Pasivos contingentes explícitos

4.5.4.1 Magnetic separation - pyroxene

Further microscopic analysis of the magnetic fraction revealed grains partially bound by clay-sized particles as a result of the crushing process. To remove the ne particles, each sample was subjected to 90-120 minutes (rinsing every ve minutes) in an ultrasonic bath, at VUW,with a frequency of 35 kHz, power at 80%, and temperature of 25°C until the water ran clear (Figure 4.26). The samples were then oven-dried at 50°C.

76 CHAPTER 4. METHODS

Figure 4.26: The samples undergoing an ultrasonic bath at VUW to disaggregate any attached clay-sized particles.

The magnetic fraction of each sample was once again run through the Frantz at various amperages, slide angles, and slide tilts, in an attempt to extract clean pyroxene minerals. For each sample that had a pyroxene constituent, the pyroxene was found to be non-magnetic at 0.15 amps and magnetic at 0.4 amps (though this is subjective to the specic Frantz being used). This method produced samples with clean pyroxene minerals, but it was not possible to separate out some other minerals that shared the same magnetic susceptibility e.g. chlorite.

Heavy liquid separation, which provides an alternative method for mineral separation was not used because the densities ofthe major minerals within each sample were too similar for the process to be feasible.

4.5.4.2 Pyroxene cleaning procedure

The four samples that contained pyroxenes were crushed individually using VUW's RockLabs Ring Mill pulverizer to enhance the exposure ofcontaminated zones (i.e. weathering pits containing meteoric10_{Be; sensu Blard et al., 2008; Collins, 2015).}

The following procedure outlines the crushing process: ˆ Crush each sample for 20 seconds and sieve to <90µm.

4.5. TCND 77 Note: If the rst 20 seconds crushed <50% of the sample, increase this crushing time by a few seconds.

ˆ Continue this process till the >90µmfraction is less than the minimum amount required for the Ring Mill (refer to specic lab instructions for minimum and maximum samples weights).

ˆ Hand crush any leftover >90µmmaterial with a mortar and pestle and add to the sample.

This procedure resulted in four samples, with dierent amounts of pyroxene (Ta- ble 4.2), ready for chemistry. While the proportion of pyroxene is not crucial for this method (K. Norton, personal communication, August, 2016), it is important to know the mineralogy of the samples.

Table 4.2: Pyroxene constituent of each sample. Original sample

name Simpliedsample name Approximate amount ofpure pyroxene (%)

EG B CS 1 pA 5

EG O CS 3 pB 5

EG N CS 3 pC 20

EG S CS 9 pD 80

4.5.4.3 Pyroxene 10_{beryllium separation chemistry}

This technique is based on Blard et al., (2008) who pioneered the idea of extract- ing 10_{Be from pyroxene minerals though had large errors in the results. Collins}

(2015) successfully dated pyroxene minerals with relatively large errors though had no quartz mineral controls from the same sample. In this study, one sample (EG N CS 3) had both pyroxene and quartz minerals. During the chemistry steps, small adjustments to Collins' methods were made to account for low pyroxene percentages and equipment limitations. These include: adjustments to centrifuge rpm, rinsing with water as opposed to hydroxylammonium-chloride (NH2OH.HCl) in Leach 1

as we did not need to measure the 10_{Be concentration of the solution, and chem-}

ical amounts were adjusted based on sample size. The leaching steps are outlined in 4.5.4.4 and 4.5.4.5. Note that a simplied step-by-step version of the pyroxene leaching process is supplied in Appendix A.

4.5.4.4 Leach 1 - hydroxylammonium-chloride

The following leach steps dissolve any metal oxides and releases grain-absorbed me- teoric 10_{Be without removing the in situ} 10_{Be (Blard et al., 2008; Collins, 2015).}

78 CHAPTER 4. METHODS Approximately 5 g of each sample was put into 180 ml Savillex beakers of a known weight and total weights recorded. The samples were leached for 12 hours at 95°C in 25 ml of a 0.04M solution of NH2OH.HCl in 25% acetic acid. After decanting

excess solution into a waste container, the sediment samples were transferred into 50 ml centrifuge tubes. These were centrifuged three times for 5 minutes at 3000 rpm, rinsing the supernate into the waste container with ultrapure DI water (MilliQ) each time. The sediment samples were transferred into their original Savillex beakers and dried for two hours on a 100°C hotplate. Once dry, the samples were precisely weighed to determine the mass of material dissolved (Table 4.3).

4.5.4.5 Leach 2 - hydrochloric acid

The remaining sediment was transferred into the original 50 ml centrifuge tubes and topped up to 25 ml with 1M HCl. The tubes were shaken gently on `hot dog' rollers at 20°C for 24 hours. The samples were centrifuged three times for 5 minutes at 3000 rpm, again rinsing with MilliQ and discarding the supernate each time. The sediment samples were rinsed back into the original 180 ml Savillex beakers and dried overnight in a 70°C oven before being precisely weighed once again (Table 4.3).

Table 4.3: Pyroxene sample weights before and after the chemistry steps and total mass of material dissolved/lost from each sample.

Simplied sample name Sample weight (g) Post-leach

1 (g) Post-leach2 (g) Post-HF(g) Totalmass lost (%) pA 5 4.8964 4.6117 3.675 26.5 pB 5.2 4.8831 4.4066 3.091 40.5577 pC 5.1 4.8685 4.5217 4.2503 16.6608 pD 5 4.7964 4.5173 3.6415 27.17 4.5.4.6 Leach eectiveness

The total mass lost from samples pA-pD was compared with Collins' total mass lost (from her Table 2.1, p. 54) to clarify whether the leach steps had been eective enough to continue with processes without needing to perform a subsequent leach. The mass dissolved here falls within the bounds of Collins' data, therefore is su- cient to continue with the next steps.

The nal stages of pyroxene separation were performed by Dr. Kevin Norton at VUW following the hydrouoric acid dissolution, carrier addition, and column steps proposed by Collins (2015) and Jones (2015).

4.5. TCND 79