Incidencia de complicaciones en los pacientes con IAM en el postoperatorio de cirugía cardíaca valvular

4.7.1 Partial Melting, Crystal Accumulation and Melt-Rock Reactions

Of the three possible origins for dunite and harzburgite in an arc-setting outlined in Chapter 3, the samples from Tubaf are also thought to be residues of very high degrees of depletion after multiple melt extraction events (Dick & Bullen, 1984; Bonatti & Michael, 1989). The exception to this is sample 136309-A which is thought to be a dunitic melt channel, formed through the crystallisation of olivine at the expense of pyroxene in the upper mantle (Kelemen et al., 1990).

Sensitive indicators of crystallisation and melt extraction indices of peridotites are known to be forsterite content and modal percentage of olivine, Cr#

and Mg# in spinel and Al2O3 in orthopyroxene (Jaques & Green, 1980; Dick &

Bullen, 1984; Bonatti & Michael, 1989; Arai, 1994). If the peridotites have experienced partial melting, residual peridotites will have high abundances of

forsteritic olivine, high Cr# in spinel, and low Al2O3 in orthopyroxene. In contrast,

crystal fractionation over a wide range of temperatures and melt compositions will yield cumulate peridotites with low abundances of less forsteritic olivine, and spinel with low Mg# and Cr# (Dick & Bullen, 1984). If the peridotite is a product of melt-rock reaction it is expected to have high modal orthopyroxene content and high Ni content in forsteritic olivine (Kelemen et al., 1998).

On these criteria, the dunite and harzburgite samples from Tubaf (with the exception of 136309-1A) are refractory (Fo# 90-91 in olivine, Cr# 45.0-69.1 and Mg#

27.8-85.6 in spinel, Al2O3 <2.5 wt% in orthopyroxene) that fall within the olivine-

spinel mantle array (OSMA) of Arai (1994) and the partial melting/residual mantle trends of Dick and Bullen (1984). Primary clinopyroxene is absent, and the small amount of secondary clinopyroxene occurs only as interstitial crystals in the fine- grained matrix so there is no truly residual clinopyroxene left, with these rocks most likely melted beyond the point of clinopyroxene disappearance from the residue (~20-25% melt extraction; Hellebrand et al., 2002; Barth et al., 2003).

The Tubaf sample 136309-1A has uniformly forsteritic olivine and Cr-rich spinel compositions consistent with formation through the melt-rock reaction

process (Dick & Bullen, 1984; Kelemen et al., 1990). The Ni content in olivine of 136309-1A is low (<0.3 wt% NiO), as would be expected from a sample that has been completely re-crystallised, and is now in the form of a dunitic melt channel. The melt-rock reaction process is described as a two-step process proposed in Kelemen (1998). First, peridotite with high Mg# and low orthopyroxene content is created by large degrees of polybaric melting. Later, these depleted residues were enriched in orthopyroxene by interaction with siliceous melts.

In similar processes outlined for the contaminated Ritter samples, the contaminated Tubaf peridotites are likely to have originally formed through a combination of the two methods presented above. It is likely that a majority of the contaminated samples were depleted residues and have been subsequently modified by the metasomatism and melt invasion which now observed.

4.7.2 Temperature

The Ballhaus, Berry & Green (1991) calibration of the reaction is well suited to the Cr-rich spinel that occurs in peridotite xenoliths such as the Tubaf and Ritter suites, and utilises the two major minerals (olivine and orthopyroxene) in these depleted rocks. Temperatures have been calculated for comparison using the Fe-Mg exchange geothermometer of O’Neill & Wall (1987), the two-pyroxene method of Wells (1977) and Brey, Kohler & Nickel (1990), and the Ca-in- orthopyroxene method of Kohler & Brey (1990). Two-pyroxene thermometry is not chosen for the peridotites of this study as textural evidence shows that clinopyroxene is a late-stage mineral, and coarse orthopyroxene and olivine grains may not be fully equilibrated with the interstitial clinopyroxene. Secondly, olivine and spinel is ubiquitous in the sample suite and allows temperature (and redox conditions) to be calculated for the greatest number of samples. Overall, the temperatures from the Fe-Mg exchange thermometers appear to be realistic and correspond to variable stages of cooling in the lithospheric source of the xenoliths. The calculated temperatures of the peridotite suite range from 755 to 836 °C, lower, but consistent with the spinel peridotite stability field and previous calculations of 790 to 1034 °C (using the two pyroxene method) by McInnes et al., (2001) for proximal Lihir Island. Calculated temperatures and oxygen fugacities are presented for the Tubaf suite (Table #) and further information on the method

4.7.3 Oxygen Fugacity

The fO2 calculations made at equilibrium temperatures for the peridotites

in this study are calculated using the Ballhaus et al. (1991) version of the olivine- spinel Fe-Mg exchange geothermometer at a pressure of 1.5 GPa. The absence of garnet and anorthite in the peridotites constrains their pressure of equilibration to between 1.0 and 2.2 GPa, and an arbitrary assumed pressure of 1.5 GPa was used

for fO2 calculations (see Chapter 3 for further detail on calculation and method for

determining redox conditions).

