Mass spectra were collected in the low-gain (LG) and high-gain (HG) channel (see Riedo et al.
2013a, b, c) from all ten locations on the sample. As an example the mass spectrum from spot number 6 is shown in Fig. 5. The LG channel records the major elements, which in this case are H, C, O, Na, Mg, Al, Si, S, K, Ca, Fe, Co and Cu. These mass spectra can be interpreted as CaCO3 with additions of quartz, clinochlore and other minor minerals. Mn was associated with CaCO3 and is identified in the LMS data where it is present already in the LG channel and much better seen in the HG channel (Fig. 5), which confirms that Mn is not a major, but a minor, species in the CaCO3 mineral. The HG channel shows the ratio D/H2 and the minor elements Li, B, C, O, Na, Mg, S, Mn, Fe, Ni and carbon clusters, which are all identified in Fig. 5.
Fig. 5. Mass spectrum of the LG and HG channels showing major and minor elements present in the sample in spot number 6. With observed dynamic range close to 106 the detection of elements with the concentrations down to ppm level are expected.
In the following, we discuss the variation of elemental abundances within the investigated depth layers and locations with relevance to mineralogy. Figures 6–10 summarize the results of the measurements performed at each location, 0–9, and the variation of the mass peak intensities of several elements are plotted between the location numbers. The mass spectra provide information on chemical composition change from one to the other location and also compositional changes with the depth. The apparent differences in mass peak intensities between ablation points 0–4 and 5–9 are due to the different ablation rates at these locations, which affected the measurement sensitivity. An increase of laser irradiance would be necessary to increase the ablation rate at the calcite site, but this would result in a decrease in spectral quality on millerite surfaces. The composition of the surrounding host material in the amygdale was known and a decision was made to optimize the laser fluence to the TFS area instead.
Fig. 6. Relative variation of oxygen with depth and proximity to the centre of TFS showing ablation points 0–4 in (a) and ablation points 5–9 in (b).
Fig. 7. Relative variation of (a) sulphur and (b) iron with depth and proximity to the centre of TFS.
Fig. 8. Relative element composition from LMS mass spectra for all ablation points showed in Fig. 5b represented both laterally by the integers at the x-axis giving the ablation point, and with depth (data between the integers with lowest number representing the surface). (a) Shows the relative abundances of Si, S and Fe, (b) O, Al, Ca and (c) Mg, Ni and Co.
Fig. 9. Relative element composition from LMS mass spectra for all ablation points showing the correlation of elements making up specific minerals identified in the sample using Raman spectroscopy. The format is the same as in Fig. 7. (a) shows arrows pointing on locations with overlaps of Co, Ni and S indicative of millerite, (b) shows arrows pointing on locations with overlaps of Ca, C and O indicative of calcite, and (c) shows arrows pointing on locations with overlaps of O and Si indicative of quartz.
Fig. 10. Relative elemental composition from LMS mass spectra for all ablation points showing the correlation of elements in minerals identified by Raman spectroscopy. The format is the same as in Fig. 7. (a) arrows pointing on locations with overlaps of Fe and O indicative of iron oxide, (b) shows data for Ti and O and the locations where the O/Ti ratio = 2 indicative for anastase, and (c) shows the data for Fe, Mg and Al.
The measurements at the first four locations were performed in the pure amygdale calcite, where the mass peaks of the elements C, Ca and O were observed to be the most intense in the mass spectrum and other elements are present in the mass spectra at a minor level. Calcium and O are present and coincide also throughout the entire ablation profile with several other elements. Their abundance ratios are consistent with the composition of calcite.
The oxidizing conditions increase linearly with the proximity to the TFS centre (Fig. 6), which is consistent also with the formation processes associated with the amygdaloidal pillow basalts. The negative correlation between S and Fe in contrast to the ESEM analysis indicates that there are no pyrite in the sample, which is confirmed by the Raman spectra where no pyrite could be observed (Fig. 7(a) and (b)). ESEM analyses are semi-quantitative and has a broad beam spot that may include elements to the spectra outside the target area, which is why Fe and S seems to appear together in the ESEM mapping figure (Fig. 3).
The amount of S increases linearly with the proximity to the TFS centre, Fig. 7(a), but the spatial variation for S and Fe does not show the same pattern. Instead, Fe and O coincide perfectly, which is consistent with the presence of iron oxides (Fig. 7(b)). The major elements appearing in the ESEM maps are S, Si, Fe, O, Ca, Al, Ni, Co and Mg are also present in mass spectra of all ablation points. Fig. 8 shows the analysis of all mass spectra for these elements in three panels for the locations given in Fig. 1(b).
Figure 8(c), shows that Ni and Co are strongly correlated throughout the entire data set, confirming the Raman observations of millerite (Fig. 4). Cobalt is closely correlated to Ni in the entire profile, but since no Co-bearing mineral phase was found with Raman spectroscopy this is likely a substitution of Ni for Co, which is a common feature in millerite (Ineson 2014). This means that both Ni and Co will be present and positively correlated with S because of their mutual association in the mineral.
To confirm the presence of the minerals derived from the Raman analyses, element correlation analyses are presented assuming that characteristic elements of these minerals can be identified in the LMS mass spectra, which are shown in Figs 9 and 10. In Fig. 9(a), the elements Ni, Co and S are positively correlated almost throughout the entire data set but it is most pronounced in the ablation points 2–6. Thereafter, there are some points in which the correlation is negative, where S is very high compared with Ni and Co. From these analyses of the mass spectra one can conclude that millerite is present at least on the edges of the TFS, but likely also inside the TFS. Calcite is mostly identified in the ablation points 0–5, as expected, since calcite is the dominating mineral there and the interferences of other elements are not so profound see Fig. 9(b). Quartz can possibly be observed as increased Si signals correlated with an increased O signal, see Fig. 9(c).
