You might get some useful information on this piece if you can get an X-ray Fluoroscope analysis. The process leaves no marks at all on the object. On your piece one would analyze that crusty scale in several places. There is no need to grind off the scale to clean metal, though if you're not watching a laboratory guy might do so.
Look for tin and arsenic, along with whatever else is there (maybe silver, antimony, lead). One that I used to have access to is the Innov-X Systems Model #XT-245S spectrometer. My employer used it to analyze the specialty nickel alloys which they sold. I understand this, or similar devices, is also to determine authenticity of old oil paintings, based on what metallic elements are in the paint.
The reason for such analysis would be that European bronze age weapons were cast of copper alloyed with a few percent, maybe 5 to 15%, of tin. The tin is what turned copper into bronze. Much of the tin used was mined in England.
South American bronze, to my fuzzy recollection, used arsenic rather than tin, to harden the copper.
I do not know what typical minor elements are present in native copper, such as we have here in Michigan. Such information, European and Inca (also Aztec, I suppose) bronze, native copper analyses, should be available with some searching.
During the conquest of Mexico Cortéz' men used the arquebus and crossbow in roughly similar quantities. When they ran out of gunpowder, well the musketeers weren't much use. However they set their Indian allies to making crossbow bolts, thousands of them, from copper which was available to them.
I got this from The Discovery and Conquest of Mexico 1517 - 1521, by Bernal Díaz del Castillo. I quote: p274: ". . . long lances . . .and, as they possessed much copper, to make for each one two metal points." p383 " . . . and over five loads of arrow heads made of copper, so that we could always make more arrows . . ." p391: " . . . Cortéz sent to advise all the friendly pueblos near Texcoco to make eight thousand arrow heads of copper in each pueblo . . ."
I would think your arrow head of sufficient archaeological interest that some local university would do the analyses.
Ahh, but your personal body must be present in any laboratory that might do the analyses. I know that as an American you speak English, and you probably believe that the laboratory technicians do as well. Not so. It may sound like English, but I can assure you from long personal experience that they absolutely speak a different language. There will be no respect for the antiquity of your item, and this includes amongst the archaeologists. One or the other will happily grind off an area to bright metal, or even slice it in half to get a better metallurgical analyses. Yes, even an archaeologist cannot be trusted not to destroy the item. I promise you this is so. You yourself must personally be within reach of the item. Neither the professors nor the graduate students mean any harm, they simply have no concept of its personal value to you.
However if you can get a non-destructive chemical analysis then one might delve further into the literature to determine its meaning. i have some papers on old copper alloys but have not read them in years. In Michigan it is possible I might get some native copper from our peninsula & get it analyzed where I used to work. They have done a nice job for me on 19th century bronze firearms parts. Also they sort of need my experience these days. Michigan copper may not be Aztec copper but it is a start.
All this to determine if some Aztec made it in the 16th century, or whether it came along much earlier with those bearded guys from the direction of the sunrise.
I am a retired metallurgist who, in his young life almost became an archaeologist. I have some interest in historical metallurgy.
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