Oxygen barometry undertaken on these samples shows a range from reduced (-1.26) to oxidised (+0.86) conditions relative to the FMQ buffer. Previous

olivine-orthopyroxene-spinel oxybarometry by McInnes et al., (2001) gives fO2

ranging from ∆FMQ = -0.3 to +1.0 (average ∆FMQ = 0.06 ± 0.33) for xenoliths from

proximal Lihir Island. In comparison to peridotite suites from other subduction zone localities (e.g. Brandon & Draper; and references therein), the majority of

Tubaf xenoliths are more reduced than the Cascades (∆FMQ = +0.98 +/- 0.30),

Japanese arc (delta FMQ = 0.76 ± 0.33) xenoliths, peridotites from southwestern USA (delta FMQ = -0.39 ± 0.36), and British Columbia (delta FMQ = -0.22 +/- 0.40) localities.

Figure 4.20. a) Cr# (spi) vs. ∆log fO2 (FMQ) for Tubaf in comparison to abyssal

peridotites and Ritter samples.

Previous work (e.g. Parkinson & Arculus, 1999) provides evidence that the mantle below subduction zones is ubiquitously oxidised relative to oceanic and

Abyssal Peridotites Ritter- Contaminated Ritter- Re-equilibrated Ritter- Residual Ritter- Pyroxenite Tubaf Δ log fO 2 (FMQ) 0 1 2 −1 −2 Cr# (spinel) 0.2 0.4 0.6 0.8 1.0

ancient cratonic mantle and it is suggested that the ultimate source of the oxygen into the mantle wedge is from the subducted slab. Evidence of reduced peridotite material beneath a subduction zone raises the question of whether the mantle peridotite sampled by these xenoliths is arc-related at all and is reminiscent of the ‘life raft’ model involving buoyant Archaean peridotite preserved in the mantle

(Griffin et al., 2003). It is noted that it is also possible for these fO2 determinations

to be inherited, and not relate to conditions under which magmas are generated.

However, whilst reduced relative to FMQ, fO2 values for the Tubaf xenoliths plot

with the majority of the Ritter xenoliths which span the range of calculated fugacities from other arc localities (Parkinson & Arculus, 1999; Wood & Virgo, 1989; Umino & Yoshizawa, 1996; Parkinson et al., 1998; Ionov, 2010; Parkinson et al., 2003; Blatter & Carmichael, 1998; Pearce et al., 2000; McInnes et al., 2001;

Barsdell & Smith, 1989; Johnson et al., 1996), but within the fO2 range of the

abyssal peridotites (Fig. 4.20) (Bryndzia & Wood, 1990).

4.7.4 Metasomatism

Both petrography and geochemistry indicate that metasomatism has affected almost all of the peridotites in the Tubaf suite, in particular the contaminated samples. The simple observation that these rocks exhibit extremely refractory lithologies coupled with the presence of secondary clinopyroxene and orthopyroxene must mean that like the Ritter suite, the Tubaf suite has experienced varying levels of metasomatism. The main type of metasomatism to have affected the Tubaf suite is also most likely to be either hydrous metasomatism or silica enrichment.

Effects of metasomatism in mantle peridotite can be either modal or cryptic. In modal metasomatism, new minerals such as phlogopite and amphibole are formed, or in the case of Tubaf, pyroxene, with the presence of these minerals in peridotite xenoliths considered strong evidence of metasomatic processes in the mantle. In cryptic metasomatism, mineral compositions are changed, or introduced elements are concentrated on grain boundaries and the peridotite mineralogy appears unchanged. Cryptic metasomatism may be caused as percolating or rising melts interact with surrounding peridotite, and compositions of both the melt and the peridotite change. Cryptic metasomatism is reported by Grégoire et al., (2001)

modifications or to concentration of incompatible elements on grain boundaries by a metasomatic agent (Ishimaru et al., 2007).

The postulation that it is silica enrichment or hydrous metasomatism that has created the contamination in the Tubaf xenoliths is due to clear petrographic evidence of reactive percolation of siliceous fluid/melt leading to orthopyroxene crystallisation at the expense of olivine. Hydrous metasomatism is also suspected, as there is secondary hydrous mineral formation (phlogopite and amphibole) in the Tubaf suite, with LREE enrichment observed in the trace element patterns of clinopyroxene.

A mechanism for silica enrichment in the mantle has been proposed by several authors and is discussed in detail within Chapter 3. Textural observations indicate that the Tubaf xenoliths have experienced at least two observable stages of mineralisation that post-date formation of the residual protolith. These involve the formation of orthopyroxene along olivine grain boundaries and in monomineralic veins that cross-cut coarse olivine as well as the final-stage formation of fine-grained fibrous orthopyroxene. As outlined in Ionov (2010), this secondary, fine-grained orthopyroxene is thought to be the least significant event in relation to modal and major element variation, rather than being the main indice of subduction metasomatism, as proposed by Arai et al. (2003) and Ishimaru et al. (2007). As outlined by Grégoire (2001), the host-trachybasalt was formed by melting of the subducted slab at a relatively shallow mantle depth and similarities between the trace-element signature of the trachybasalt and the veining of the Tubaf samples have been put forward as evidence that both were derived from the same source. It is thought that the host-magma has reacted with the peridotite similar to the contamination in the Ritter suite. The spatial distribution of the metasomatic overprint by the trachybasalt is primarily controlled by pathways, such as grain boundaries or fractures and is a pervasive alteration feature of these samples.