics are crosscut by subhorizontal centimeter thick shear bands. The igneous contacts include irregular boundaries between coarse grained and medium grained olivine gabbro. The coarser grained gabbro is partially recrystallized whereas the medium grained gabbro is not. The magmatic textures have plagioclase core and mantle structures and very little if any recrystallization in olivine and pyroxene. The crystal plastic overprint observed in thin sections demonstrates that primary igneous textures described macroscopically actually have recorded some deformation.
The microbiology team completed ATP analysis of samples from Cores 36R through 44R. Measurable ATP was detected in Sections 41R-3 and 42R-2, and concentrations of 0.3 pg ATP/ml are one order of magnitude less than those in surface waters at Site U1473, which is tentatively encouraging.
Daily Science Report for 1 January 2016
Location: Underway from Site U1473 towards Mauritius for a medical evacuation.
Science Update: We continued our transit towards Mauritius for a medical evacuation, completing 230 nmi at an average speed of 9.6 kt. The somewhat reduced speed was due to heavy seas. We expect to arrive at the tentative rendezvous site at 1800 h on 2 January.
All 183 working section halves available so far from Hole U1473A were laid out across the core laboratories and scientists discussed group sampling and began flagging their requested samples for postcruise research.
The Metamorphic Petrology team reported the occurrence of a biotite or stilpnomelane corona around relic olivine in a gabbro intruded by felsic material, based on thin section analysis. Furthermore, they have analyzed the mineralogy of ~50 samples so far, mostly veins, using the onboard X-ray diffractometer. Quartz was identified in some of the felsic veins, occurring together with plagioclase. Clinozoisite/epidote were detected in some felsic and amphibole veins. Different clay mineral groups were identified in some altered felsic veins and fracture fillings.
The Paleomagnetic team continued their measurements on discrete samples, including low temperature (liquid nitrogen) demagnetization on samples from Cores U1473A-13R to 31R prior to subsequent thermal demagnetization. Analysis of remanence data collected from archive section halves using the SRM system continues to demonstrate that the sampled interval carries a reversed polarity magnetization that is steeper than the axial geocentric dipole inclination expected at Site U1473, reflecting significant footwall rotation.
The Physical Properties team continued their measurements on discrete samples. Thermal conductivity measured on 35 gabbroic rock samples ranges from 1.77 to 2.38 W/(m·K) and averages 2.20 W/(m·K).