Figure 10(a)–(c), show the relative abundance of elements indicative of the location of iron oxides, anatase and chlorite minerals. The O abundance is loosely tracing the Fe abundance, especially at ablation points 7 and 8, suggesting that iron oxides are present in the sample, which is consistent with the literature from the area (Rex & Scott 1987). However, the location of anatase and chlorite is not unambiguous as many elements show high abundance in the TFS. Our Raman spectrum showed the presence of both anatase and chlorite within the TFS and the elements appearing in those minerals correlate also in the LMS element data (Fig. 10(b) and (c)). In Fig. 10(b), markings have been added to show the positions where the O/Ti ratio is 2 (which represents the anatase O/Ti ratio). From those points, it can be seen that the O/Ti ratio is 2 mostly outside the TFS, indicating that the anatase is coupled to the calcite rather than to the TFS. This indicates that anatase is syngenetic with the calcite and that the TFS is a later alteration product. The increase of O in the TFS with increasing ablation numbers (Fig. 8(b) and Fig. 9(b) and (c)) shows that at least some parts of the TFS are oxidized, which is also confirmed by the presence of iron oxides and alteration products such as clinochlore.
A detailed depth profile analysis is presented for location 4 and 7 in Fig. 11. Typically, at one measurement location the measured mass peak intensities of the analysed elements are observed to decay smoothly with the ablation progress (ablation number layer) when homogenous material is investigated. In the current studies, the peak intensity envelope is generally more complex. The measurements yield relatively large peak intensity variations are indicating a deficit or increase of the concentration of particular elements for particular ablation layer. From the analysis of element correlation (element deficit, element concentration increase) one can get an insight into mineralogical composition of the investigated layer.
Fig. 11. Variation of mass peak intensities of various elements with ablation depth measured at location 4(a) and 7(b).
The chemical compositional analysis of the first uppermost surface layers measured at location 4 shows no significant relative variation of elemental peak intensities. All peak intensities decay with relatively similar rate, and only very small variations can be seen for these uppermost surface layers. However, at location 7, larger intensity variations are observed. The peak intensities of the elements B, C, O and S decrease steadily with the ablation progress while the mass peak intensities of the other elements are observed to increase; they reach their highest intensities after ablation of approximately 20 layers, thereafter the intensities decrease.
Occasionally, at some depths (at some ablation layers) the peak intensities of some of the elements were observed to increase and others to decrease. At location 4 an increase of peak intensities was observed in the ablation layer ranges 814–820, 830–846, 894–910 and 952–968, respectively. In the depth layer range 865–874, the sulphur intensity is readily decreasing while intensities of other element increase. Large peak intensity variations were observed at location 7 for several ablation layers including 1435–1445, 1465–1488, 1535–1540 and 1570–1590. For these depth layers, an increase in intensity of several elements (including B, C, O, Na, S and Ti) is accompanying a decrease in the peak intensities of other measured elements (Mg, Al. Si, K, Ca, Cr, Fe, Ni and Co). The correlation and anticorrelation indicate presence of chemically different layers or particles. While changes of the intensities of several elements at location 4 (left panel) are relatively well-correlated, at location 7 (right panel) for some of the elements the intensities are anticorrelated. The depth scale is roughly estimated for these locations and leads to the conclusion that the size of these layers/particles is in the range 1–3 µm. The correlation of elements in ablation point number 4 indicates a fast precipitation, in which the calcite was precipitating quickly together with all other elements and no other minerals had the time to form. At ablation point 7, several minerals are present, indicating enough time for minerals (in this case, mainly secondary) to form and precipitate. The elements Mg, Si, Al, V, Ca, K, Co, Cr and Ni are anticorrelating with C, B, O, Na, S and Ti, which suggests a secondary mineralogy forming anatase, sodium carbonates and sulphates, whereas the heavier metals remained within the carbonates during the alteration of the amygdales. This is contradictory to the interpretation of the O/Ti ratio, where anatase was suggested to be syngenetic with the calcite. The solubility of Na, Ti, S, B and C ions is higher than that of the other elements, and it is therefore more likely that anatase is syngenetic with the secondary mineral precipitation than with the calcite. The oxidation of the minerals at point 7 is much higher than at point 4, suggesting that the alteration of the amygdales is through oxidative fluids.
The relative abundances of elements measured at the location 3–7 are presented in Table 1 and displayed in Fig. 12. The elemental analysis at each location was computed from the mass spectrum obtained by summing up the spectra of individual ablation layers.
Fig. 12. Elemental abundance fractions at several locations for major (a), minor (b) and trace elements (c, d).
Table 1. Abundance of elements for locations 3–7 (see Fig. 12) determined from the mass spectrometric analysis. Elements with concentrations down to ppm level are measured (Ca, F and Cl)
In general, many elements are correlated at the same ablation points, and only large deviations may be considered as tenable mineral identification. The size of the ablation crater is likely playing a role in that the ablated materials are picking up signals not only from the structures but also to a large extent from the surrounding host minerals, since the ablation spot size is larger than the thickness of the filaments.
The analyses of depth profiles from each spot are shown between the integer ablation numbers on the x-axis in Figs 8–10. The oxidation of the surface is clearly visible as high O signals at the surface followed by a rapid decrease with depth, except from ablation points 7 and 8, where the O is high throughout the investigated depth layers, indicating the presence of oxidized minerals such as iron oxides. With the first layer discarded, there are still strong sulphur signals with depth, showing repeated occurrence of S-rich grains with minor additions of Ni and Co (Fig. 12).