@@@ NQNT frequently uses the instrumental where English uses “because”, e.g. poldorelyanen “because of your strength, (lit.) by means of your strength”. NQNT generally uses pan for “since, because” when used as a conjugation joining phrases: pan pennelte savie “since they lacked faith”. Tamas Ferencz suggests that an “for” might also be used this way, though I think the meaning would be slightly different. However, Tolkien himself seems to have used an in this way in the Ambidexters Sentence: úsië, an cé mo quernë cendelë númenna, ve senya i hyarma tentanë Melcorello: “on the contrary, for [because] if one turned the face westward, as was usual the left hand pointed away from Melkor”.
This word probably should not be used temporally, as in the English “since then”. Sami Paldanius suggested the phrase lúmello yá “from a time when ...” for that purpose.
pan adv. "since" (in the sense of because) (VT49:17, 18). The word comes from a text that was later struck out; we cannot know whether Tolkien rejected the word as such.